[RFC PATCH v3.1] xSplice design.

[RFC PATCH v3.1] xSplice design.

[Konrad Rzeszutek Wilk]

Hey!

Since v3 [http://lists.xen.org/archives/html/xen-devel/2015-07/msg00990.html]
 - Nailed down the comments, ingested them in.
 - Wrote and tested some code.
RFC v2 [http://lists.xen.org/archives/html/xen-devel/2015-05/msg02142.html]
 - Ingested every review comment in.


The patches for the code are a shell - there is no patching done at all and
it is very much just to test out the design and hypercalls. The hard parts
are yet to come :-)

At the Seattle LinuxCon/Xen Summit I will be presenting about xSplice and
referring to this URL. There is also an slot for brainstorming to talk
in details about things we disagree - and there is ample time to talk
during dinner. Martin who has been heavily reviewing the design will be there
and I hope other folks will be there as well to shape the design and
how we want this to work.

The big outstanding issues are how we want to handle preemption. That
is the problem of making an hypercall and waiting for the hypervisor
to do its job (and the VCPU is blocked). In the past some XSAs have come
out to resolve this and I would very much like this to have it addressed at start.

I think the other issues that have been raised should also be discussed
naturally, but the above is crucial (at least for me). I've attached the
patches on how I thought the preemption part could be solved by having an 'worker'
in hypervisor acting on the commands - and we just poll on the status to see
what the hypervisor has done so far.

Lastly, I also plan to add an Wiki to outline the dependency implementation
parts that so far bubbled up - I figured Wiki would be better as some folks
could put their name behind it.

Now please excuse the roughness of the patch and this giant one huge having
everything in it. It ought to be split in three at least: hypervisor, toolstacks
(libxc and libxl) - that is to be done later.

 docs/misc/xsplice.h           |   80 +++
 docs/misc/xsplice.markdown    | 1230 +++++++++++++++++++++++++++++++++++++++++
 docs/misc/xsplice_test.c      |   78 +++
 tools/libxc/include/xenctrl.h |   16 +
 tools/libxc/xc_misc.c         |  183 ++++++
 tools/libxc/xc_private.c      |    3 +
 tools/misc/Makefile           |    4 +
 tools/misc/xen-xsplice.c      |  385 +++++++++++++
 xen/common/Makefile           |    1 +
 xen/common/kernel.c           |   11 +
 xen/common/keyhandler.c       |    8 +-
 xen/common/sysctl.c           |    5 +
 xen/common/version.c          |    5 +
 xen/common/xsplice.c          |  405 ++++++++++++++
 xen/include/public/sysctl.h   |   66 +++
 xen/include/public/version.h  |    4 +
 xen/include/xen/compile.h.in  |    1 +
 xen/include/xen/version.h     |    1 +
 xen/include/xen/xsplice.h     |    9 +
 19 files changed, 2494 insertions(+), 1 deletion(-)

Konrad Rzeszutek Wilk (2):
      xsplice: rfc.v3.1
      xsplice: Add hook for build_id

[RFC PATCH v3.1 1/2] xsplice: rfc.v3.1

[Konrad Rzeszutek Wilk]

*TODO*:
 - XSM add
 - Contributs/authors on xsplice.markdown
 - Figure out the preemption method (rfc.v4 target). Talk in Seattle?
 - Further work - write out an Wiki detailing what implementation
   pieces to be done for individual contributions. Do it before
   Seattle?

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 docs/misc/xsplice.h           |   80 +++
 docs/misc/xsplice.markdown    | 1230 +++++++++++++++++++++++++++++++++++++++++
 docs/misc/xsplice_test.c      |   78 +++
 tools/libxc/include/xenctrl.h |   16 +
 tools/libxc/xc_misc.c         |  183 ++++++
 tools/misc/Makefile           |    4 +
 tools/misc/xen-xsplice.c      |  360 ++++++++++++
 xen/common/Makefile           |    1 +
 xen/common/keyhandler.c       |    8 +-
 xen/common/sysctl.c           |    5 +
 xen/common/xsplice.c          |  405 ++++++++++++++
 xen/include/public/sysctl.h   |   66 +++
 xen/include/xen/xsplice.h     |    9 +
 13 files changed, 2444 insertions(+), 1 deletion(-)
 create mode 100644 docs/misc/xsplice.h
 create mode 100644 docs/misc/xsplice.markdown
 create mode 100644 docs/misc/xsplice_test.c
 create mode 100644 tools/misc/xen-xsplice.c
 create mode 100644 xen/common/xsplice.c
 create mode 100644 xen/include/xen/xsplice.h

diff --git a/docs/misc/xsplice.h b/docs/misc/xsplice.h
new file mode 100644
index 0000000..00061fc
--- /dev/null
+++ b/docs/misc/xsplice.h
@@ -0,0 +1,80 @@
+
+#include <stdint.h>
+#include <sys/types.h>
+
+#define XSPLICE_HOWTO_INLINE        0x1 /* It is an inline replacement. */  
+#define XSPLICE_HOWTO_RELOC_PATCH   0x2 /* Add an trampoline. */  
+
+#define XSPLICE_HOWTO_FLAG_PC_REL    0x1 /* Is PC relative. */  
+#define XSPLICE_HOWOT_FLAG_SIGN      0x2 /* Should the new value be treated as signed value. */  
+
+struct xsplice_reloc_howto {  
+    uint32_t    howto;/* XSPLICE_HOWTO_* */  
+    uint32_t    flag; /* XSPLICE_HOWTO_FLAG_* */  
+    uint32_t    size; /* Size, in bytes, of the item to be relocated. */  
+    uint32_t    r_shift; /* The value the final relocation is shifted right by; used to drop unwanted data from the relocation. */  
+    uint64_t    mask; /* Bitmask for which parts of the instruction or data are replaced with the relocated value. */  
+    uint8_t     pad[8]; /* Must be zero. */  
+};  
+
+struct xsplice_symbol {  
+    const char *name; /* The ELF name of the symbol. */  
+    const char *label; /* A unique xSplice name for the symbol. */  
+    uint8_t pad[16]; /* Must be zero. */  
+};  
+#define XSPLICE_PATCH_INLINE_TEXT   0x1
+#define XSPLICE_PATCH_INLINE_DATA   0x2
+#define XSPLICE_PATCH_RELOC_TEXT    0x3
+
+struct xsplice_patch {  
+    uint32_t type; /* XSPLICE_PATCH_* .*/  
+    uint32_t size; /* Size of patch. */  
+    uint64_t addr; /* The address of the inline new code (or data). */  
+    void *content; /* The bytes to be installed. */  
+    uint8_t pad[40]; /* Must be zero. */  
+};
+
+#define XSPLICE_SECTION_TEXT   0x00000001 /* Section is in .text */  
+#define XSPLICE_SECTION_RODATA 0x00000002 /* Section is in .rodata */  
+#define XSPLICE_SECTION_DATA   0x00000004 /* Section is in .data */  
+#define XSPLICE_SECTION_STRING 0x00000008 /* Section is in .str */  
+
+#define XSPLICE_SECTION_TEXT_INLINE 0x00000200 /* Change is to be inline. */   
+#define XSPLICE_SECTION_MATCH_EXACT 0x00000400 /* Must match exactly. */  
+#define XSPLICE_SECTION_NO_STACKCHECK 0x00000800 /* Do not check the stack. */  
+
+struct xsplice_section {  
+    struct xsplice_symbol *symbol; /* The symbol associated with this change. */  
+    uint64_t address; /* The address of the section (if known). */  
+    uint32_t size; /* The size of the section. */  
+    uint32_t flags; /* Various XSPLICE_SECTION_* flags. */
+    uint8_t pad[12]; /* To be zero. */  
+};
+
+struct xsplice_reloc {  
+    uint64_t addr; /* The address of the relocation (if known). */  
+    struct xsplice_symbol *symbol; /* Symbol for this relocation. */  
+    int64_t isns_target; /* rest of the ELF addend.  This is equal to the offset against the symbol that the relocation refers to. */  
+    struct xsplice_reloc_howto  *howto; /* Pointer to the above structure. */  
+    int64_t isns_added; /* ELF addend resulting from quirks of instruction one of whose operands is the relocation. For example, this is -4 on x86 pc-relative jumps. */  
+    uint8_t pad[24];  /* Must be zero. */  
+};  
+
+struct xsplice_code {  
+    struct xsplice_reloc *relocs; /* How to patch it. */  
+    uint32_t n_relocs;
+    struct xsplice_section *sections; /* Safety data. */  
+    uint32_t n_sections;
+    struct xsplice_patch *patches; /* Patch code and data */  
+    uint32_t n_patches;
+    uint8_t pad[28]; /* Must be zero. */
+};
+struct xsplice {
+    uint32_t version;
+    const char *name; /* A sensible name for the patch. Up to 40 characters. */  
+    const char *id; /* ID of the hypervisor this binary was built against. */  
+    uint32_t id_size;
+    struct xsplice_code *new; /* Pointer to the new code to be patched. */  
+    struct xsplice_code *old; /* Pointer to the old code to be checked against. */  
+    uint8_t pad[24];  /* Must be zero. */  
+};
diff --git a/docs/misc/xsplice.markdown b/docs/misc/xsplice.markdown
new file mode 100644
index 0000000..02fd4d1
--- /dev/null
+++ b/docs/misc/xsplice.markdown
@@ -0,0 +1,1230 @@
+# xSplice Design v1 (EXTERNAL RFC v3)
+
+## Rationale
+
+A mechanism is required to binarily patch the running hypervisor with new
+opcodes that have come about due to primarily security updates.
+
+This document describes the design of the API that would allow us to
+upload to the hypervisor binary patches.
+
+The document is split in four sections:
+ - Detailed descriptions of the problem statement.
+ - Design of the data structures.
+ - Design of the hypercalls.
+ - Implementation notes that should be taken into consideration.
+
+
+## Glossary
+
+ * splice - patch in the binary code with new opcodes
+ * trampoline - a jump to a new instruction.
+ * payload - telemetries of the old code along with binary blob of the new
+   function (if needed).
+ * reloc - telemetries contained in the payload to construct proper trampoline.
+
+## Multiple ways to patch
+
+The mechanism needs to be flexible to patch the hypervisor in multiple ways
+and be as simple as possible. The compiled code is contiguous in memory with
+no gaps - so we have no luxury of 'moving' existing code and must either
+insert a trampoline to the new code to be executed - or only modify in-place
+the code if there is sufficient space. The placement of new code has to be done
+by hypervisor and the virtual address for the new code is allocated dynamically.
+
+This implies that the hypervisor must compute the new offsets when splicing
+in the new trampoline code. Where the trampoline is added (inside
+the function we are patching or just the callers?) is also important.
+
+To lessen the amount of code in hypervisor, the consumer of the API
+is responsible for identifying which mechanism to employ and how many locations
+to patch. Combinations of modifying in-place code, adding trampoline, etc
+has to be supported. The API should allow read/write any memory within
+the hypervisor virtual address space.
+
+We must also have a mechanism to query what has been applied and a mechanism
+to revert it if needed.
+
+We must also have a mechanism to: (optional) provide an copy of the old code - so
+that the hypervisor can verify it against the code in memory; the new code;
+the symbol name of the function to be patched; or offset from the symbol;
+or virtual address.
+
+The complications that this design will encounter are explained later
+in this document.
+
+## Workflow
+
+
+The expected workflows of higher-level tools that manage multiple patches
+on production machines would be:
+
+ * The first obvious task is loading all available / suggested
+   hotpatches around system start.
+ * Whenever new hotpatches are installed, they should be loaded too.
+ * One wants to query which modules have been loaded at runtime.
+ * If unloading is deemed safe (see unloading below), one may want to
+   support a workflow where a specific hotpatch is marked as bad and
+   unloaded.
+ * If we do no restrict module activation order and want to report tboot
+   state on sequences, we might have a complexity explosion problem, in
+   what system hashes should be considered acceptable.
+
+## Patching code
+
+The first mechanism to patch that comes in mind is in-place replacement.
+That is replace the affected code with new code. Unfortunately the x86
+ISA is variable size which places limits on how much space we have available
+to replace the instructions. That is not a problem if the change is smaller
+than the original opcode and we can fill it with nops. Problems will
+appear if the replacement code is longer.
+
+The second mechanism is by replacing the call or jump to the
+old function with the address of the new function.
+
+A third mechanism is to add a jump to the new function at the
+start of the old function.
+
+### Example of trampoline and in-place splicing
+
+As example we will assume the hypervisor does not have XSA-132 (see
+*domctl/sysctl: don't leak hypervisor stack to toolstacks*
+4ff3449f0e9d175ceb9551d3f2aecb59273f639d) and we would like to binary patch
+the hypervisor with it. The original code looks as so:
+
+<pre>
+   48 89 e0                  mov    %rsp,%rax  
+   48 25 00 80 ff ff         and    $0xffffffffffff8000,%rax  
+</pre>
+
+while the new patched hypervisor would be:
+
+<pre>
+   48 c7 45 b8 00 00 00 00   movq   $0x0,-0x48(%rbp)  
+   48 c7 45 c0 00 00 00 00   movq   $0x0,-0x40(%rbp)  
+   48 c7 45 c8 00 00 00 00   movq   $0x0,-0x38(%rbp)  
+   48 89 e0                  mov    %rsp,%rax  
+   48 25 00 80 ff ff         and    $0xffffffffffff8000,%rax  
+</pre>
+
+This is inside the arch_do_domctl. This new change adds 21 extra
+bytes of code which alters all the offsets inside the function. To alter
+these offsets and add the extra 21 bytes of code we might not have enough
+space in .text to squeeze this in.
+
+As such we could simplify this problem by only patching the site
+which calls arch_do_domctl:
+
+<pre>
+<do_domctl>:  
+ e8 4b b1 05 00          callq  ffff82d08015fbb9 <arch_do_domctl>  
+</pre>
+
+with a new address for where the new `arch_do_domctl` would be (this
+area would be allocated dynamically).
+
+Astute readers will wonder what we need to do if we were to patch `do_domctl`
+- which is not called directly by hypervisor but on behalf of the guests via
+the `compat_hypercall_table` and `hypercall_table`.
+Patching the offset in `hypercall_table` for `do_domctl:
+(ffff82d080103079 <do_domctl>:)
+<pre>
+
+ ffff82d08024d490:   79 30  
+ ffff82d08024d492:   10 80 d0 82 ff ff   
+
+</pre>
+with the new address where the new `do_domctl` is possible. The other
+place where it is used is in `hvm_hypercall64_table` which would need
+to be patched in a similar way. This would require an in-place splicing
+of the new virtual address of `arch_do_domctl`.
+
+In summary this example patched the callee of the affected function by
+ * allocating memory for the new code to live in,
+ * changing the virtual address in all the functions which called the old
+   code (computing the new offset, patching the callq with a new callq).
+ * changing the function pointer tables with the new virtual address of
+   the function (splicing in the new virtual address). Since this table
+   resides in the .rodata section we would need to temporarily change the
+   page table permissions during this part.
+
+
+However it has severe drawbacks - the safety checks which have to make sure
+the function is not on the stack - must also check every caller. For some
+patches this could mean - if there were an sufficient large amount of
+callers - that we would never be able to apply the update.
+
+### Example of different trampoline patching.
+
+An alternative mechanism exists where we can insert a trampoline in the
+existing function to be patched to jump directly to the new code. This
+lessens the locations to be patched to one but it puts pressure on the
+CPU branching logic (I-cache, but it is just one unconditional jump).
+
+For this example we will assume that the hypervisor has not been compiled
+with fe2e079f642effb3d24a6e1a7096ef26e691d93e (XSA-125: *pre-fill structures
+for certain HYPERVISOR_xen_version sub-ops*) which mem-sets an structure
+in `xen_version` hypercall. This function is not called **anywhere** in
+the hypervisor (it is called by the guest) but referenced in the
+`compat_hypercall_table` and `hypercall_table` (and indirectly called
+from that). Patching the offset in `hypercall_table` for the old
+`do_xen_version` (ffff82d080112f9e <do_xen_version>)
+
+</pre>
+ ffff82d08024b270 <hypercall_table>  
+ ...  
+ ffff82d08024b2f8:   9e 2f 11 80 d0 82 ff ff  
+
+</pre>
+with the new address where the new `do_xen_version` is possible. The other
+place where it is used is in `hvm_hypercall64_table` which would need
+to be patched in a similar way. This would require an in-place splicing
+of the new virtual address of `do_xen_version`.
+
+An alternative solution would be to patch insert a trampoline in the
+old `do_xen_version' function to directly jump to the new `do_xen_version`.
+
+<pre>
+ ffff82d080112f9e <do_xen_version>:  
+ ffff82d080112f9e:       48 c7 c0 da ff ff ff    mov    $0xffffffffffffffda,%rax  
+ ffff82d080112fa5:       83 ff 09                cmp    $0x9,%edi  
+ ffff82d080112fa8:       0f 87 24 05 00 00       ja     ffff82d0801134d2 <do_xen_version+0x534>  
+</pre>
+
+with:
+
+<pre>
+ ffff82d080112f9e <do_xen_version>:  
+ ffff82d080112f9e:       e9 XX YY ZZ QQ          jmpq   [new do_xen_version]  
+</pre>
+
+which would lessen the amount of patching to just one location.
+
+In summary this example patched the affected function to jump to the
+new replacement function which required:
+ * allocating memory for the new code to live in,
+ * inserting trampoline with new offset in the old function to point to the
+   new function.
+ * Optionally we can insert in the old function a trampoline jump to an function
+   providing an BUG_ON to catch errant code.
+
+The disadvantage of this are that the unconditional jump will consume a small
+I-cache penalty. However the simplicity of the patching and higher chance
+of passing safety checks make this a worthwhile option.
+
+### Security
+
+With this method we can re-write the hypervisor - and as such we **MUST** be
+diligent in only allowing certain guests to perform this operation.
+
+Furthermore with SecureBoot or tboot, we **MUST** also verify the signature
+of the payload to be certain it came from a trusted source and integrity
+was intact.
+
+As such the hypercall **MUST** support an XSM policy to limit what the guest
+is allowed to invoke. If the system is booted with signature checking the
+signature checking will be enforced.
+
+## Design of payload format
+
+The payload **MUST** contain enough data to allow us to apply the update
+and also safely reverse it. As such we **MUST** know:
+
+ * (optional) What the old code is expected to be. We **MUST** be able verify it
+   against the runtime code if old code is included in the payload.
+ * Verify the build-id of hypervisor against the payload build-id.
+ * The locations in memory to be patched. This can be determined dynamically
+   via symbols or via virtual addresses.
+ * The new code (or data) that will be patched in.
+ * Signature to verify the payload.
+
+This binary format can be constructed using an custom binary format but
+there are severe disadvantages of it:
+
+ * The format might need to be changed and we need an mechanism to accommodate
+   that.
+ * It has to be platform agnostic.
+ * Easily constructed using existing tools.
+
+As such having the payload in an ELF file is the sensible way. We would be
+carrying the various sets of structures (and data) in the ELF sections under
+different names and with definitions. The prefix for the ELF section name
+would always be: *.xsplice* to match up to the names of the structures.
+
+Note that every structure has padding. This is added so that the hypervisor
+can re-use those fields as it sees fit.
+
+Earlier design attempted to ineptly explain the relations of the ELF sections
+to each other without using proper ELF mechanism (sh_info, sh_link, data
+structures using Elf_* types, etc). This design will explain in detail
+the structures and how they are used together and not dig in the ELF
+format - except mention that the section names should match the
+structure names.
+
+### ASCII art of structures.
+
+The diagram below is omitting some entries to easy the relationship explanation.
+
+<pre>
+                                                                          /---------------------\  
+                                                                       +->| xsplice_reloc_howto |  
+                                                                      /   \---------------------/  
+                                                /---------------\ 1:1/  
+                                             +->| xsplice_reloc |   /  
+                                            /   | - howto       +--/  1:1 /----------------\  
+                                           /    | - symbol      +-------->| xsplice_symbol |  
+                                     1:N  /     \---------------/       / \----------------/  
+/----------\        /--------------\     /                             /  
+| xsplice  |  1:1   | xsplice_code |    /                          1:1/  
+| - new    +------->|  - relocs    +---/  1:N   /-----------------\  /  
+| - old    +------->|  - sections  +----------->| xsplice_section | /  
+\----------/        |  - patches   +--\         | - symbol        +/ 1:1   /----------------\  
+                    \--------------/   \        | - addr          +------->| .text or .data |  
+                                        \       \----------------/         \----------------/  
+                                         \  
+                                      1:N \  
+                                           \    /----------------\  
+                                            +-->| xsplice_patch  |  1:1  /----------------\  
+                                                | - content      +------>| binary code or |  
+                                                \----------------/       | data           |  
+                                                                         \----------------/  
+
+</pre>
+
+### xsplice structures
+
+From the top (or left in the above diagram) the structures are:
+
+ *  `xsplice`. The top most structure - contains the the name of the update,
+    the id to match against the hypervisor, the pointer to the metadata for
+    the new code and optionally the metadata for the old code.
+
+ * `xsplice_code`. The structure that ties all of this together and defines
+   the payload. Contains arrays of `xsplice_reloc`, `xsplice_section`, and
+   `xsplice_patch`.
+
+ * `xsplice_reloc` contains telemetry used for patching - which describes the
+   targets to be patched and how to do it.
+
+ * `xsplice_section` - the safety data for the code. Contains pointer to the
+   symbol (`xsplice_symbols`) and pointer to the code (`.text`) or data (`.data`),
+   which are to be used during safety and dependency checking.
+
+ * `xsplice_patch`: the description of the new function to be patched in
+   along with the binary code or data.
+
+ * ` xsplice_reloc_howto`: the howto properly construct trampolines for an patch.
+   We may have multiple locations for which we need to insert a trampoline for a
+   payload and each location might require a different way of handling it.
+
+ * `xsplice_symbols `.  The symbol that will be patched.
+
+In short the *.xsplice* sections (with `xsplice` being the top) represent
+various structures to define the new code and safety checks for the old
+code (optional). The ELF provides the mechanism to glue it all together when
+loaded in memory.
+
+
+Note that a lot of these ideas are borrowed from kSplice which is
+available at: https://github.com/jirislaby/ksplice
+
+### struct xsplice
+
+The top most structure is quite simple. It defines the name, the id
+of the hypervisor, pointer to the new code & data and an pointer to
+the old code & data (optional).
+
+The `new` code uses all of the `xsplice_*` structures while the
+`old`  does not use the `xsplice_reloc` structures.
+
+The sections defining the structures will explicitly state
+when they are not used.
+
+<pre>
+struct xsplice {
+    uint32_t version; /* Version of payload. */
+    const char *name; /* A sensible name for the patch. Up to 40 characters. */  
+    const char *id; /* ID of the hypervisor this binary was built against. */  
+    uint32_t id_size; /* Size of the ID. */  
+    struct xsplice_code *new; /* Pointer to the new code & data to be patched. */  
+    struct xsplice_code *old; /* Pointer to the old code & data to be checked against. */  
+    uint8_t pad[24];  /* Must be zero. */  
+};
+</pre>
+
+The size of this structure should be 64 bytes.
+
+### xsplice_code
+
+The structure embedded within this section ties the other
+structures together. It has the pointers with an start and end
+address for each set of structures. This means that an update
+can be split in multiple changes - for example to accomodate
+an update that contains both code and data and will need patching
+in both .text and .data sections.
+
+<pre>
+struct xsplice_code {  
+    struct xsplice_reloc *relocs; /* How to patch it. */  
+    uint32_t n_relocs;
+    struct xsplice_section *sections; /* Safety data. */  
+    uint32_t n_sections;
+    struct xsplice_patch *patches; /* Patch code and data */  
+    uint32_t n_patches;  
+    uint8_t pad[28]; /* Must be zero. */
+};
+</pre>
+
+The size of this structure is 64 bytes.
+
+There can be at most two of those structures in the payload.
+One for the `new` and another for the `old` (optional).
+
+If it is for the old code the relocs, and relocs_end values will be ignored.
+
+
+### xsplice_reloc
+
+The `xsplice_code` defines an array of these structures. As such
+an singular structure defines an singular point where to patch the
+hypervisor.
+
+The structure contains the address of the hypervisor (if known),
+the symbol associated with this address, how the patching is to
+be done, and platform specific details.
+
+The `isns_added` is an value to be used to compute the new offset
+due to the quirks of the operands of the instruction. For example
+to patch in an jump operation to the new code - the offset is relative
+to the program counter of the next instruction - hence the offset
+value has to be subtracted by four bytes - hence this would contain -4 .
+
+The `isns_target` is the offset against the symbol.
+
+The relation of this structure with `xsplice_patch` is 1:1, even
+for inline patches. See the section detailing the structure
+`xsplice_reloc_howto`.
+
+The relation of this structure with `xsplice_section` is 1:1.
+
+This structure is as follow:
+
+<pre>
+struct xsplice_reloc {  
+    uint64_t addr; /* The address of the relocation (if known). */  
+    struct xsplice_symbol *symbol; /* Symbol for this relocation. */  
+    int64_t isns_target; /* rest of the ELF addend.  This is equal to the offset against the symbol that the relocation refers to. */  
+    struct xsplice_reloc_howto  *howto; /* Pointer to the above structure. */  
+    int64_t isns_added; /* ELF addend resulting from quirks of instruction one of whose operands is the relocation. For example, this is -4 on x86 pc-relative jumps. */  
+    uint8_t pad[24];  /* Must be zero. */  
+};  
+
+</pre>
+
+The size of this structure is 64 bytes.
+
+### xsplice_section
+
+The structure defined in this section is used during pre-patching and
+during patching. Pre-patching it is used to verify that it is safe
+to update with the new changes - and contains safety data on the old code
+and what kind of matching we are to expect.
+
+That is whether the address (either provided or resolved when payload is
+loaded by referencing the symbols) is:
+
+ * in memory,
+ * correct size,
+ * in it's proper ELF section,
+ * has been already patched (or not),
+ * is expected not to be on any CPU stack - (or if it is OK for it be on the CPU stack).
+
+with what we expect it to be.
+
+Some of the checks can be relaxed, as such the `flag` values
+can be or-ed together.
+
+Depending on the time when patching is done, stack checking might not
+be required.
+<pre>
+
+#define XSPLICE_SECTION_TEXT   0x00000001 /* Section is in .text */  
+#define XSPLICE_SECTION_RODATA 0x00000002 /* Section is in .rodata */  
+#define XSPLICE_SECTION_DATA   0x00000004 /* Section is in .data */  
+#define XSPLICE_SECTION_STRING 0x00000008 /* Section is in .str */  
+
+#define XSPLICE_SECTION_TEXT_INLINE 0x00000200 /* Change is to be inline. */   
+#define XSPLICE_SECTION_MATCH_EXACT 0x00000400 /* Must match exactly. */  
+#define XSPLICE_SECTION_NO_STACKCHECK 0x00000800 /* Do not check the stack. */  
+
+
+struct xsplice_section {  
+    struct xsplice_symbol *symbol; /* The symbol associated with this change. */  
+    uint64_t address; /* The address of the section (if known). */  
+    uint32_t size; /* The size of the section. */  
+    uint32_t flags; /* Various XSPLICE_SECTION_* flags. */
+    uint8_t pad[12]; /* To be zero. */  
+};
+
+</pre>
+
+The size of this structure is 32 bytes.
+
+### xsplice_patch
+
+This structure has the binary code (or data) to be patched. Depending on the
+type it can either an inline patch (data or text) or require an relocation
+change (which requires a trampoline). Naturally it also points to a blob
+of the binary data to patch in, and the size of the patch.
+
+The `addr` is used when the patch is for inline change. It can be
+an virtual address or an offset from the symbol start.
+
+If it is an relocation (requiring a trampoline), the `addr` should be zero.
+
+There must be an corresponding ` struct xsplice_reloc` and
+`struct xsplice_section` describing this patch.
+
+<pre>
+#define XSPLICE_PATCH_INLINE_TEXT   0x1
+#define XSPLICE_PATCH_INLINE_DATA   0x2
+#define XSPLICE_PATCH_RELOC_TEXT    0x3
+
+struct xsplice_patch {  
+    uint32_t type; /* XSPLICE_PATCH_* .*/  
+    uint32_t size; /* Size of patch. */  
+    uint64_t addr; /* The address (or offset from symbol) of the inline new code (or data). */  
+    void *content; /* The bytes to be installed. */  
+    uint8_t pad[40]; /* Must be zero. */  
+};
+
+</pre>
+
+The size of this structure is 64 bytes.
+
+### xsplice_symbols
+
+The structure contains an pointer to the name of the ELF symbol
+to be patched and as well an unique name for the symbol.
+
+The `label` is used for diagnostic purposes - such as including the
+name and the offset.
+
+The structure is as follow:
+
+<pre>
+struct xsplice_symbol {  
+    const char *name; /* The ELF name of the symbol. */  
+    const char *label; /* A unique xSplice name for the symbol. */  
+    uint8_t pad[16]; /* Must be zero. */  
+};  
+</pre>
+
+The size of this structure is 32 bytes.
+
+
+### xsplice_reloc_howto
+
+The howto defines in the detail the change. It contains the type,
+whether the relocation is relative, the size of the relocation,
+bitmask for which parts of the instruction or data are to be replaced,
+amount the final relocation is shifted by (to drop unwanted data), and
+whether the replacement should be interpreted as signed value.
+
+The structure is as follow:
+
+<pre>
+#define XSPLICE_HOWTO_INLINE        0x1 /* It is an inline replacement. */  
+#define XSPLICE_HOWTO_RELOC_PATCH   0x2 /* Add a trampoline. */  
+
+#define XSPLICE_HOWTO_FLAG_PC_REL    0x1 /* Is PC relative. */  
+#define XSPLICE_HOWOT_FLAG_SIGN      0x2 /* Should the new value be treated as signed value. */  
+
+struct xsplice_reloc_howto {  
+    uint32_t    howto; /* XSPLICE_HOWTO_* */  
+    uint32_t    flag; /* XSPLICE_HOWTO_FLAG_* */  
+    uint32_t    size; /* Size, in bytes, of the item to be relocated. */  
+    uint32_t    r_shift; /* The value the final relocation is shifted right by; used to drop unwanted data from the relocation. */  
+    uint64_t    mask; /* Bitmask for which parts of the instruction or data are replaced with the relocated value. */  
+    uint8_t     pad[8]; /* Must be zero. */  
+};  
+
+</pre>
+
+The size of this structure is 32 bytes.
+
+### Example
+
+There is a wealth of information that the payload must have to define a simple
+patch.  For this example we will assume that the hypervisor has not been compiled
+with fe2e079f642effb3d24a6e1a7096ef26e691d93e (XSA-125: *pre-fill structures
+for certain HYPERVISOR_xen_version sub-ops*) which mem-sets an structure
+in `xen_version` hypercall. This function is not called **anywhere** in
+the hypervisor (it is called by the guest) but referenced in the
+`compat_hypercall_table` and `hypercall_table` (and indirectly called
+from that). There are two ways to patch this:
+inline patch `hvm_hypercall64_table` and `hvm_hypercall` with a new
+address for the new `do_xen_version` , or insert
+trampoline in `do_xen_version` code. The example will focus on the later.
+
+The `do_xen_version` code is located at virtual address ffff82d080112f9e.
+
+<pre>
+struct xsplice_code xsplice_xsa125;  
+struct xsplice_reloc relocs[1];  
+struct xsplice_section sections[1];  
+struct xsplice_patch patches[1];  
+struct xsplice_symbol do_xen_version_symbol;  
+struct xsplice_reloc_howto do_xen_version_howto;  
+char do_xen_version_new_code[1728];  
+
+#ifndef HYPERVISOR_ID  
+#define HYPERVISOR_ID "92dd05a61556c554155b1508c9cf67d993336d28"
+#endif  
+
+struct xsplice xsa125 = {  
+    .name = "xsa125",  
+    .id = HYPERVISOR_ID,  
+    .old = NULL,  
+    .new = &xsplice_xsa125,  
+};  
+
+struct xsplice_code xsplice_xsa125 = {  
+    .relocs = &relocs[0],  
+    .n_relocs = 1,  
+    .sections = &sections[0],  
+    .n_sections = 1,  
+    .patches = &patches[0],  
+    .n_patches = 1,   
+};
+
+struct xsplice_reloc relocs[1] = {  
+    {  
+        .addr = 0xffff82d080112f9e,  
+        .symbol = &do_xen_version_symbol,  
+        .isns_target = 0,  
+        .howto = &do_xen_version_howto,  
+        .isns_added = -4,  
+    },  
+};  
+
+struct xsplice_symbol do_xen_version_symbol = {  
+    .name = "do_xen_version",  
+    .label = "do_xen_version+<0x0>",  
+};  
+
+struct xsplice_reloc_howto do_xen_version_howto = {  
+    .type = XSPLICE_HOWTO_RELOC_PATCH,  
+    .flag = XSPLICE_HOWTO_FLAG_PC_REL,  
+    .r_shift = 0,  
+    .mask = (-1ULL),  
+};  
+
+
+struct xsplice_section sections[1] = {  
+    {  
+        .symbol = &do_xen_version_symbol,  
+        .address = 0xffff82d080112f9e,  
+        .size = 1728,  
+        .flags = XSPLICE_SECTION_TEXT,  
+    },  
+};  
+
+struct xsplice_patch patches[1] = {  
+    {  
+        .type = XSPLICE_PATCH_RELOC_TEXT,  
+        .size = 1728,  
+        .addr = 0,  
+        .content = &do_xen_version_new_code,  
+    },  
+};  
+
+char do_xen_version_new_code[1728] = { 0x83, 0xff, 0x09, /* And more code. */};  
+</pre>
+
+
+## Signature checking requirements.
+
+The signature checking requires that the layout of the data in memory
+**MUST** be same for signature to be verified. This means that the payload
+data layout in ELF format **MUST** match what the hypervisor would be
+expecting such that it can properly do signature verification.
+
+The signature is based on the all of the payloads continuously laid out
+in memory. The signature is to be appended at the end of the ELF payload
+prefixed with the string '~Module signature appended~\n', followed by
+an signature header then followed by the signature, key identifier, and signers
+name.
+
+Specifically the signature header would be:
+
+<pre>
+#define PKEY_ALGO_DSA       0  
+#define PKEY_ALGO_RSA       1  
+
+#define PKEY_ID_PGP         0 /* OpenPGP generated key ID */  
+#define PKEY_ID_X509        1 /* X.509 arbitrary subjectKeyIdentifier */  
+
+#define HASH_ALGO_MD4          0  
+#define HASH_ALGO_MD5          1  
+#define HASH_ALGO_SHA1         2  
+#define HASH_ALGO_RIPE_MD_160  3  
+#define HASH_ALGO_SHA256       4  
+#define HASH_ALGO_SHA384       5  
+#define HASH_ALGO_SHA512       6  
+#define HASH_ALGO_SHA224       7  
+#define HASH_ALGO_RIPE_MD_128  8  
+#define HASH_ALGO_RIPE_MD_256  9  
+#define HASH_ALGO_RIPE_MD_320 10  
+#define HASH_ALGO_WP_256      11  
+#define HASH_ALGO_WP_384      12  
+#define HASH_ALGO_WP_512      13  
+#define HASH_ALGO_TGR_128     14  
+#define HASH_ALGO_TGR_160     15  
+#define HASH_ALGO_TGR_192     16  
+
+
+struct elf_payload_signature {  
+	u8	algo;		/* Public-key crypto algorithm PKEY_ALGO_*. */  
+	u8	hash;		/* Digest algorithm: HASH_ALGO_*. */  
+	u8	id_type;	/* Key identifier type PKEY_ID*. */  
+	u8	signer_len;	/* Length of signer's name */  
+	u8	key_id_len;	/* Length of key identifier */  
+	u8	__pad[3];  
+	__be32	sig_len;	/* Length of signature data */  
+};
+
+</pre>
+(Note that this has been borrowed from Linux module signature code.).
+
+
+## Hypercalls
+
+We will employ the sub operations of the system management hypercall (sysctl).
+There are to be four sub-operations:
+
+ * upload the payloads.
+ * listing of payloads summary uploaded and their state.
+ * getting an particular payload summary and its state.
+ * command to apply, delete, or revert the payload.
+ * querying of the hypervisor ID (TODO).
+
+Most of the actions are asynchronous therefore the caller is responsible
+to verify that it has been applied properly by retrieving the summary of it
+and verifying that there are no error codes associated with the payload.
+
+We **MUST** make some of them asynchronous due to the nature of patching
+it requires every physical CPU to be lock-step with each other.
+The patching mechanism while an implementation detail, is not an short
+operation and as such the design **MUST** assume it will be an long-running
+operation.
+
+The sub-operations will spell out how preemption is to be handled (if at all).
+
+Furthermore it is possible to have multiple different payloads for the same
+function. As such an unique id per payload has to be visible to allow proper manipulation.
+
+The hypercall is part of the `xen_sysctl`. The top level structure contains
+one uint32_t to determine the sub-operations:
+
+<pre>
+struct xen_sysctl_xsplice_op {  
+    uint32_t cmd;  
+	union {  
+          ... see below ...  
+        } u;  
+};  
+
+</pre>
+while the rest of hypercall specific structures are part of the this structure.
+
+### XEN_SYSCTL_XSPLICE_UPLOAD (0)
+
+Upload a payload to the hypervisor. The payload is verified and if there
+are any issues the proper return code will be returned. The payload is
+not applied at this time - that is controlled by *XEN_SYSCTL_XSPLICE_ACTION*.
+
+The caller provides:
+
+ * `id` unique id.
+ * `payload` the virtual address of where the ELF payload is.
+
+The `id` could be an UUID in mind that stays fixed forever for a given
+hotpatch. It can be embedded into the Elf payload at creation time
+and extracted by tools.
+
+The return value is zero if the payload was succesfully uploaded and the
+signature was verified. Otherwise an EXX return value is provided.
+Duplicate `id` are not supported.
+
+The `payload` is the ELF payload as mentioned in the `Payload format` section.
+
+This operation can be preempted by the hypercall returning EAGAIN.
+This is due to the nature of signature verification - which may require
+SecureBoot firmware calls which are unbounded.
+
+The structure is as follow:
+
+<pre>
+struct xen_sysctl_xsplice_upload {  
+    char id[40];  /* IN, name of the patch. */  
+    uint64_t size; /* IN, size of the ELF file. */
+    XEN_GUEST_HANDLE_64(uint8) payload; /* ELF file. */  
+}; 
+</pre>
+
+### XEN_SYSCTL_XSPLICE_GET (1)
+
+Retrieve an summary of an specific payload. This caller provides:
+
+ * `id` the unique id.
+ * `status` *MUST* be set to zero.
+
+The `summary` structure contains an summary of payload which includes:
+
+ * `id` the unique id.
+ * `status` - whether it has been:
+ 1. *XSPLICE_STATUS_LOADED* (0x1) has been loaded.
+ 2. *XSPLICE_STATUS_PROGRESS* (0x2) acting on the **XEN_SYSCTL_XSPLICE_ACTION** command.
+ 3. *XSPLICE_STATUS_CHECKED*  (0x4) the ELF payload safety checks passed.
+ 4. *XSPLICE_STATUS_APPLIED* (0x8) loaded, checked, and applied.
+ 5. *XSPLICE_STATUS_REVERTED* (0x10) loaded, checked, applied and then also reverted.
+ 6. Negative values is an error. The error would be of EXX format.
+
+The return value is zero on success and EXX on failure. This operation
+is synchronous and does not require preemption.
+
+The structure is as follow:
+
+<pre>
+#define XSPLICE_STATUS_LOADED    0x1  
+#define XSPLICE_STATUS_PROGRESS  0x2  
+#define XSPLICE_STATUS_CHECKED   0x4  
+#define XSPLICE_STATUS_APPLIED   0x8  
+#define XSPLICE_STATUS_REVERTED  0x10  
+
+struct xen_sysctl_xsplice_summary {  
+    char id[40];  /* IN/OUT, name of the patch. */  
+    int32_t status;   /* OUT */  
+}; 
+</pre>
+
+### XEN_SYSCTL_XSPLICE_LIST (2)
+
+Retrieve an array of abbreviated summary of payloads that are loaded in the
+hypervisor.
+
+The caller provides:
+
+ * `version`. Initially it *MUST* be zero.
+ * `idx` index iterator. On first call *MUST* be zero, subsequent calls varies.
+ * `count` the max number of entries to populate.
+ * `summary` virtual address of where to write payload summaries.
+
+The hypercall returns zero on success and updates the `idx` (index) iterator
+with the number of payloads returned, `count` with the number of remaining
+payloads, and `summary` with an number of payload summaries. The `version`
+is updated on every hypercall - if it varies from one hypercall to another
+the data is stale and further calls could fail.
+
+
+If the hypercall returns E2BIG the `count` is too big and should be
+lowered.
+
+Note that due to the asynchronous nature of hypercalls the domain might have
+added or removed the number of payloads making this information stale. It is
+the responsibility of the toolstack to use the `version` field to check
+between each invocation. if the version differs it should discard the stale
+data and start from scratch.
+
+This operation is synchronous and does not require preemption.
+
+The `summary` structure contains an summary of payload which includes:
+
+ * `version` version of the data.
+ * `id` unique id per payload.
+ * `status` - whether it has been:
+ 1. *XSPLICE_STATUS_LOADED* (0x1) has been loaded.
+ 2. *XSPLICE_STATUS_PROGRESS* (0x2) acting on the **XEN_SYSCTL_XSPLICE_ACTION** command.
+ 3. *XSPLICE_STATUS_CHECKED*  (0x4) the ELF payload safety checks passed.
+ 4. *XSPLICE_STATUS_APPLIED* (0x8) loaded, checked, and applied.
+ 5. *XSPLICE_STATUS_REVERTED* (0x10) loaded, checked, applied and then also reverted.
+ 6. Any negative values means there has been error. The value is in EXX format.
+
+The structure is as follow:
+
+<pre>
+struct xen_sysctl_xsplice_list {  
+    uint32_t version; /* OUT */
+    uint32_t idx;  /* IN/OUT */  
+    uint32_t count;  /* IN/OUT */
+    XEN_GUEST_HANDLE_64(xen_sysctl_xsplice_summary) summary;  /* OUT */  
+};  
+
+struct xen_sysctl_xsplice_summary {  
+    char id[40];  /* OUT, name of the patch. */  
+    int32_t status;  /* OUT */  
+};  
+
+</pre>
+### XEN_SYSCTL_XSPLICE_ACTION (3)
+
+Perform an operation on the payload structure referenced by the `id` field.
+The operation request is asynchronous and the status should be retrieved
+by using either **XEN_SYSCTL_XSPLICE_GET** or **XEN_SYSCTL_XSPLICE_LIST** hypercall.
+
+There are two ways about doing preemption. Either via returning back EBUSY
+or the mechanism outlined here.
+
+Doing it in userland would remove any tracking of states in
+the hypervisor - except the simple commands apply, unload, and revert.
+
+However we would not be able to patch all the code that is invoked while
+this hypercall is in progress. That is - the do_domctl, the spinlocks,
+anything put on the stack, etc.
+
+The disadvantage of the mechanism outlined here is that the hypervisor
+code has to keep the state atomic and have an upper bound of time
+on actions. If within the time the operation does not succeed the
+operation would go in error state.
+
+ * `id` the unique id.
+ * `time` the upper bound of time the cmd should take. Zero means infinite.
+ * `cmd` the command requested:
+  1. *XSPLICE_ACTION_CHECK* (1) check that the payload will apply properly.
+  2. *XSPLICE_ACTION_UNLOAD* (2) unload the payload.
+   Any further hypercalls against the `id` will result in failure unless
+   **XEN_SYSCTL_XSPLICE_UPLOAD** hypercall is perfomed with same `id`.
+  3. *XSPLICE_ACTION_REVERT* (3) revert the payload. If the operation takes
+  more time than the upper bound of time the `status` will EBUSY.
+  4. *XSPLICE_ACTION_APPLY* (4) apply the payload. If the operation takes
+  more time than the upper bound of time the `status` will be EBUSY.
+  5. *XSPLICE_ACTION_LOADED* is an initial state and cannot be requested.  
+
+The return value will be zero unless the provided fields are incorrect.
+
+The structure is as follow:
+
+<pre>
+#define XSPLICE_ACTION_CHECK  1  
+#define XSPLICE_ACTION_UNLOAD 2  
+#define XSPLICE_ACTION_REVERT 3  
+#define XSPLICE_ACTION_APPLY  4  
+
+struct xen_sysctl_xsplice_action {  
+    char id[40];  /* IN, name of the patch. */  
+    uint64_t time; /* IN, upper bound of time (ms) for the operation to take. */  
+    uint32_t cmd; /* IN */  
+};  
+
+</pre>
+
+## State diagrams of XSPLICE_ACTION values.
+
+There is a strict ordering state of what the commands can be.
+The XSPLICE_ACTION prefix has been dropped to easy reading:
+
+<pre>
+                        /->\  
+                        \  /  
+             /-------< CHECK  
+             |          |  
+             |          +  
+             |       UNLOAD<--\  
+             |                 \  
+             |                   \  
+      /-> APPLY -----------> REVERT --\  
+      |                               |  
+      \-------------------------------/  
+
+</pre>
+Or an state transition table of valid states:
+<pre>
++-------+-------+--------+--------+---------+-------+------------------+  
+| CHECK | APPLY | REVERT | UNLOAD | Current | Next  | Result           |  
++-------+-------+--------+--------+---------+-------+------------------+  
+|   x   |       |        |        | LOADED  | CHECK | Check payload.   |  
++-------+-------+--------+--------+---------+-------+------------------+  
+|       |       |        |   x    | LOADED  | UNLOAD| unload payload.  |  
++-------+-------+--------+--------+---------+-------+------------------+  
+|   x   |       |        |        | CHECK   | CHECK | Check payload.   |  
++-------+-------+--------+--------+---------+-------+------------------+  
+|       |   x   |        |        | CHECK   | APPLY | Apply payload.   |  
++-------+-------+--------+--------+---------+-------+------------------+  
+|       |       |        |   x    | CHECK   | UNLOAD| Unload payload.  |  
++-------+-------+--------+--------+---------+-------+------------------+  
+|       |       |   x    |        | APPLY   | REVERT| Revert payload.  |  
++-------+-------+--------+--------+---------+-------+------------------+  
+|       |   x   |        |        | REVERT  | APPLY | Apply payload.   |  
++-------+-------+--------+--------+---------+-------+------------------+  
+|       |       |        |   x    | REVERT  | UNLOAD| Unload payload.  |  
++-------+-------+--------+--------+---------+-------+------------------+  
+</pre>
+All the other state transitions are invalid.
+
+## Sequence of events.
+
+The normal sequence of events is to:
+
+ 1. *XEN_SYSCTL_XSPLICE_UPLOAD* to upload the payload. If there are errors *STOP* here.
+ 2. *XEN_SYSCTL_XSPLICE_GET* to check the `->status`. If in *XSPLICE_STATUS_PROGRESS* spin. If in *XSPLICE_STATUS_LOADED* go to next step.
+ 3. *XEN_SYSCTL_XSPLICE_ACTION* with *XSPLICE_ACTION_CHECK* command to verify that the payload can be succesfully applied.
+ 4. *XEN_SYSCTL_XSPLICE_GET* to check the `->status`. If in *XSPLICE_STATUS_PROGRESS* spin. If in *XSPLICE_STATUS_CHECKED* go to next step.
+ 5. *XEN_SYSCTL_XSPLICE_ACTION* with *XSPLICE_ACTION_APPLY* to apply the patch.
+ 6. *XEN_SYSCTL_XSPLICE_GET* to check the `->status`. If in *XSPLICE_STATUS_PROGRESS* spin. If in *XSPLICE_STATUS_APPLIED* exit with success.
+
+ 
+## Addendum
+
+Implementation quirks should not be discussed in a design document.
+
+However these observations can provide aid when developing against this
+document.
+
+
+### Alternative assembler
+
+Alternative assembler is a mechanism to use different instructions depending
+on what the CPU supports. This is done by providing multiple streams of code
+that can be patched in - or if the CPU does not support it - padded with
+`nop` operations. The alternative assembler macros cause the compiler to
+expand the code to place a most generic code in place - emit a special
+ELF .section header to tag this location. During run-time the hypervisor
+can leave the areas alone or patch them with an better suited opcodes.
+
+However these sections are part of .init. and as such can't reasonably be
+subject to patching.
+
+### .rodata sections
+
+The patching might require strings to be updated as well. As such we must be
+also able to patch the strings as needed. This sounds simple - but the compiler
+has a habit of coalescing strings that are the same - which means if we in-place
+alter the strings - other users will be inadvertently affected as well.
+
+This is also where pointers to functions live - and we may need to patch this
+as well. And switch-style jump tables.
+
+To guard against that we must be prepared to do patching similar to
+trampoline patching or in-line depending on the flavour. If we can
+do in-line patching we would need to:
+
+ * alter `.rodata` to be writeable.
+ * inline patch.
+ * alter `.rodata` to be read-only.
+
+If are doing trampoline patching we would need to:
+
+ * allocate a new memory location for the string.
+ * all locations which use this string will have to be updated to use the
+   offset to the string.
+ * mark the region RO when we are done.
+
+### .bss and .data sections.
+
+Patching writable data is not suitable as it is unclear what should be done
+depending on the current state of data. As such it should not be attempted.
+
+
+### Patching code which is in the stack.
+
+We should not patch the code which is on the stack. That can lead
+to corruption.
+
+### Inline patching
+
+The hypervisor should verify that the in-place patching would fit within
+the code or data.
+
+### Trampoline (e9 opcode)
+
+The e9 opcode used for jmpq uses a 32-bit signed displacement. That means
+we are limited to up to 2GB of virtual address to place the new code
+from the old code. That should not be a problem since Xen hypervisor has
+a very small footprint.
+
+However if we need - we can always add two trampolines. One at the 2GB
+limit that calls the next trampoline.
+
+Please note there is a small limitation for trampolines in
+function entries: The target function (+ trailing padding) must be able
+to accomodate the trampoline. On x86 with +-2 GB relative jumps,
+this means 5 bytes are  required.
+
+Depending on compiler settings, there are several functions in Xen that
+are smaller (without inter-function padding).
+
+<pre> 
+readelf -sW xen-syms | grep " FUNC " | \
+    awk '{ if ($3 < 5) print $3, $4, $5, $8 }'
+
+...
+3 FUNC LOCAL wbinvd_ipi
+3 FUNC LOCAL shadow_l1_index
+...
+</pre>
+A compile-time check for, e.g., a minimum alignment of functions or a
+runtime check that verifies symbol size (+ padding to next symbols) for
+that in the hypervisor is advised.
+
+### When to patch
+
+During the discussion on the design two candidates bubbled where
+the call stack for each CPU would be deterministic. This would
+minimize the chance of the patch not being applied due to safety
+checks failing.
+
+#### Rendezvous code instead of stop_machine for patching
+
+The hypervisor's time rendezvous code runs synchronously across all CPUs
+every second. Using the stop_machine to patch can stall the time rendezvous
+code and result in NMI. As such having the patching be done at the tail
+of rendezvous code should avoid this problem.
+
+However the entrance point for that code is
+do_softirq->timer_softirq_action->time_calibration
+which ends up calling on_selected_cpus on remote CPUs.
+
+The remote CPUs receive CALL_FUNCTION_VECTOR IPI and execute the
+desired function.
+
+
+#### Before entering the guest code.
+
+Before we call VMXResume we check whether any soft IRQs need to be executed.
+This is a good spot because all Xen stacks are effectively empty at
+that point.
+
+To randezvous all the CPUs an barrier with an maximum timeout (which
+could be adjusted), combined with forcing all other CPUs through the
+hypervisor with IPIs, can be utilized to have all the CPUs be lockstep.
+
+The approach is similar in concept to stop_machine and the time rendezvous
+but is time-bound. However the local CPU stack is much shorter and
+a lot more deterministic.
+
+### Compiling the hypervisor code
+
+Hotpatch generation often requires support for compiling the target
+with -ffunction-sections / -fdata-sections.  Changes would have to
+be done to the linker scripts to support this.
+
+
+### Generation of xSplice ELF payloads
+
+The design of that is not discussed in this design.
+
+The author of this design envisions objdump and objcopy along
+with special GCC parameters (see above) to create .o.xsplice files
+which can be used to splice an ELF with the new payload.
+
+The ksplice code can provide inspiration.
+
+### Exception tables and symbol tables growth
+
+We may need support for adapting or augmenting exception tables if
+patching such code.  Hotpatches may need to bring their own small
+exception tables (similar to how Linux modules support this).
+
+If supporting hotpatches that introduce additional exception-locations
+is not important, one could also change the exception table in-place
+and reorder it afterwards.
+
+
+### xSplice interdependencies
+
+xSplice patches interdependencies are tricky.
+
+There are the ways this can be addressed:
+ * A single large patch that subsumes and replaces all previous ones.
+   Over the life-time of patching the hypervisor this large patch
+   grows to accumulate all the code changes.
+ * Hotpatch stack - where an mechanism exists that loads the hotpatches
+   in the same order they were built in. We would need an build-id
+   of the hypevisor to make sure the hot-patches are build against the
+   correct build.
+ * Payload containing the old code to check against that. That allows
+   the hotpatches to be loaded indepedently (if they don't overlap) - or
+   if the old code also containst previously patched code - even if they
+   overlap.
+
+The disadvantage of the first large patch is that it can grow over
+time and not provide an bisection mechanism to identify faulty patches.
+
+The hot-patch stack puts stricts requirements on the order of the patches
+being loaded and requires an hypervisor build-id to match against.
+
+The old code allows much more flexibility and an additional guard,
+but is more complex to implement.
+
+### Hypervisor ID (buid-id)
+
+The build-id can help with:
+
+  * Prevent loading of wrong hotpatches (intended for other builds)
+
+  * Allow to identify suitable hotpatches on disk and help with runtime
+    tooling (if laid out using build ID)
+
+The build-id (aka hypervisor id) can be easily obtained by utilizing
+the ld --build-id operatin which (copied from ld):
+
+<pre>
+--build-id  
+    --build-id=style  
+        Request creation of ".note.gnu.build-id" ELF note section.  The contents of the note are unique bits identifying this  
+        linked file.  style can be "uuid" to use 128 random bits, "sha1" to use a 160-bit SHA1 hash on the normative parts of the  
+        output contents, "md5" to use a 128-bit MD5 hash on the normative parts of the output contents, or "0xhexstring" to use a  
+        chosen bit string specified as an even number of hexadecimal digits ("-" and ":" characters between digit pairs are  
+        ignored).  If style is omitted, "sha1" is used.  
+
+        The "md5" and "sha1" styles produces an identifier that is always the same in an identical output file, but will be  
+        unique among all nonidentical output files.  It is not intended to be compared as a checksum for the file's contents.  A  
+        linked file may be changed later by other tools, but the build ID bit string identifying the original linked file does  
+        not change.  
+
+        Passing "none" for style disables the setting from any "--build-id" options earlier on the command line.  
+
+</pre>
+
+### Symbol names
+
+
+Xen as it is now, has a couple of non-unique symbol names which will
+make runtime symbol identification hard.  Sometimes, static symbols
+simply have the same name in C files, sometimes such symbols get
+included via header files, and some C files are also compiled
+multiple times and linked under different names (guest_walk.c).
+
+As such we need to modify the linker to make sure that the symbol
+table qualifies also symbols by their source file name.
+
+For the awkward situations in which C-files are compiled multiple
+times patches we would need to some modification in the Xen code.
+
+
+The convention for file-type symbols (that would allow to map many
+symbols to their compilation unit) says that only the basename (i.e.,
+without directories) is embedded.  This creates another layer of
+confusion for duplicate file names in the build tree.
+
+That would have to be resolved.
+
+<pre>
+> find . -name \*.c -print0 | xargs -0 -n1 basename | sort | uniq -c | sort -n | tail -n10
+      3 shutdown.c
+      3 sysctl.c
+      3 time.c
+      3 xenoprof.c
+      4 gdbstub.c
+      4 irq.c
+      5 domain.c
+      5 mm.c
+      5 pci.c
+      5 traps.c
+</pre>
+
+### Security
+
+Only the privileged domain should be allowed to do this operation.
+
diff --git a/docs/misc/xsplice_test.c b/docs/misc/xsplice_test.c
new file mode 100644
index 0000000..6e0cf93
--- /dev/null
+++ b/docs/misc/xsplice_test.c
@@ -0,0 +1,78 @@
+#include "xsplice.h"
+#include <stdio.h>
+
+struct xsplice_code xsplice_xsa125;  
+struct xsplice_reloc relocs[1];  
+struct xsplice_section sections[1];  
+struct xsplice_patch patches[1];  
+struct xsplice_symbol do_xen_version_symbol;  
+struct xsplice_reloc_howto do_xen_version_howto;  
+char do_xen_version_new_code[1728];  
+
+#ifndef HYPERVISOR_ID  
+#define HYPERVISOR_ID "Xen 4.6-unstable-g9348394"  
+#endif  
+
+struct xsplice xsa125 = {  
+    .name = "xsa125",  
+    .id = HYPERVISOR_ID,  
+    .old = NULL,  
+    .new = &xsplice_xsa125,  
+};  
+
+struct xsplice_code xsplice_xsa125 = {  
+    .relocs = &relocs[0],  
+    .n_relocs = 1,
+    .sections = &sections[0],  
+    .n_sections = 1,
+    .patches = &patches[0],  
+    .n_patches = 1,
+};
+
+struct xsplice_reloc relocs[1] = {  
+    {  
+        .addr = 0xffff82d080112f9e,  
+        .symbol = &do_xen_version_symbol,  
+        .isns_target = 0,  
+        .howto = &do_xen_version_howto,  
+        .isns_added = -4,  
+    },  
+};  
+
+struct xsplice_symbol do_xen_version_symbol = {  
+    .name = "do_xen_version",  
+    .label = "do_xen_version+<0x0>",  
+};  
+
+struct xsplice_reloc_howto do_xen_version_howto = {  
+    .howto = XSPLICE_HOWTO_RELOC_PATCH,  
+    .flag = XSPLICE_HOWTO_FLAG_PC_REL,  
+    .r_shift = 0,  
+    .mask = (-1ULL),  
+};  
+
+
+struct xsplice_section sections[1] = {  
+    {  
+        .symbol = &do_xen_version_symbol,  
+        .address = 0xffff82d080112f9e,  
+        .size = 1728,  
+        .flags = XSPLICE_SECTION_TEXT,  
+    },  
+};  
+
+struct xsplice_patch patches[1] = {  
+    {  
+        .type = XSPLICE_PATCH_RELOC_TEXT,  
+        .size = 1728,  
+        .addr = 0,  
+        .content = &do_xen_version_new_code,  
+    },  
+};  
+
+char do_xen_version_new_code[1728] = { 0x83, 0xff, 0x09, };  
+
+void main()
+{
+   printf("%s\n", xsa125.name); 
+}
diff --git a/tools/libxc/include/xenctrl.h b/tools/libxc/include/xenctrl.h
index ac7e5fd..9ef39fd 100644
--- a/tools/libxc/include/xenctrl.h
+++ b/tools/libxc/include/xenctrl.h
@@ -2828,6 +2828,22 @@ int xc_psr_cat_get_l3_info(xc_interface *xch, uint32_t socket,
                            uint32_t *cos_max, uint32_t *cbm_len);
 #endif
 
+int xc_xsplice_upload(xc_interface *xch,
+                      char *id, char *payload, uint32_t size);
+
+int xc_xsplice_get(xc_interface *xch,
+                   char *id,
+                   xen_sysctl_xsplice_summary_t *summary);
+
+int xc_xsplice_list(xc_interface *xch, unsigned int max, unsigned int start,
+                    xen_sysctl_xsplice_summary_t *info, unsigned int *done,
+                    unsigned int *left);
+
+int xc_xsplice_apply(xc_interface *xch, char *id);
+int xc_xsplice_revert(xc_interface *xch, char *id);
+int xc_xsplice_unload(xc_interface *xch, char *id);
+int xc_xsplice_check(xc_interface *xch, char *id);
+
 #endif /* XENCTRL_H */
 
 /*
diff --git a/tools/libxc/xc_misc.c b/tools/libxc/xc_misc.c
index b827bbb..bb91930 100644
--- a/tools/libxc/xc_misc.c
+++ b/tools/libxc/xc_misc.c
@@ -719,6 +719,189 @@ int xc_hvm_inject_trap(
     return rc;
 }
 
+int xc_xsplice_upload(xc_interface *xch,
+                      char *id,
+                      char *payload,
+                      uint32_t size)
+{
+    int rc;
+    DECLARE_SYSCTL;
+    DECLARE_HYPERCALL_BOUNCE(payload, size, XC_HYPERCALL_BUFFER_BOUNCE_IN);
+
+    if ( !id || !payload )
+        return -1;
+
+    if ( xc_hypercall_bounce_pre(xch, payload) )
+        return -1;
+
+    sysctl.cmd = XEN_SYSCTL_xsplice_op;
+    sysctl.u.xsplice.cmd = XEN_SYSCTL_XSPLICE_UPLOAD;
+    sysctl.u.xsplice.u.upload.size = size;
+    memcpy(sysctl.u.xsplice.u.upload.id, id, XEN_XSPLICE_ID_SIZE);
+    set_xen_guest_handle(sysctl.u.xsplice.u.upload.payload, payload);
+
+    rc = do_sysctl(xch, &sysctl);
+
+    xc_hypercall_bounce_post(xch, payload);
+
+    return rc;
+}
+
+int xc_xsplice_get(xc_interface *xch,
+                   char *id,
+                   xen_sysctl_xsplice_summary_t *summary)
+{
+    int rc;
+    DECLARE_SYSCTL;
+
+    if ( !id )
+        return -1;
+
+    sysctl.cmd = XEN_SYSCTL_xsplice_op;
+    sysctl.u.xsplice.cmd = XEN_SYSCTL_XSPLICE_GET;
+    sysctl.u.xsplice.u.get.status = 0;
+    memcpy(sysctl.u.xsplice.u.get.id, id, XEN_XSPLICE_ID_SIZE);
+
+    rc = do_sysctl(xch, &sysctl);
+
+    memcpy(summary, &sysctl.u.xsplice.u.get, sizeof(*summary));
+
+    return rc;
+}
+
+int xc_xsplice_list(xc_interface *xch, unsigned int max, unsigned int start,
+                    xen_sysctl_xsplice_summary_t *info, unsigned int *done,
+                    unsigned int *left)
+{
+    int rc;
+    DECLARE_SYSCTL;
+    DECLARE_HYPERCALL_BOUNCE(info, 0 /* adjust later. */, XC_HYPERCALL_BUFFER_BOUNCE_OUT);
+    uint32_t max_batch_sz, nr;
+    uint32_t version = 0, retries = 0;
+    uint32_t adjust = 0;
+
+    if ( !max )
+        return -1;
+
+    sysctl.cmd = XEN_SYSCTL_xsplice_op;
+    sysctl.u.xsplice.cmd = XEN_SYSCTL_XSPLICE_LIST;
+    sysctl.u.xsplice.u.list.version = 0;
+    sysctl.u.xsplice.u.list.idx = start;
+
+    max_batch_sz = max;
+
+    *done = 0;
+    *left = 0;
+    do {
+        if ( adjust )
+            adjust = 0; /* Used when adjusting the 'max_batch_sz' or 'retries'. */
+
+        nr = min(max - *done, max_batch_sz);
+
+        sysctl.u.xsplice.u.list.nr = nr;
+        HYPERCALL_BOUNCE_SET_SIZE(info, nr * sizeof(*info));
+
+        /* Move the pointer to proper offset into 'info'. */
+        (HYPERCALL_BUFFER(info))->ubuf = info + *done;
+        if ( xc_hypercall_bounce_pre(xch, info) )
+            return -1;
+
+        set_xen_guest_handle(sysctl.u.xsplice.u.list.summary, info);
+
+        rc = do_sysctl(xch, &sysctl);
+        if ( rc < 0 && errno == E2BIG )
+        {
+            if ( max_batch_sz <= 1 )
+                break;
+            max_batch_sz >>= 1;
+            adjust = 1; /* For the loop conditional to let us loop again. */
+            xc_hypercall_bounce_post(xch, info); /* No memory leaks! */
+            continue;
+        }
+        if ( rc < 0 ) /* For all other errors we bail out. */
+            break;
+
+        if ( !version )
+            version = sysctl.u.xsplice.u.list.version;
+
+        if ( sysctl.u.xsplice.u.list.version != version )
+        {
+            /* TODO: retries should be configurable? */
+            if ( retries++ > 3 )
+            {
+                rc = -1;
+                errno = EBUSY;
+                break;
+            }
+            *done = 0; /* Retry from scratch. */
+            version = sysctl.u.xsplice.u.list.version;
+            adjust = 1; /* And make sure we continue in the loop. */
+            xc_hypercall_bounce_post(xch, info); /* No memory leaks! */
+            continue;
+        }
+
+        /* We should never hit this, but just in case. */
+        if ( rc > nr )
+        {
+            errno = EINVAL; /* Overflow! */
+            rc = -1;
+            break;
+        }
+        *left = sysctl.u.xsplice.u.list.nr; /* Total remaining count. */
+        /* Copy only up 'rc' of data' - we could add 'min(rc,nr) if desired. */
+        HYPERCALL_BOUNCE_SET_SIZE(info, (rc * sizeof(*info)));
+        /* Bounce the data and free the bounce buffer. */
+        xc_hypercall_bounce_post(xch, info);
+        /* And update how many elements of info we have copied into. */
+        *done += rc;
+        /* Update idx. */
+        sysctl.u.xsplice.u.list.idx = rc;
+    } while ( adjust || (*done < max && *left != 0) );
+
+    return rc > 0 ? 0 : rc;
+}
+
+static int _xc_xsplice_action(xc_interface *xch,
+                              char *id,
+                              unsigned int action)
+{
+    int rc;
+    DECLARE_SYSCTL;
+
+    if ( !id )
+        return -1;
+
+    sysctl.cmd = XEN_SYSCTL_xsplice_op;
+    sysctl.u.xsplice.cmd = XEN_SYSCTL_XSPLICE_ACTION;
+    sysctl.u.xsplice.u.action.cmd = action;
+    sysctl.u.xsplice.u.action.time = 0; /* TODO */
+    memcpy(sysctl.u.xsplice.u.action.id, id, XEN_XSPLICE_ID_SIZE);
+
+    rc = do_sysctl(xch, &sysctl);
+
+    return rc;
+}
+
+int xc_xsplice_apply(xc_interface *xch, char *id)
+{
+    return _xc_xsplice_action(xch, id, XSPLICE_ACTION_APPLY);
+}
+
+int xc_xsplice_revert(xc_interface *xch, char *id)
+{
+    return _xc_xsplice_action(xch, id, XSPLICE_ACTION_REVERT);
+}
+
+int xc_xsplice_unload(xc_interface *xch, char *id)
+{
+    return _xc_xsplice_action(xch, id, XSPLICE_ACTION_UNLOAD);
+}
+
+int xc_xsplice_check(xc_interface *xch, char *id)
+{
+    return _xc_xsplice_action(xch, id, XSPLICE_ACTION_CHECK);
+}
+
 /*
  * Local variables:
  * mode: C
diff --git a/tools/misc/Makefile b/tools/misc/Makefile
index c4490f3..c46873e 100644
--- a/tools/misc/Makefile
+++ b/tools/misc/Makefile
@@ -30,6 +30,7 @@ INSTALL_SBIN                   += xenlockprof
 INSTALL_SBIN                   += xenperf
 INSTALL_SBIN                   += xenpm
 INSTALL_SBIN                   += xenwatchdogd
+INSTALL_SBIN                   += xen-xsplice
 INSTALL_SBIN += $(INSTALL_SBIN-y)
 
 # Everything to be installed in a private bin/
@@ -98,6 +99,9 @@ xen-mfndump: xen-mfndump.o
 xenwatchdogd: xenwatchdogd.o
 	$(CC) $(LDFLAGS) -o $@ $< $(LDLIBS_libxenctrl) $(APPEND_LDFLAGS)
 
+xen-xsplice: xen-xsplice.o
+	$(CC) $(LDFLAGS) -o $@ $< $(LDLIBS_libxenctrl) $(APPEND_LDFLAGS)
+
 xen-lowmemd: xen-lowmemd.o
 	$(CC) $(LDFLAGS) -o $@ $< $(LDLIBS_libxenctrl) $(LDLIBS_libxenstore) $(APPEND_LDFLAGS)
 
diff --git a/tools/misc/xen-xsplice.c b/tools/misc/xen-xsplice.c
new file mode 100644
index 0000000..7cf9879
--- /dev/null
+++ b/tools/misc/xen-xsplice.c
@@ -0,0 +1,360 @@
+#include <xenctrl.h>
+#include <xenstore.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdio.h>
+#include <sys/stat.h>
+#include <sys/mman.h>
+#include <fcntl.h>
+static xc_interface *xch;
+
+void show_help(void)
+{
+    fprintf(stderr,
+            "xen-xsplice: Xsplice test tool\n"
+            "Usage: xen-xsplice <command> [args]\n"
+            " <id> An unique name of payload. Up to 40 characters.\n"
+            "Commands:\n"
+            "  help                 display this help\n"
+            "  upload <id> <file>   upload file <cpuid> with <id> name\n"
+            "  list                 list payloads uploaded.\n"
+            "  apply <id>           apply <id> patch.\n"
+            "  revert <id>          revert id <id> patch.\n"
+            "  unload <id>          unload id <id> patch.\n"
+            "  check <id>           check id <id> patch.\n"
+           );
+}
+
+/* wrapper function */
+static int help_func(int argc, char *argv[])
+{
+    show_help();
+    return 0;
+}
+
+#define ARRAY_SIZE(a) (sizeof (a) / sizeof ((a)[0]))
+
+static const char *status2str(long status)
+{
+#define STATUS(x) [XSPLICE_STATUS_##x] = #x
+    static const char *const names[] = {
+            STATUS(LOADED),
+            STATUS(PROGRESS),
+            STATUS(CHECKED),
+            STATUS(APPLIED),
+            STATUS(REVERTED),
+    };
+
+    if (status >= ARRAY_SIZE(names))
+        return "unknown";
+
+    if (status < 0)
+        return "-EXX";
+
+    if (!names[status])
+        return "unknown";
+
+    return names[status];
+}
+
+#define MAX 11
+static int list_func(int argc, char *argv[])
+{
+    unsigned int idx, done, left, rc, i;
+    xen_sysctl_xsplice_summary_t *info;
+
+    if ( argc )
+    {
+        show_help();
+        return -1;
+    }
+    idx = left = 0;
+    info = malloc(sizeof(*info) * MAX);
+    if ( !info )
+    {
+        fprintf(stderr, "Could not allocate buffer!\n");
+        return ENOMEM;
+    }
+    fprintf(stdout," ID                                     | status\n"
+                   "----------------------------------------+------------\n");
+    do {
+        done = 0;
+        memset(info, 'A', sizeof(*info) * MAX); /* Optional. */
+        rc = xc_xsplice_list(xch, MAX, idx, info, &done, &left);
+        if ( rc )
+        {
+            fprintf(stderr, "Failed to list %d/%d: %d(%s)!\n", idx, left, errno, strerror(errno));
+            return errno;
+        }
+        for ( i = 0; i < done; i++ )
+        {
+            fprintf(stdout, "%-40s| ", info[i].id);
+            if ( info[i].status < 0 )
+                fprintf(stdout, "%s\n", strerror(info[i].status));
+            else
+                fprintf(stdout, "%s\n", status2str(info[i].status));
+        }
+        idx += done;
+    } while ( left );
+
+    return 0;
+}
+
+static int get_id(int argc, char *argv[], char *id)
+{
+    ssize_t len = strlen(argv[0]);
+    if ( len > XEN_XSPLICE_ID_SIZE )
+    {
+        fprintf(stderr, "ID MUST be %d characters!\n", XEN_XSPLICE_ID_SIZE);
+        errno = EINVAL;
+        return errno;
+    }
+    /* Don't want any funny strings from the stack. */
+    memset(id, 0, XEN_XSPLICE_ID_SIZE);
+    strncpy(id, argv[0], len);
+    return 0;
+}
+
+static int upload_func(int argc, char *argv[])
+{
+    char *filename;
+    char id[XEN_XSPLICE_ID_SIZE];
+    int fd = 0, rc;
+    struct stat buf;
+    unsigned char *fbuf;
+    ssize_t len;
+    DECLARE_HYPERCALL_BUFFER(char, payload);
+
+    if ( argc != 2 )
+    {
+        show_help();
+        return -1;
+    }
+
+    if ( get_id(argc, argv, id) )
+        return EINVAL;
+
+    filename = argv[1];
+    fd = open(filename, O_RDONLY);
+    if ( fd < 0 )
+    {
+        fprintf(stderr, "Could not open %s, error: %d(%s)\n",
+                filename, errno, strerror(errno));
+        return errno;
+    }
+    if ( stat(filename, &buf) != 0 )
+    {
+        fprintf(stderr, "Could not get right size %s, error: %d(%s)\n",
+                filename, errno, strerror(errno));
+        close(fd);
+        return errno;
+    }
+
+    len = buf.st_size;
+    fbuf = mmap(0, len, PROT_READ, MAP_PRIVATE, fd, 0);
+    if ( fbuf == MAP_FAILED )
+    {
+        fprintf(stderr,"Could not map: %s, error: %d(%s)\n",
+                filename, errno, strerror(errno));
+        close (fd);
+        return errno;
+    }
+    printf("Uploading %s (%ld bytes)\n", filename, len);
+    payload = xc_hypercall_buffer_alloc(xch, payload, len);
+    memcpy(payload, fbuf, len);
+
+    rc = xc_xsplice_upload(xch, id, payload, len);
+    if ( rc )
+    {
+        fprintf(stderr, "Upload failed: %s, error: %d(%s)!\n",
+                filename, errno, strerror(errno));
+        goto out;
+    }
+    xc_hypercall_buffer_free(xch, payload);
+
+out:
+    if ( munmap( fbuf, len) )
+    {
+        fprintf(stderr, "Could not unmap!? error: %d(%s)!\n",
+                errno, strerror(errno));
+        rc = errno;
+    }
+    close(fd);
+
+    return rc;
+}
+
+struct {
+    int allow; /* State it must be in to call function. */
+    int expected; /* The state to be in after the function. */
+    const char *name;
+    int (*function)(xc_interface *xch, char *id);
+    unsigned int executed; /* Has the function been called?. */
+} action_options[] = {
+    {   .allow = XSPLICE_STATUS_CHECKED | XSPLICE_STATUS_REVERTED,
+        .expected = XSPLICE_STATUS_APPLIED,
+        .name = "apply",
+        .function = xc_xsplice_apply,
+    },
+    {   .allow = XSPLICE_STATUS_APPLIED,
+        .expected = XSPLICE_STATUS_REVERTED,
+        .name = "revert",
+        .function = xc_xsplice_revert,
+    },
+    {   .allow = XSPLICE_STATUS_CHECKED | XSPLICE_STATUS_REVERTED | XSPLICE_STATUS_LOADED,
+        .expected = ENOENT,
+        .name = "unload",
+        .function = xc_xsplice_unload,
+    },
+    {   .allow = XSPLICE_STATUS_CHECKED | XSPLICE_STATUS_LOADED,
+        .expected = XSPLICE_STATUS_CHECKED,
+        .name = "check",
+        .function = xc_xsplice_check
+    },
+};
+
+int action_func(int argc, char *argv[], unsigned int idx)
+{
+    char id[40];
+    int rc;
+    xen_sysctl_xsplice_summary_t summary;
+    unsigned int retry = 0;
+
+    if ( argc != 1 )
+    {
+        show_help();
+        return -1;
+    }
+
+    if ( get_id(argc, argv, id) )
+        return EINVAL;
+
+    do {
+        rc = xc_xsplice_get(xch, id, &summary);
+        /* N.B. Successfull unload will return ENOENT. */
+        if ( rc )
+        {
+            rc = errno; /* rc is just -1 and we want proper EXX. */
+            break;
+        }
+
+        if ( summary.status < 0 )
+        { /* We report it outside the loop. */
+            rc = summary.status;
+            break;
+        }
+        if ( summary.status == XSPLICE_STATUS_PROGRESS )
+        {
+            if ( !action_options[idx].executed )
+            {
+                printf("%s is in progress and we didn't initiate it!\n", id);
+                errno = EBUSY;
+                rc = -1;
+                break;
+            }
+            if ( retry++ < 30 )
+            {
+                printf(".");
+                sleep(1);
+                continue;
+            }
+            printf("%s: Waited more than 30 seconds! Bailing out.\n", id);
+            errno = EBUSY;
+            rc = -1;
+            break;
+        }
+        /* We use rc outside loop to deal with EXX type expected values. */
+        rc = summary.status;
+        if ( action_options[idx].expected == rc ) /* Yeey! */
+            break;
+
+        if ( action_options[idx].allow & rc )
+        {
+            if ( action_options[idx].executed )
+            {
+                printf(" (0x%x vs 0x%x) state not reached!?\n",
+                       action_options[idx].expected, rc);
+                errno = EINVAL;
+                break;
+            }
+            printf("%s: State is 0x%x, ok are 0x%x. Commencing %s:",
+                   id, rc, action_options[idx].allow,
+                   action_options[idx].name);
+
+            rc = action_options[idx].function(xch, id);
+            if ( rc ) /* We report it outside the loop. */
+                break;
+
+            action_options[idx].executed = 1;
+            rc = 1; /* Loop again so we can display the dots. */
+        } else {
+            printf("%s: in wrong state (0x%x), expected 0x%x\n",
+                   id, rc, action_options[idx].expected);
+            errno = EINVAL;
+            rc = -1;
+            break;
+        }
+    } while ( rc > 0 );
+
+    if ( action_options[idx].expected == rc )
+    {
+            printf("completed!\n");
+            rc = 0;
+    } else
+        printf("%s failed with %d(%s)\n", id, errno, strerror(errno));
+
+    return rc;
+}
+struct {
+    const char *name;
+    int (*function)(int argc, char *argv[]);
+} main_options[] = {
+    { "help", help_func },
+    { "list", list_func },
+    { "upload", upload_func },
+};
+
+int main(int argc, char *argv[])
+{
+    int i, j, ret;
+
+    if ( argc  <= 1 )
+    {
+        show_help();
+        return 0;
+    }
+    for ( i = 0; i < ARRAY_SIZE(main_options); i++ )
+        if (!strncmp(main_options[i].name, argv[1], strlen(argv[1])))
+            break;
+
+    if ( i == ARRAY_SIZE(main_options) )
+    {
+        for ( j = 0; j < ARRAY_SIZE(action_options); j++ )
+            if (!strncmp(action_options[j].name, argv[1], strlen(argv[1])))
+                break;
+
+        if ( j == ARRAY_SIZE(action_options) )
+        {
+            fprintf(stderr, "Unrecognised command '%s' -- try "
+                   "'xen-xsplice help'\n", argv[1]);
+            return 1;
+        }
+    }
+
+    xch = xc_interface_open(0,0,0);
+    if ( !xch )
+    {
+        fprintf(stderr, "failed to get the handler\n");
+        return 0;
+    }
+
+    if ( i == ARRAY_SIZE(main_options) )
+        ret = action_func(argc -2, argv + 2, j);
+    else
+        ret = main_options[i].function(argc -2, argv + 2);
+
+    xc_interface_close(xch);
+
+    return !!ret;
+}
diff --git a/xen/common/Makefile b/xen/common/Makefile
index 3fdf931..7769e5c 100644
--- a/xen/common/Makefile
+++ b/xen/common/Makefile
@@ -55,6 +55,7 @@ obj-y += vmap.o
 obj-y += vsprintf.o
 obj-y += wait.o
 obj-y += xmalloc_tlsf.o
+obj-y += xsplice.o
 
 obj-bin-$(CONFIG_X86) += $(foreach n,decompress bunzip2 unxz unlzma unlzo unlz4 earlycpio,$(n).init.o)
 
diff --git a/xen/common/keyhandler.c b/xen/common/keyhandler.c
index 5d21e48..1d4574a 100644
--- a/xen/common/keyhandler.c
+++ b/xen/common/keyhandler.c
@@ -18,6 +18,7 @@
 #include <xen/mm.h>
 #include <xen/watchdog.h>
 #include <xen/init.h>
+#include <xen/xsplice.h>
 #include <asm/debugger.h>
 #include <asm/div64.h>
 
@@ -455,6 +456,11 @@ static struct keyhandler spinlock_reset_keyhandler = {
     .desc = "reset lock profile info"
 };
 #endif
+static struct keyhandler xsplice_printall_keyhandler = {
+    .diagnostic = 1,
+    .u.fn = xsplice_printall,
+    .desc = "print splicing information"
+};
 
 static void run_all_nonirq_keyhandlers(unsigned long unused)
 {
@@ -567,7 +573,7 @@ void __init initialize_keytable(void)
     register_keyhandler('l', &spinlock_printall_keyhandler);
     register_keyhandler('L', &spinlock_reset_keyhandler);
 #endif
-
+    register_keyhandler('x', &xsplice_printall_keyhandler);
 }
 
 /*
diff --git a/xen/common/sysctl.c b/xen/common/sysctl.c
index f1c0c76..641bb25 100644
--- a/xen/common/sysctl.c
+++ b/xen/common/sysctl.c
@@ -27,6 +27,7 @@
 #include <xsm/xsm.h>
 #include <xen/pmstat.h>
 #include <xen/gcov.h>
+#include <xen/xsplice.h>
 
 long do_sysctl(XEN_GUEST_HANDLE_PARAM(xen_sysctl_t) u_sysctl)
 {
@@ -399,6 +400,10 @@ long do_sysctl(XEN_GUEST_HANDLE_PARAM(xen_sysctl_t) u_sysctl)
         ret = sysctl_coverage_op(&op->u.coverage_op);
         break;
 #endif
+    case XEN_SYSCTL_xsplice_op:
+        ret = xsplice_control(&op->u.xsplice);
+        copyback = 1;
+        break;
 
 #ifdef HAS_PCI
     case XEN_SYSCTL_pcitopoinfo:
diff --git a/xen/common/xsplice.c b/xen/common/xsplice.c
new file mode 100644
index 0000000..e816394
--- /dev/null
+++ b/xen/common/xsplice.c
@@ -0,0 +1,405 @@
+/*
+ * xSplice - Copyright Oracle Corp. Inc 2015.
+ *
+ * Author: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+ */
+
+/* TODO: Sort includes .*/
+#include <xen/smp.h>
+#include <xen/keyhandler.h>
+#include <xen/spinlock.h>
+#include <xen/mm.h>
+#include <xen/list.h>
+#include <xen/guest_access.h>
+#include <xen/stdbool.h>
+#include <xen/sched.h>
+#include <xen/lib.h>
+#include <xen/xsplice.h>
+#include <public/sysctl.h>
+
+#include <asm/event.h>
+
+static DEFINE_SPINLOCK(payload_list_lock);
+static LIST_HEAD(payload_list);
+static unsigned int payload_cnt;
+static unsigned int payload_version = 1;
+static int _debug = 1;
+#define where() { if (_debug) printk("%s:%d\n", __func__, __LINE__); }
+
+struct payload {
+    char  id[XEN_XSPLICE_ID_SIZE]; /* Unique id. */
+    int32_t    status; /* XSPLICE_STATUS_* or Exx type value. */
+    int32_t    old_status; /* XSPLICE_STATUS_* or Exx type value. */
+
+    uint32_t    cmd; /* Action request. XSPLICE_ACTION_* */
+    struct spinlock cmd_lock; /* Lock against the action. */
+
+    uint8_t     *raw;   /* Pointer to Elf file. */
+    ssize_t     size;   /* Size of 'raw'. */
+
+    struct tasklet tasklet;
+    struct list_head   list;   /* Linked to 'payload_list'. */
+};
+
+static const char *status2str(int64_t status)
+{
+#define STATUS(x) [XSPLICE_STATUS_##x] = #x
+    static const char *const names[] = {
+            STATUS(LOADED),
+            STATUS(PROGRESS),
+            STATUS(CHECKED),
+            STATUS(APPLIED),
+            STATUS(REVERTED),
+    };
+
+    if (status >= ARRAY_SIZE(names))
+        return "unknown";
+
+    if (status < 0)
+        return "-EXX";
+
+    if (!names[status])
+        return "unknown";
+
+    return names[status];
+}
+
+void xsplice_printall(unsigned char key)
+{
+    struct payload *data;
+
+    spin_lock(&payload_list_lock);
+
+    list_for_each_entry ( data, &payload_list, list )
+    {
+        printk(" id=%s status=%s(%d,old=%d): \n", data->id,
+               status2str(data->status), data->status, data->old_status);
+    }
+    spin_unlock(&payload_list_lock);
+}
+
+struct payload *find_payload(const char *id, bool_t need_lock)
+{
+    struct payload *data, *found = NULL;
+
+    if ( need_lock )
+        spin_lock(&payload_list_lock);
+
+    list_for_each_entry ( data, &payload_list, list )
+    {
+        if ( !strncmp(data->id, id, XEN_XSPLICE_ID_SIZE) )
+        {
+            found = data;
+            break;
+        }
+    }
+
+    if ( need_lock )
+        spin_unlock(&payload_list_lock);
+
+    return found;
+}
+
+static int verify_payload(xen_sysctl_xsplice_upload_t *upload)
+{
+    if ( upload->id[0] == '\0' )
+    {
+        where();
+        return -EINVAL;
+    }
+    if ( upload->size == 0 )
+    {
+        where();
+        return -EINVAL;
+    }
+    if ( !guest_handle_okay(upload->payload, upload->size) )
+    {
+        where();
+        return -EFAULT;
+    }
+
+    return 0;
+}
+
+/*
+ * We MUST be holding the spinlock.
+ */
+static void __free_payload(struct payload *data)
+{
+
+    free_xenheap_pages(data->raw, get_order_from_bytes(data->size));
+    list_del(&data->list);
+    payload_cnt --;
+    payload_version ++;
+    tasklet_kill(&data->tasklet);
+    xfree(data);
+}
+#include <xen/delay.h>
+static void xsplice_tasklet(unsigned long _data)
+{
+    struct payload *data = (struct payload *)_data;
+
+    /* TODO: Remove it. */
+    mdelay(1000);
+
+    spin_lock(&data->cmd_lock);
+    switch ( data->cmd ) {
+    case XSPLICE_ACTION_CHECK:
+            /* Do the operation here. */
+            data->status = XSPLICE_STATUS_CHECKED;
+            break;
+    case XSPLICE_ACTION_APPLY:
+            /* TODO: Well, do the work :-) */
+            data->status = XSPLICE_STATUS_APPLIED;
+            break;
+    case XSPLICE_ACTION_REVERT:
+            /* TODO: Well, do the work :-) */
+            data->status = XSPLICE_STATUS_REVERTED;
+            break;
+    default:
+            data->status = -EINVAL;
+    }
+    spin_unlock(&data->cmd_lock);
+}
+
+static int xsplice_upload(xen_sysctl_xsplice_upload_t *upload)
+{
+    struct payload *data;
+    uint8_t *raw = NULL;
+    int rc;
+
+    rc = verify_payload(upload);
+    if ( rc )
+        return rc;
+
+    /*
+     * Compute the size of the structures which need to be verified.
+     */
+
+    data = find_payload(upload->id, true);
+    if ( data )
+    {
+        where();
+        return -EEXIST;
+    }
+    rc = -ENOMEM;
+    data = xzalloc(struct payload);
+    if ( !data )
+    {
+        where();
+        return rc;
+    }
+
+    raw = alloc_xenheap_pages(get_order_from_bytes(upload->size), 0);
+    if ( !raw )
+    {
+        printk("%s: alloc for %ld bytes, %d order\n", __func__, upload->size, get_order_from_bytes(upload->size));
+        xfree(data);
+        return rc;
+    }
+    if ( copy_from_guest(raw, upload->payload, upload->size) )
+    {
+        rc = -EFAULT;
+        goto err_out;
+    }
+
+    printk("%s: size %ld %p [%02x %02x ..] \n", __func__, upload->size,
+           raw, (unsigned int)raw[0], (unsigned int)raw[1]);
+
+    /* TODO: Verify signature . */
+    memcpy(data->id, upload->id, XEN_XSPLICE_ID_SIZE);
+    data->status = XSPLICE_STATUS_LOADED;
+    INIT_LIST_HEAD(&data->list);
+    data->raw = raw;
+    data->size = upload->size;
+    spin_lock_init(&data->cmd_lock);
+    data->cmd = 0;
+    tasklet_init(&data->tasklet, xsplice_tasklet, (unsigned long)data);
+
+    spin_lock(&payload_list_lock);
+    list_add_tail(&data->list, &payload_list);
+    payload_cnt ++;
+    payload_version ++;
+    spin_unlock(&payload_list_lock);
+
+    return 0;
+
+ err_out:
+    if ( raw )
+        free_xenheap_pages(raw, get_order_from_bytes(upload->size));
+    if ( data )
+        xfree(data);
+    return rc;
+}
+
+static int xsplice_get(xen_sysctl_xsplice_summary_t *summary)
+{
+    struct payload *data;
+
+    if ( summary->status )
+        return -EINVAL;
+
+    data = find_payload(summary->id, true);
+    if ( !data )
+        return -ENOENT;
+
+    summary->status = data->status;
+
+    return 0;
+}
+
+static int xsplice_list(xen_sysctl_xsplice_list_t *list)
+{
+    xen_sysctl_xsplice_summary_t summary;
+    struct payload *data;
+    unsigned int idx = 0, i = 0;
+    int rc = 0;
+    unsigned int ver = payload_version;
+
+    // TODO: Increase to 64. Leave 4 for debug.
+    if ( list->nr > 4 )
+        return -E2BIG;
+
+    if ( guest_handle_is_null(list->summary) )
+        return -EINVAL;
+
+    spin_lock(&payload_list_lock);
+    if ( list->idx > payload_cnt )
+    {
+        spin_unlock(&payload_list_lock);
+        where();
+        return -EINVAL;
+    }
+
+    list_for_each_entry( data, &payload_list, list )
+    {
+        if ( list->idx > i++ )
+            continue;
+
+        /* Copy all of the bytes avoid leaking stack data. */
+        memcpy(summary.id, data->id, XEN_XSPLICE_ID_SIZE);
+        summary.status = data->status;
+
+        /* N.B. 'idx' != 'i'. */
+        if ( copy_to_guest_offset(list->summary, idx++, &summary, 1) )
+        {
+            rc = -EFAULT;
+            break;
+        }
+        if ( hypercall_preempt_check() || (idx + 1 > list->nr) )
+        {
+            break;
+        }
+    }
+    list->nr = payload_cnt - i; /* Remaining amount. */
+    spin_unlock(&payload_list_lock);
+    list->version = ver;
+
+    /* And how many we have processed. */
+    return rc ? rc : idx;
+}
+
+static int xsplice_action(xen_sysctl_xsplice_action_t *action)
+{
+    struct payload *data;
+    int rc = -EINVAL;
+
+    if ( action->id[0] == '\0' )
+        return rc;
+
+    spin_lock(&payload_list_lock);
+    data = find_payload(action->id, false /* we are holding the lock. */);
+    if ( !data )
+    {
+        rc = -ENOENT;
+        goto out;
+    }
+    if ( action->cmd != XSPLICE_ACTION_UNLOAD )
+        spin_lock(&data->cmd_lock);
+
+    switch ( action->cmd )
+    {
+    case XSPLICE_ACTION_CHECK:
+        if ( ( data->status == XSPLICE_STATUS_LOADED ) )
+        {
+            data->old_status = data->status;
+            data->status = XSPLICE_STATUS_PROGRESS;
+            data->cmd = action->cmd;
+            tasklet_schedule(&data->tasklet);
+            rc = 0;
+        } else if ( data->status == XSPLICE_STATUS_CHECKED )
+        {
+            rc = 0;
+        }
+        break;
+    case XSPLICE_ACTION_UNLOAD:
+        if ( ( data->status == XSPLICE_STATUS_REVERTED ) ||
+             ( data->status == XSPLICE_STATUS_LOADED ) ||
+             ( data->status == XSPLICE_STATUS_CHECKED ) )
+        {
+            __free_payload(data);
+            /* No touching 'data' from here on! */
+            rc = 0;
+        }
+        break;
+    case XSPLICE_ACTION_REVERT:
+        if ( data->status == XSPLICE_STATUS_APPLIED )
+        {
+            data->old_status = data->status;
+            data->status = XSPLICE_STATUS_PROGRESS;
+            data->cmd = action->cmd;
+            rc = 0;
+            /* TODO: Tasklet is not good for this. We need a different vehicle. */
+            tasklet_schedule(&data->tasklet);
+        }
+        break;
+    case XSPLICE_ACTION_APPLY:
+        if ( ( data->status == XSPLICE_STATUS_CHECKED ) ||
+             ( data->status == XSPLICE_STATUS_REVERTED ))
+        {
+            data->old_status = data->status;
+            data->status = XSPLICE_STATUS_PROGRESS;
+            data->cmd = action->cmd;
+            rc = 0;
+            /* TODO: Tasklet is not good for this. We need a different vehicle. */
+            tasklet_schedule(&data->tasklet);
+        }
+        break;
+    default:
+        rc = -ENOSYS;
+        break;
+    }
+
+    if ( action->cmd != XSPLICE_ACTION_UNLOAD )
+        spin_unlock(&data->cmd_lock);
+ out:
+    spin_unlock(&payload_list_lock);
+
+    return rc;
+}
+
+int xsplice_control(xen_sysctl_xsplice_op_t *xsplice)
+{
+    int rc;
+
+    switch ( xsplice->cmd )
+    {
+    case XEN_SYSCTL_XSPLICE_UPLOAD:
+        rc = xsplice_upload(&xsplice->u.upload);
+        break;
+    case XEN_SYSCTL_XSPLICE_GET:
+        rc = xsplice_get(&xsplice->u.get);
+        break;
+    case XEN_SYSCTL_XSPLICE_LIST:
+        rc = xsplice_list(&xsplice->u.list);
+        break;
+    case XEN_SYSCTL_XSPLICE_ACTION:
+        rc = xsplice_action(&xsplice->u.action);
+        break;
+    default:
+        rc = -ENOSYS;
+        break;
+   }
+
+    return rc;
+}
diff --git a/xen/include/public/sysctl.h b/xen/include/public/sysctl.h
index 58c9be2..48dd511 100644
--- a/xen/include/public/sysctl.h
+++ b/xen/include/public/sysctl.h
@@ -710,6 +710,70 @@ struct xen_sysctl_psr_cat_op {
 typedef struct xen_sysctl_psr_cat_op xen_sysctl_psr_cat_op_t;
 DEFINE_XEN_GUEST_HANDLE(xen_sysctl_psr_cat_op_t);
 
+/*
+ * XEN_SYSCTL_XSPLICE_op
+ *
+ */
+#define XEN_SYSCTL_XSPLICE_UPLOAD 0
+#define XEN_XSPLICE_ID_SIZE 40
+
+struct xen_sysctl_xsplice_upload {
+    char        id[XEN_XSPLICE_ID_SIZE];    /* IN, name of the patch. */
+    uint64_t    size;                       /* IN, size of the ELF file. */
+    XEN_GUEST_HANDLE_64(uint8) payload;     /* IN, the ELF file. */
+};
+typedef struct xen_sysctl_xsplice_upload xen_sysctl_xsplice_upload_t;
+DEFINE_XEN_GUEST_HANDLE(xen_sysctl_xsplice_upload_t);
+
+#define XEN_SYSCTL_XSPLICE_GET 1
+struct xen_sysctl_xsplice_summary {
+    char        id[XEN_XSPLICE_ID_SIZE];    /* IN, name of the patch. */
+#define XSPLICE_STATUS_LOADED       0x01
+#define XSPLICE_STATUS_PROGRESS     0x02
+#define XSPLICE_STATUS_CHECKED      0x04
+#define XSPLICE_STATUS_APPLIED      0x08
+#define XSPLICE_STATUS_REVERTED     0x10
+ /* Any negative value is an error. The error would be in -EXX format. */
+	int32_t status;   /* OUT, On IN has to be zero. */
+};
+typedef struct xen_sysctl_xsplice_summary xen_sysctl_xsplice_summary_t;
+DEFINE_XEN_GUEST_HANDLE(xen_sysctl_xsplice_summary_t);
+
+#define XEN_SYSCTL_XSPLICE_LIST 2
+struct xen_sysctl_xsplice_list {
+    uint32_t version;           /* OUT. */
+    uint32_t idx;               /* IN/OUT */
+    uint32_t nr;                /* IN/OUT */
+    XEN_GUEST_HANDLE_64(xen_sysctl_xsplice_summary_t) summary;  /* OUT */
+};
+typedef struct xen_sysctl_xsplice_list xen_sysctl_xsplice_list_t;
+DEFINE_XEN_GUEST_HANDLE(xen_sysctl_xsplice_list_t);
+
+#define XEN_SYSCTL_XSPLICE_ACTION 3
+struct xen_sysctl_xsplice_action {
+    char        id[XEN_XSPLICE_ID_SIZE];    /* IN, name of the patch. */
+#define XSPLICE_ACTION_CHECK        1
+#define XSPLICE_ACTION_UNLOAD       2
+#define XSPLICE_ACTION_REVERT       3
+#define XSPLICE_ACTION_APPLY        4
+    uint32_t    cmd; /* IN */
+    uint64_aligned_t time;  /* IN */
+};
+typedef struct xen_sysctl_xsplice_action xen_sysctl_xsplice_action_t;
+DEFINE_XEN_GUEST_HANDLE(xen_sysctl_xsplice_action_t);
+
+struct xen_sysctl_xsplice_op {
+    uint32_t cmd; /* IN */
+    union {
+        xen_sysctl_xsplice_upload_t upload;
+        xen_sysctl_xsplice_list_t list;
+        xen_sysctl_xsplice_summary_t get;
+        xen_sysctl_xsplice_action_t action;
+    } u;
+};
+typedef struct xen_sysctl_xsplice_op xen_sysctl_xsplice_op_t;
+DEFINE_XEN_GUEST_HANDLE(xen_sysctl_xsplice_op_t);
+
 struct xen_sysctl {
     uint32_t cmd;
 #define XEN_SYSCTL_readconsole                    1
@@ -734,6 +798,7 @@ struct xen_sysctl {
 #define XEN_SYSCTL_psr_cmt_op                    21
 #define XEN_SYSCTL_pcitopoinfo                   22
 #define XEN_SYSCTL_psr_cat_op                    23
+#define XEN_SYSCTL_xsplice_op                    24
     uint32_t interface_version; /* XEN_SYSCTL_INTERFACE_VERSION */
     union {
         struct xen_sysctl_readconsole       readconsole;
@@ -758,6 +823,7 @@ struct xen_sysctl {
         struct xen_sysctl_coverage_op       coverage_op;
         struct xen_sysctl_psr_cmt_op        psr_cmt_op;
         struct xen_sysctl_psr_cat_op        psr_cat_op;
+        struct xen_sysctl_xsplice_op        xsplice;
         uint8_t                             pad[128];
     } u;
 };
diff --git a/xen/include/xen/xsplice.h b/xen/include/xen/xsplice.h
new file mode 100644
index 0000000..41e28da
--- /dev/null
+++ b/xen/include/xen/xsplice.h
@@ -0,0 +1,9 @@
+#ifndef __XEN_XSPLICE_H__
+#define __XEN_XSPLICE_H__
+
+struct xen_sysctl_xsplice_op;
+int xsplice_control(struct xen_sysctl_xsplice_op *);
+
+extern void xsplice_printall(unsigned char key);
+
+#endif /* __XEN_XSPLICE_H__ */
-- 
1.8.4.2

Re: [RFC PATCH v3.1 1/2] xsplice: rfc.v3.1

[Johannes Erdfelt]

Thanks for the work on this. I had some comments and questions on the
latest draft.

On Mon, Jul 27, 2015, Konrad Rzeszutek Wilk <konrad@kernel.org> wrote:
> +#define XSPLICE_HOWTO_FLAG_PC_REL    0x1 /* Is PC relative. */  
> +#define XSPLICE_HOWOT_FLAG_SIGN      0x2 /* Should the new value be treated as signed value. */  

s/HOWOT/HOWTO/

> +struct xsplice_reloc_howto {  
> +    uint32_t    howto; /* XSPLICE_HOWTO_* */  
> +    uint32_t    flag; /* XSPLICE_HOWTO_FLAG_* */  
> +    uint32_t    size; /* Size, in bytes, of the item to be relocated. */  
> +    uint32_t    r_shift; /* The value the final relocation is shifted right by; used to drop unwanted data from the relocation. */  
> +    uint64_t    mask; /* Bitmask for which parts of the instruction or data are replaced with the relocated value. */  
> +    uint8_t     pad[8]; /* Must be zero. */  
> +};  

I'm curious how r_shift and mask are used. I'm familiar with x86 and
x86_64 and I'm not sure how these fit in. Is this to support other
architectures?

> +### Trampoline (e9 opcode)
> +
> +The e9 opcode used for jmpq uses a 32-bit signed displacement. That means
> +we are limited to up to 2GB of virtual address to place the new code
> +from the old code. That should not be a problem since Xen hypervisor has
> +a very small footprint.
> +
> +However if we need - we can always add two trampolines. One at the 2GB
> +limit that calls the next trampoline.

I'm not sure I understand the concern. At least on x86_64, there is a
ton of unused virtual address space where the hypervisor image is
mapped. Why not simply map memory at the end of virtual address space?

There shouldn't be a concern with 2GB jumps then.

> +Please note there is a small limitation for trampolines in
> +function entries: The target function (+ trailing padding) must be able
> +to accomodate the trampoline. On x86 with +-2 GB relative jumps,
> +this means 5 bytes are  required.
> +
> +Depending on compiler settings, there are several functions in Xen that
> +are smaller (without inter-function padding).
> +
> +<pre> 
> +readelf -sW xen-syms | grep " FUNC " | \
> +    awk '{ if ($3 < 5) print $3, $4, $5, $8 }'
> +
> +...
> +3 FUNC LOCAL wbinvd_ipi
> +3 FUNC LOCAL shadow_l1_index
> +...
> +</pre>
> +A compile-time check for, e.g., a minimum alignment of functions or a
> +runtime check that verifies symbol size (+ padding to next symbols) for
> +that in the hypervisor is advised.

Is this really necessary? The way Xen is currently compiled results in
functions being aligned at 16-byte boundaries. The extra space is padded
with NOPs. Even if a function is only 3 bytes, it still has at least 16
bytes of space to use.

This can be controlled with the -falign-functions option to gcc.

Also, there are ways to get a 5-byte NOP added before the function.
This is what the Linux kernel does for ftrace which is what the recent
Linux kernel live patching support is built on.

It seems like it would be easier to be explicit during the build process
than do runtime checks to ensure there is enough space.

> +### When to patch
> +
> +During the discussion on the design two candidates bubbled where
> +the call stack for each CPU would be deterministic. This would
> +minimize the chance of the patch not being applied due to safety
> +checks failing.

It would be nice to have the consistency model be more explicit.

Maybe using the terminology from this LKML post?

https://lkml.org/lkml/2014/11/7/354

> +To randezvous all the CPUs an barrier with an maximum timeout (which
> +could be adjusted), combined with forcing all other CPUs through the
> +hypervisor with IPIs, can be utilized to have all the CPUs be lockstep.

s/randezvous/rendezvous/

> +### Compiling the hypervisor code
> +
> +Hotpatch generation often requires support for compiling the target
> +with -ffunction-sections / -fdata-sections.  Changes would have to
> +be done to the linker scripts to support this.

Is this for correctness reasons?

I understand this would be a good idea to reduce the size of patches,
but I wanted to make sure I'm not missing something.

> +### Symbol names
> +
> +
> +Xen as it is now, has a couple of non-unique symbol names which will
> +make runtime symbol identification hard.  Sometimes, static symbols
> +simply have the same name in C files, sometimes such symbols get
> +included via header files, and some C files are also compiled
> +multiple times and linked under different names (guest_walk.c).

I'm not sure I understand the problem with static symbols. They aren't
visible outside of the .c file, so when performing the linking against
the target xen image, there shouldn't be any conflicts.

JE

Re: [RFC PATCH v3.1 1/2] xsplice: rfc.v3.1

[Konrad Rzeszutek Wilk]

On Thu, Jul 30, 2015 at 09:47:40AM -0700, Johannes Erdfelt wrote:
> Thanks for the work on this. I had some comments and questions on the
> latest draft.

Hey Johannes!

Thank you for your review!
> 
> On Mon, Jul 27, 2015, Konrad Rzeszutek Wilk <konrad@kernel.org> wrote:
> > +#define XSPLICE_HOWTO_FLAG_PC_REL    0x1 /* Is PC relative. */  
> > +#define XSPLICE_HOWOT_FLAG_SIGN      0x2 /* Should the new value be treated as signed value. */  
> 
> s/HOWOT/HOWTO/
> 
> > +struct xsplice_reloc_howto {  
> > +    uint32_t    howto; /* XSPLICE_HOWTO_* */  
> > +    uint32_t    flag; /* XSPLICE_HOWTO_FLAG_* */  
> > +    uint32_t    size; /* Size, in bytes, of the item to be relocated. */  
> > +    uint32_t    r_shift; /* The value the final relocation is shifted right by; used to drop unwanted data from the relocation. */  
> > +    uint64_t    mask; /* Bitmask for which parts of the instruction or data are replaced with the relocated value. */  
> > +    uint8_t     pad[8]; /* Must be zero. */  
> > +};  
> 
> I'm curious how r_shift and mask are used. I'm familiar with x86 and
> x86_64 and I'm not sure how these fit in. Is this to support other
> architectures?

It is to patch up data. We can specify the exact mask for an unsigned
int - so we only patch specific bits. Ditto if we want to remove certain
values.
> 
> > +### Trampoline (e9 opcode)
> > +
> > +The e9 opcode used for jmpq uses a 32-bit signed displacement. That means
> > +we are limited to up to 2GB of virtual address to place the new code
> > +from the old code. That should not be a problem since Xen hypervisor has
> > +a very small footprint.
> > +
> > +However if we need - we can always add two trampolines. One at the 2GB
> > +limit that calls the next trampoline.
> 
> I'm not sure I understand the concern. At least on x86_64, there is a
> ton of unused virtual address space where the hypervisor image is
> mapped. Why not simply map memory at the end of virtual address space?
> 
> There shouldn't be a concern with 2GB jumps then.

Correct. This is added on if the hypervisor ends up gobbling up tons
of memory and having those virtual addresses eaten up.

> 
> > +Please note there is a small limitation for trampolines in
> > +function entries: The target function (+ trailing padding) must be able
> > +to accomodate the trampoline. On x86 with +-2 GB relative jumps,
> > +this means 5 bytes are  required.
> > +
> > +Depending on compiler settings, there are several functions in Xen that
> > +are smaller (without inter-function padding).
> > +
> > +<pre> 
> > +readelf -sW xen-syms | grep " FUNC " | \
> > +    awk '{ if ($3 < 5) print $3, $4, $5, $8 }'
> > +
> > +...
> > +3 FUNC LOCAL wbinvd_ipi
> > +3 FUNC LOCAL shadow_l1_index
> > +...
> > +</pre>
> > +A compile-time check for, e.g., a minimum alignment of functions or a
> > +runtime check that verifies symbol size (+ padding to next symbols) for
> > +that in the hypervisor is advised.
> 
> Is this really necessary? The way Xen is currently compiled results in
> functions being aligned at 16-byte boundaries. The extra space is padded
> with NOPs. Even if a function is only 3 bytes, it still has at least 16
> bytes of space to use.
> 
> This can be controlled with the -falign-functions option to gcc.

Right. The 'compile-time' check can be just to make sure that the
compiler is controlled by that - otherwise we will halt the compilation.

> 
> Also, there are ways to get a 5-byte NOP added before the function.
> This is what the Linux kernel does for ftrace which is what the recent
> Linux kernel live patching support is built on.
> 
> It seems like it would be easier to be explicit during the build process
> than do runtime checks to ensure there is enough space.

Correct.
> 
> > +### When to patch
> > +
> > +During the discussion on the design two candidates bubbled where
> > +the call stack for each CPU would be deterministic. This would
> > +minimize the chance of the patch not being applied due to safety
> > +checks failing.
> 
> It would be nice to have the consistency model be more explicit.
> 
> Maybe using the terminology from this LKML post?
> 
> https://lkml.org/lkml/2014/11/7/354

Certainy we can borrow.
> 
> > +To randezvous all the CPUs an barrier with an maximum timeout (which
> > +could be adjusted), combined with forcing all other CPUs through the
> > +hypervisor with IPIs, can be utilized to have all the CPUs be lockstep.
> 
> s/randezvous/rendezvous/
> 
> > +### Compiling the hypervisor code
> > +
> > +Hotpatch generation often requires support for compiling the target
> > +with -ffunction-sections / -fdata-sections.  Changes would have to
> > +be done to the linker scripts to support this.
> 
> Is this for correctness reasons?

Sanity mostly. Without that having the objdump and tools to figure out
what piece of code is for what would be complicated.
> 
> I understand this would be a good idea to reduce the size of patches,
> but I wanted to make sure I'm not missing something.

> 
> > +### Symbol names
> > +
> > +
> > +Xen as it is now, has a couple of non-unique symbol names which will
> > +make runtime symbol identification hard.  Sometimes, static symbols
> > +simply have the same name in C files, sometimes such symbols get
> > +included via header files, and some C files are also compiled
> > +multiple times and linked under different names (guest_walk.c).
> 
> I'm not sure I understand the problem with static symbols. They aren't
> visible outside of the .c file, so when performing the linking against
> the target xen image, there shouldn't be any conflicts.

To do run-time checking based on symbols. We may get information that
we need to patch 'xen_someting_static+<0x1f>' and we have not been supplied
the virtual address. As such if xen_something_static may not
show up at all - and we can't patch it. We need some mechanism to
tweak the build so that those symbols do bubble up.
> 
> JE
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel

Re: [RFC PATCH v3.1 1/2] xsplice: rfc.v3.1

[Jan Beulich]

>>> On 31.07.15 at 17:46, <konrad.wilk@oracle.com> wrote:
> On Thu, Jul 30, 2015 at 09:47:40AM -0700, Johannes Erdfelt wrote:
>> On Mon, Jul 27, 2015, Konrad Rzeszutek Wilk <konrad@kernel.org> wrote:
>> > +struct xsplice_reloc_howto {  
>> > +    uint32_t    howto; /* XSPLICE_HOWTO_* */  
>> > +    uint32_t    flag; /* XSPLICE_HOWTO_FLAG_* */  
>> > +    uint32_t    size; /* Size, in bytes, of the item to be relocated. */  
>> > +    uint32_t    r_shift; /* The value the final relocation is shifted right by; used to drop unwanted data from the relocation. */  
>> > +    uint64_t    mask; /* Bitmask for which parts of the instruction or data are replaced with the relocated value. */  
>> > +    uint8_t     pad[8]; /* Must be zero. */  
>> > +};  
>> 
>> I'm curious how r_shift and mask are used. I'm familiar with x86 and
>> x86_64 and I'm not sure how these fit in. Is this to support other
>> architectures?
> 
> It is to patch up data. We can specify the exact mask for an unsigned
> int - so we only patch specific bits. Ditto if we want to remove certain
> values.

Still I don't see a practical use: What relocated item would (on x86)
be stored starting at other than bit 0 of a byte/word? Also, wouldn't
a shift count be redundant with the mask value anyway?

Jan

[RFC PATCH v3.1 2/2] xsplice: Add hook for build_id

[Konrad Rzeszutek Wilk]

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 tools/libxc/xc_private.c     |  3 +++
 tools/misc/xen-xsplice.c     | 25 +++++++++++++++++++++++++
 xen/common/kernel.c          | 11 +++++++++++
 xen/common/version.c         |  5 +++++
 xen/include/public/version.h |  4 ++++
 xen/include/xen/compile.h.in |  1 +
 xen/include/xen/version.h    |  1 +
 7 files changed, 50 insertions(+)

diff --git a/tools/libxc/xc_private.c b/tools/libxc/xc_private.c
index 2ffebd9..7c039ca 100644
--- a/tools/libxc/xc_private.c
+++ b/tools/libxc/xc_private.c
@@ -713,6 +713,9 @@ int xc_version(xc_interface *xch, int cmd, void *arg)
     case XENVER_commandline:
         sz = sizeof(xen_commandline_t);
         break;
+    case XENVER_build_id:
+        sz = sizeof(xen_build_id_t);
+        break;
     default:
         ERROR("xc_version: unknown command %d\n", cmd);
         return -EINVAL;
diff --git a/tools/misc/xen-xsplice.c b/tools/misc/xen-xsplice.c
index 7cf9879..dd8266c 100644
--- a/tools/misc/xen-xsplice.c
+++ b/tools/misc/xen-xsplice.c
@@ -17,6 +17,7 @@ void show_help(void)
             " <id> An unique name of payload. Up to 40 characters.\n"
             "Commands:\n"
             "  help                 display this help\n"
+            "  build-id             display build-id of hypervisor.\n"
             "  upload <id> <file>   upload file <cpuid> with <id> name\n"
             "  list                 list payloads uploaded.\n"
             "  apply <id>           apply <id> patch.\n"
@@ -306,12 +307,36 @@ int action_func(int argc, char *argv[], unsigned int idx)
 
     return rc;
 }
+
+static int build_id_func(int argc, char *argv[])
+{
+    xen_build_id_t build_id;
+
+    if ( argc )
+    {
+        show_help();
+        return -1;
+    }
+
+    memset(build_id, 0, sizeof(*build_id));
+
+    if ( xc_version(xch, XENVER_build_id, &build_id) < 0 )
+    {
+        printf("Failed to get build_id: %d(%s)\n", errno, strerror(errno));
+        return -1;
+    }
+
+    printf("%s\n", build_id);
+    return 0;
+}
+
 struct {
     const char *name;
     int (*function)(int argc, char *argv[]);
 } main_options[] = {
     { "help", help_func },
     { "list", list_func },
+    { "build-id", build_id_func },
     { "upload", upload_func },
 };
 
diff --git a/xen/common/kernel.c b/xen/common/kernel.c
index 6a3196a..e9d41b6 100644
--- a/xen/common/kernel.c
+++ b/xen/common/kernel.c
@@ -357,6 +357,17 @@ DO(xen_version)(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
         if ( copy_to_guest(arg, saved_cmdline, ARRAY_SIZE(saved_cmdline)) )
             return -EFAULT;
         return 0;
+
+    case XENVER_build_id:
+    {
+        xen_build_id_t build_id;
+
+        memset(build_id, 0, sizeof(build_id));
+        safe_strcpy(build_id, xen_build_id());
+        if ( copy_to_guest(arg, build_id, ARRAY_SIZE(build_id)) )
+            return -EFAULT;
+        return 0;
+    }
     }
 
     return -ENOSYS;
diff --git a/xen/common/version.c b/xen/common/version.c
index b152e27..5c3dbb0 100644
--- a/xen/common/version.c
+++ b/xen/common/version.c
@@ -55,3 +55,8 @@ const char *xen_banner(void)
 {
     return XEN_BANNER;
 }
+
+const char *xen_build_id(void)
+{
+    return XEN_BUILD_ID;
+}
diff --git a/xen/include/public/version.h b/xen/include/public/version.h
index 44f26b0..c863393 100644
--- a/xen/include/public/version.h
+++ b/xen/include/public/version.h
@@ -83,6 +83,10 @@ typedef struct xen_feature_info xen_feature_info_t;
 #define XENVER_commandline 9
 typedef char xen_commandline_t[1024];
 
+#define XENVER_build_id 10
+typedef char xen_build_id_t[1024];
+#define XEN_BUILD_ID_LEN (sizeof(xen_build_id_t))
+
 #endif /* __XEN_PUBLIC_VERSION_H__ */
 
 /*
diff --git a/xen/include/xen/compile.h.in b/xen/include/xen/compile.h.in
index 440ecb2..939685e 100644
--- a/xen/include/xen/compile.h.in
+++ b/xen/include/xen/compile.h.in
@@ -10,4 +10,5 @@
 #define XEN_EXTRAVERSION	"@@extraversion@@"
 
 #define XEN_CHANGESET		"@@changeset@@"
+#define XEN_BUILD_ID        "@@changeset@@"
 #define XEN_BANNER		\
diff --git a/xen/include/xen/version.h b/xen/include/xen/version.h
index 81a3c7d..02f9585 100644
--- a/xen/include/xen/version.h
+++ b/xen/include/xen/version.h
@@ -12,5 +12,6 @@ unsigned int xen_minor_version(void);
 const char *xen_extra_version(void);
 const char *xen_changeset(void);
 const char *xen_banner(void);
+const char *xen_build_id(void);
 
 #endif /* __XEN_VERSION_H__ */
-- 
1.8.4.2

Re: [RFC PATCH v3.1 2/2] xsplice: Add hook for build_id

[Andrew Cooper]

On 27/07/15 20:20, Konrad Rzeszutek Wilk wrote:
> diff --git a/xen/include/xen/compile.h.in b/xen/include/xen/compile.h.in
> index 440ecb2..939685e 100644
> --- a/xen/include/xen/compile.h.in
> +++ b/xen/include/xen/compile.h.in
> @@ -10,4 +10,5 @@
>  #define XEN_EXTRAVERSION	"@@extraversion@@"
>  
>  #define XEN_CHANGESET		"@@changeset@@"
> +#define XEN_BUILD_ID        "@@changeset@@"

I am not sure putting the changeset into the build id field is sensible.

On the other hand, linking .note.gnu.build-id and exposing it as
XENVER_build_id seems like a very sensible thing to do.  It would
probably make sense to expose it in `xl info`.

~Andrew

Re: [RFC PATCH v3.1 2/2] xsplice: Add hook for build_id

[Konrad Rzeszutek Wilk]

On July 28, 2015 11:51:00 AM EDT, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>On 27/07/15 20:20, Konrad Rzeszutek Wilk wrote:
>> diff --git a/xen/include/xen/compile.h.in
>b/xen/include/xen/compile.h.in
>> index 440ecb2..939685e 100644
>> --- a/xen/include/xen/compile.h.in
>> +++ b/xen/include/xen/compile.h.in
>> @@ -10,4 +10,5 @@
>>  #define XEN_EXTRAVERSION	"@@extraversion@@"
>>  
>>  #define XEN_CHANGESET		"@@changeset@@"
>> +#define XEN_BUILD_ID        "@@changeset@@"
>
>I am not sure putting the changeset into the build id field is
>sensible.
>

Correct, it is unfinished.

>On the other hand, linking .note.gnu.build-id and exposing it as
>XENVER_build_id seems like a very sensible thing to do.  It would
>probably make sense to expose it in `xl info`.

That can certainly be done.
>
>~Andrew

Re: [RFC PATCH v3.1 2/2] xsplice: Add hook for build_id

[Martin Pohlack]

On 27.07.2015 21:20, Konrad Rzeszutek Wilk wrote:
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> ---
>  tools/libxc/xc_private.c     |  3 +++
>  tools/misc/xen-xsplice.c     | 25 +++++++++++++++++++++++++
>  xen/common/kernel.c          | 11 +++++++++++
>  xen/common/version.c         |  5 +++++
>  xen/include/public/version.h |  4 ++++
>  xen/include/xen/compile.h.in |  1 +
>  xen/include/xen/version.h    |  1 +
>  7 files changed, 50 insertions(+)
> 
> diff --git a/tools/libxc/xc_private.c b/tools/libxc/xc_private.c
> index 2ffebd9..7c039ca 100644
> --- a/tools/libxc/xc_private.c
> +++ b/tools/libxc/xc_private.c
> @@ -713,6 +713,9 @@ int xc_version(xc_interface *xch, int cmd, void *arg)
>      case XENVER_commandline:
>          sz = sizeof(xen_commandline_t);
>          break;
> +    case XENVER_build_id:
> +        sz = sizeof(xen_build_id_t);
> +        break;
>      default:
>          ERROR("xc_version: unknown command %d\n", cmd);
>          return -EINVAL;
> diff --git a/tools/misc/xen-xsplice.c b/tools/misc/xen-xsplice.c
> index 7cf9879..dd8266c 100644
> --- a/tools/misc/xen-xsplice.c
> +++ b/tools/misc/xen-xsplice.c
> @@ -17,6 +17,7 @@ void show_help(void)
>              " <id> An unique name of payload. Up to 40 characters.\n"
>              "Commands:\n"
>              "  help                 display this help\n"
> +            "  build-id             display build-id of hypervisor.\n"
>              "  upload <id> <file>   upload file <cpuid> with <id> name\n"
>              "  list                 list payloads uploaded.\n"
>              "  apply <id>           apply <id> patch.\n"
> @@ -306,12 +307,36 @@ int action_func(int argc, char *argv[], unsigned int idx)
>  
>      return rc;
>  }
> +
> +static int build_id_func(int argc, char *argv[])
> +{
> +    xen_build_id_t build_id;
> +
> +    if ( argc )
> +    {
> +        show_help();
> +        return -1;
> +    }
> +
> +    memset(build_id, 0, sizeof(*build_id));
> +
> +    if ( xc_version(xch, XENVER_build_id, &build_id) < 0 )
> +    {
> +        printf("Failed to get build_id: %d(%s)\n", errno, strerror(errno));
> +        return -1;
> +    }
> +
> +    printf("%s\n", build_id);
> +    return 0;
> +}
> +
>  struct {
>      const char *name;
>      int (*function)(int argc, char *argv[]);
>  } main_options[] = {
>      { "help", help_func },
>      { "list", list_func },
> +    { "build-id", build_id_func },
>      { "upload", upload_func },
>  };
>  
> diff --git a/xen/common/kernel.c b/xen/common/kernel.c
> index 6a3196a..e9d41b6 100644
> --- a/xen/common/kernel.c
> +++ b/xen/common/kernel.c
> @@ -357,6 +357,17 @@ DO(xen_version)(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
>          if ( copy_to_guest(arg, saved_cmdline, ARRAY_SIZE(saved_cmdline)) )
>              return -EFAULT;
>          return 0;
> +
> +    case XENVER_build_id:
> +    {
> +        xen_build_id_t build_id;
> +
> +        memset(build_id, 0, sizeof(build_id));
> +        safe_strcpy(build_id, xen_build_id());

You seem to want to store and transfer the build_id as a string.  Any
reason why we don't directly expose the build_id embedded by the linker
in binary format?

> +        if ( copy_to_guest(arg, build_id, ARRAY_SIZE(build_id)) )
> +            return -EFAULT;
> +        return 0;
> +    }

We should not expose the build_id to normal guests, but only to Dom0.

A build_id uniquely identifies a specific build and I don't see how that
information would be required from DomU.  It might actually help an
attacker to build his return-oriented programming exploit against a
specific build.

The normal version numbers should be enough to know about capabilities
and API.

>      }
>  
>      return -ENOSYS;
> diff --git a/xen/common/version.c b/xen/common/version.c
> index b152e27..5c3dbb0 100644
> --- a/xen/common/version.c
> +++ b/xen/common/version.c
> @@ -55,3 +55,8 @@ const char *xen_banner(void)
>  {
>      return XEN_BANNER;
>  }
> +
> +const char *xen_build_id(void)
> +{
> +    return XEN_BUILD_ID;
> +}
> diff --git a/xen/include/public/version.h b/xen/include/public/version.h
> index 44f26b0..c863393 100644
> --- a/xen/include/public/version.h
> +++ b/xen/include/public/version.h
> @@ -83,6 +83,10 @@ typedef struct xen_feature_info xen_feature_info_t;
>  #define XENVER_commandline 9
>  typedef char xen_commandline_t[1024];
>  
> +#define XENVER_build_id 10
> +typedef char xen_build_id_t[1024];
> +#define XEN_BUILD_ID_LEN (sizeof(xen_build_id_t))
> +
>  #endif /* __XEN_PUBLIC_VERSION_H__ */
>  
>  /*
> diff --git a/xen/include/xen/compile.h.in b/xen/include/xen/compile.h.in
> index 440ecb2..939685e 100644
> --- a/xen/include/xen/compile.h.in
> +++ b/xen/include/xen/compile.h.in
> @@ -10,4 +10,5 @@
>  #define XEN_EXTRAVERSION	"@@extraversion@@"
>  
>  #define XEN_CHANGESET		"@@changeset@@"
> +#define XEN_BUILD_ID        "@@changeset@@"

That leads to a chicken and egg problem when embedding a real build_id.
 Some linker script magic seems to be required.  I will try to refine
the patch.

>  #define XEN_BANNER		\
> diff --git a/xen/include/xen/version.h b/xen/include/xen/version.h
> index 81a3c7d..02f9585 100644
> --- a/xen/include/xen/version.h
> +++ b/xen/include/xen/version.h
> @@ -12,5 +12,6 @@ unsigned int xen_minor_version(void);
>  const char *xen_extra_version(void);
>  const char *xen_changeset(void);
>  const char *xen_banner(void);
> +const char *xen_build_id(void);
>  
>  #endif /* __XEN_VERSION_H__ */
> 

Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger
Ust-ID: DE289237879
Eingetragen am Amtsgericht Charlottenburg HRB 149173 B

Re: [RFC PATCH v3.1 2/2] xsplice: Add hook for build_id

[Andrew Cooper]

On 05/08/15 09:50, Martin Pohlack wrote:
> On 27.07.2015 21:20, Konrad Rzeszutek Wilk wrote:
>> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>> ---
>>  tools/libxc/xc_private.c     |  3 +++
>>  tools/misc/xen-xsplice.c     | 25 +++++++++++++++++++++++++
>>  xen/common/kernel.c          | 11 +++++++++++
>>  xen/common/version.c         |  5 +++++
>>  xen/include/public/version.h |  4 ++++
>>  xen/include/xen/compile.h.in |  1 +
>>  xen/include/xen/version.h    |  1 +
>>  7 files changed, 50 insertions(+)
>>
>> diff --git a/tools/libxc/xc_private.c b/tools/libxc/xc_private.c
>> index 2ffebd9..7c039ca 100644
>> --- a/tools/libxc/xc_private.c
>> +++ b/tools/libxc/xc_private.c
>> @@ -713,6 +713,9 @@ int xc_version(xc_interface *xch, int cmd, void *arg)
>>      case XENVER_commandline:
>>          sz = sizeof(xen_commandline_t);
>>          break;
>> +    case XENVER_build_id:
>> +        sz = sizeof(xen_build_id_t);
>> +        break;
>>      default:
>>          ERROR("xc_version: unknown command %d\n", cmd);
>>          return -EINVAL;
>> diff --git a/tools/misc/xen-xsplice.c b/tools/misc/xen-xsplice.c
>> index 7cf9879..dd8266c 100644
>> --- a/tools/misc/xen-xsplice.c
>> +++ b/tools/misc/xen-xsplice.c
>> @@ -17,6 +17,7 @@ void show_help(void)
>>              " <id> An unique name of payload. Up to 40 characters.\n"
>>              "Commands:\n"
>>              "  help                 display this help\n"
>> +            "  build-id             display build-id of hypervisor.\n"
>>              "  upload <id> <file>   upload file <cpuid> with <id> name\n"
>>              "  list                 list payloads uploaded.\n"
>>              "  apply <id>           apply <id> patch.\n"
>> @@ -306,12 +307,36 @@ int action_func(int argc, char *argv[], unsigned int idx)
>>  
>>      return rc;
>>  }
>> +
>> +static int build_id_func(int argc, char *argv[])
>> +{
>> +    xen_build_id_t build_id;
>> +
>> +    if ( argc )
>> +    {
>> +        show_help();
>> +        return -1;
>> +    }
>> +
>> +    memset(build_id, 0, sizeof(*build_id));
>> +
>> +    if ( xc_version(xch, XENVER_build_id, &build_id) < 0 )
>> +    {
>> +        printf("Failed to get build_id: %d(%s)\n", errno, strerror(errno));
>> +        return -1;
>> +    }
>> +
>> +    printf("%s\n", build_id);
>> +    return 0;
>> +}
>> +
>>  struct {
>>      const char *name;
>>      int (*function)(int argc, char *argv[]);
>>  } main_options[] = {
>>      { "help", help_func },
>>      { "list", list_func },
>> +    { "build-id", build_id_func },
>>      { "upload", upload_func },
>>  };
>>  
>> diff --git a/xen/common/kernel.c b/xen/common/kernel.c
>> index 6a3196a..e9d41b6 100644
>> --- a/xen/common/kernel.c
>> +++ b/xen/common/kernel.c
>> @@ -357,6 +357,17 @@ DO(xen_version)(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
>>          if ( copy_to_guest(arg, saved_cmdline, ARRAY_SIZE(saved_cmdline)) )
>>              return -EFAULT;
>>          return 0;
>> +
>> +    case XENVER_build_id:
>> +    {
>> +        xen_build_id_t build_id;
>> +
>> +        memset(build_id, 0, sizeof(build_id));
>> +        safe_strcpy(build_id, xen_build_id());
> You seem to want to store and transfer the build_id as a string.  Any
> reason why we don't directly expose the build_id embedded by the linker
> in binary format?
>
>> +        if ( copy_to_guest(arg, build_id, ARRAY_SIZE(build_id)) )
>> +            return -EFAULT;
>> +        return 0;
>> +    }
> We should not expose the build_id to normal guests, but only to Dom0.
>
> A build_id uniquely identifies a specific build and I don't see how that
> information would be required from DomU.  It might actually help an
> attacker to build his return-oriented programming exploit against a
> specific build.
>
> The normal version numbers should be enough to know about capabilities
> and API.

It will need its own XSM hook, but need not be strictly limited to just
dom0.

>
>>      }
>>  
>>      return -ENOSYS;
>> diff --git a/xen/common/version.c b/xen/common/version.c
>> index b152e27..5c3dbb0 100644
>> --- a/xen/common/version.c
>> +++ b/xen/common/version.c
>> @@ -55,3 +55,8 @@ const char *xen_banner(void)
>>  {
>>      return XEN_BANNER;
>>  }
>> +
>> +const char *xen_build_id(void)
>> +{
>> +    return XEN_BUILD_ID;
>> +}
>> diff --git a/xen/include/public/version.h b/xen/include/public/version.h
>> index 44f26b0..c863393 100644
>> --- a/xen/include/public/version.h
>> +++ b/xen/include/public/version.h
>> @@ -83,6 +83,10 @@ typedef struct xen_feature_info xen_feature_info_t;
>>  #define XENVER_commandline 9
>>  typedef char xen_commandline_t[1024];
>>  
>> +#define XENVER_build_id 10
>> +typedef char xen_build_id_t[1024];
>> +#define XEN_BUILD_ID_LEN (sizeof(xen_build_id_t))
>> +
>>  #endif /* __XEN_PUBLIC_VERSION_H__ */
>>  
>>  /*
>> diff --git a/xen/include/xen/compile.h.in b/xen/include/xen/compile.h.in
>> index 440ecb2..939685e 100644
>> --- a/xen/include/xen/compile.h.in
>> +++ b/xen/include/xen/compile.h.in
>> @@ -10,4 +10,5 @@
>>  #define XEN_EXTRAVERSION	"@@extraversion@@"
>>  
>>  #define XEN_CHANGESET		"@@changeset@@"
>> +#define XEN_BUILD_ID        "@@changeset@@"
> That leads to a chicken and egg problem when embedding a real build_id.
>  Some linker script magic seems to be required.  I will try to refine
> the patch.

So funnily enough, I tried experimenting with this and it is fairly easy
to get the basics done.

Further TODO which I havn't done yet is make the --build-id optional on
finding a compatible `ld`, and some symbol magic to directly locate
.note.gnu.build-id

However, this in addition to some of Konrad's original patch is a good
start.

~Andrew

diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile
index 5f24951..10938b2 100644
--- a/xen/arch/x86/Makefile
+++ b/xen/arch/x86/Makefile
@@ -112,7 +112,7 @@ $(TARGET)-syms: prelink.o xen.lds
$(BASEDIR)/common/symbols-dummy.o
            $(@D)/.$(@F).0.o -o $(@D)/.$(@F).1
        $(NM) -n $(@D)/.$(@F).1 | $(BASEDIR)/tools/symbols >$(@D)/.$(@F).1.S
        $(MAKE) -f $(BASEDIR)/Rules.mk $(@D)/.$(@F).1.o
-       $(LD) $(LDFLAGS) -T xen.lds -N prelink.o \
+       $(LD) $(LDFLAGS) -T xen.lds -N prelink.o --build-id \
            $(@D)/.$(@F).1.o -o $@
        rm -f $(@D)/.$(@F).[0-9]*
 
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index 6553cff..46e6546 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -68,6 +68,13 @@ SECTIONS
   } :text
 
   . = ALIGN(SMP_CACHE_BYTES);
+  .notes : {
+       __start_notes = .;
+       *(.note.*)
+       __end_notes = .;
+  } :text
+
+  . = ALIGN(SMP_CACHE_BYTES);
   .data.read_mostly : {
        /* Exception table */
        __start___ex_table = .;

Re: [RFC PATCH v3.1 2/2] xsplice: Add hook for build_id

[Martin Pohlack]

On 05.08.2015 10:58, Andrew Cooper wrote:
> On 05/08/15 09:50, Martin Pohlack wrote:
>> On 27.07.2015 21:20, Konrad Rzeszutek Wilk wrote:
>>> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>>> ---
>>>  tools/libxc/xc_private.c     |  3 +++
>>>  tools/misc/xen-xsplice.c     | 25 +++++++++++++++++++++++++
>>>  xen/common/kernel.c          | 11 +++++++++++
>>>  xen/common/version.c         |  5 +++++
>>>  xen/include/public/version.h |  4 ++++
>>>  xen/include/xen/compile.h.in |  1 +
>>>  xen/include/xen/version.h    |  1 +
>>>  7 files changed, 50 insertions(+)
>>>
>>> diff --git a/tools/libxc/xc_private.c b/tools/libxc/xc_private.c
>>> index 2ffebd9..7c039ca 100644
>>> --- a/tools/libxc/xc_private.c
>>> +++ b/tools/libxc/xc_private.c
>>> @@ -713,6 +713,9 @@ int xc_version(xc_interface *xch, int cmd, void *arg)
>>>      case XENVER_commandline:
>>>          sz = sizeof(xen_commandline_t);
>>>          break;
>>> +    case XENVER_build_id:
>>> +        sz = sizeof(xen_build_id_t);
>>> +        break;
>>>      default:
>>>          ERROR("xc_version: unknown command %d\n", cmd);
>>>          return -EINVAL;
>>> diff --git a/tools/misc/xen-xsplice.c b/tools/misc/xen-xsplice.c
>>> index 7cf9879..dd8266c 100644
>>> --- a/tools/misc/xen-xsplice.c
>>> +++ b/tools/misc/xen-xsplice.c
>>> @@ -17,6 +17,7 @@ void show_help(void)
>>>              " <id> An unique name of payload. Up to 40 characters.\n"
>>>              "Commands:\n"
>>>              "  help                 display this help\n"
>>> +            "  build-id             display build-id of hypervisor.\n"
>>>              "  upload <id> <file>   upload file <cpuid> with <id> name\n"
>>>              "  list                 list payloads uploaded.\n"
>>>              "  apply <id>           apply <id> patch.\n"
>>> @@ -306,12 +307,36 @@ int action_func(int argc, char *argv[], unsigned int idx)
>>>  
>>>      return rc;
>>>  }
>>> +
>>> +static int build_id_func(int argc, char *argv[])
>>> +{
>>> +    xen_build_id_t build_id;
>>> +
>>> +    if ( argc )
>>> +    {
>>> +        show_help();
>>> +        return -1;
>>> +    }
>>> +
>>> +    memset(build_id, 0, sizeof(*build_id));
>>> +
>>> +    if ( xc_version(xch, XENVER_build_id, &build_id) < 0 )
>>> +    {
>>> +        printf("Failed to get build_id: %d(%s)\n", errno, strerror(errno));
>>> +        return -1;
>>> +    }
>>> +
>>> +    printf("%s\n", build_id);
>>> +    return 0;
>>> +}
>>> +
>>>  struct {
>>>      const char *name;
>>>      int (*function)(int argc, char *argv[]);
>>>  } main_options[] = {
>>>      { "help", help_func },
>>>      { "list", list_func },
>>> +    { "build-id", build_id_func },
>>>      { "upload", upload_func },
>>>  };
>>>  
>>> diff --git a/xen/common/kernel.c b/xen/common/kernel.c
>>> index 6a3196a..e9d41b6 100644
>>> --- a/xen/common/kernel.c
>>> +++ b/xen/common/kernel.c
>>> @@ -357,6 +357,17 @@ DO(xen_version)(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
>>>          if ( copy_to_guest(arg, saved_cmdline, ARRAY_SIZE(saved_cmdline)) )
>>>              return -EFAULT;
>>>          return 0;
>>> +
>>> +    case XENVER_build_id:
>>> +    {
>>> +        xen_build_id_t build_id;
>>> +
>>> +        memset(build_id, 0, sizeof(build_id));
>>> +        safe_strcpy(build_id, xen_build_id());
>> You seem to want to store and transfer the build_id as a string.  Any
>> reason why we don't directly expose the build_id embedded by the linker
>> in binary format?
>>
>>> +        if ( copy_to_guest(arg, build_id, ARRAY_SIZE(build_id)) )
>>> +            return -EFAULT;
>>> +        return 0;
>>> +    }
>> We should not expose the build_id to normal guests, but only to Dom0.
>>
>> A build_id uniquely identifies a specific build and I don't see how that
>> information would be required from DomU.  It might actually help an
>> attacker to build his return-oriented programming exploit against a
>> specific build.
>>
>> The normal version numbers should be enough to know about capabilities
>> and API.
> 
> It will need its own XSM hook, but need not be strictly limited to just
> dom0.
> 
>>
>>>      }
>>>  
>>>      return -ENOSYS;
>>> diff --git a/xen/common/version.c b/xen/common/version.c
>>> index b152e27..5c3dbb0 100644
>>> --- a/xen/common/version.c
>>> +++ b/xen/common/version.c
>>> @@ -55,3 +55,8 @@ const char *xen_banner(void)
>>>  {
>>>      return XEN_BANNER;
>>>  }
>>> +
>>> +const char *xen_build_id(void)
>>> +{
>>> +    return XEN_BUILD_ID;
>>> +}
>>> diff --git a/xen/include/public/version.h b/xen/include/public/version.h
>>> index 44f26b0..c863393 100644
>>> --- a/xen/include/public/version.h
>>> +++ b/xen/include/public/version.h
>>> @@ -83,6 +83,10 @@ typedef struct xen_feature_info xen_feature_info_t;
>>>  #define XENVER_commandline 9
>>>  typedef char xen_commandline_t[1024];
>>>  
>>> +#define XENVER_build_id 10
>>> +typedef char xen_build_id_t[1024];
>>> +#define XEN_BUILD_ID_LEN (sizeof(xen_build_id_t))
>>> +
>>>  #endif /* __XEN_PUBLIC_VERSION_H__ */
>>>  
>>>  /*
>>> diff --git a/xen/include/xen/compile.h.in b/xen/include/xen/compile.h.in
>>> index 440ecb2..939685e 100644
>>> --- a/xen/include/xen/compile.h.in
>>> +++ b/xen/include/xen/compile.h.in
>>> @@ -10,4 +10,5 @@
>>>  #define XEN_EXTRAVERSION	"@@extraversion@@"
>>>  
>>>  #define XEN_CHANGESET		"@@changeset@@"
>>> +#define XEN_BUILD_ID        "@@changeset@@"
>> That leads to a chicken and egg problem when embedding a real build_id.
>>  Some linker script magic seems to be required.  I will try to refine
>> the patch.
> 
> So funnily enough, I tried experimenting with this and it is fairly easy
> to get the basics done.
> 
> Further TODO which I havn't done yet is make the --build-id optional on
> finding a compatible `ld`, and some symbol magic to directly locate
> .note.gnu.build-id
> 
> However, this in addition to some of Konrad's original patch is a good
> start.
> 
> ~Andrew
> 
> diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile
> index 5f24951..10938b2 100644
> --- a/xen/arch/x86/Makefile
> +++ b/xen/arch/x86/Makefile
> @@ -112,7 +112,7 @@ $(TARGET)-syms: prelink.o xen.lds
> $(BASEDIR)/common/symbols-dummy.o
>             $(@D)/.$(@F).0.o -o $(@D)/.$(@F).1
>         $(NM) -n $(@D)/.$(@F).1 | $(BASEDIR)/tools/symbols >$(@D)/.$(@F).1.S
>         $(MAKE) -f $(BASEDIR)/Rules.mk $(@D)/.$(@F).1.o
> -       $(LD) $(LDFLAGS) -T xen.lds -N prelink.o \
> +       $(LD) $(LDFLAGS) -T xen.lds -N prelink.o --build-id \
>             $(@D)/.$(@F).1.o -o $@
>         rm -f $(@D)/.$(@F).[0-9]*
>  
> diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
> index 6553cff..46e6546 100644
> --- a/xen/arch/x86/xen.lds.S
> +++ b/xen/arch/x86/xen.lds.S
> @@ -68,6 +68,13 @@ SECTIONS
>    } :text
>  
>    . = ALIGN(SMP_CACHE_BYTES);
> +  .notes : {
> +       __start_notes = .;
> +       *(.note.*)
> +       __end_notes = .;
> +  } :text
> +
> +  . = ALIGN(SMP_CACHE_BYTES);
>    .data.read_mostly : {
>         /* Exception table */
>         __start___ex_table = .;

And here is my version, also on-top of Konrad's series:

----------------------------------------------------------------------

[PATCH] xsplice: Use ld-embedded build-ids

Todo:
  * Should be moved to sysctl to only allow Dom0 access
  * Maybe convert to binary transport to userland instead of printable form

* use ld to actually embed the build ID
* convert to textual representation in hypervisor and report in
  printable form

Signed-off-by: Martin Pohlack <mpohlack@amazon.de>
---
 xen/arch/x86/Makefile        |  4 ++--
 xen/arch/x86/xen.lds.S       |  5 +++++
 xen/common/kernel.c          | 33 +++++++++++++++++++++++++++++----
 xen/common/version.c         |  5 -----
 xen/include/xen/compile.h.in |  1 -
 5 files changed, 36 insertions(+), 12 deletions(-)

diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile
index 5f24951..f724bd8 100644
--- a/xen/arch/x86/Makefile
+++ b/xen/arch/x86/Makefile
@@ -108,11 +108,11 @@ $(TARGET)-syms: prelink.o xen.lds
$(BASEDIR)/common/symbols-dummy.o
 	    $(BASEDIR)/common/symbols-dummy.o -o $(@D)/.$(@F).0
 	$(NM) -n $(@D)/.$(@F).0 | $(BASEDIR)/tools/symbols >$(@D)/.$(@F).0.S
 	$(MAKE) -f $(BASEDIR)/Rules.mk $(@D)/.$(@F).0.o
-	$(LD) $(LDFLAGS) -T xen.lds -N prelink.o \
+	$(LD) $(LDFLAGS) -T xen.lds -N prelink.o --build-id=sha1 \
 	    $(@D)/.$(@F).0.o -o $(@D)/.$(@F).1
 	$(NM) -n $(@D)/.$(@F).1 | $(BASEDIR)/tools/symbols >$(@D)/.$(@F).1.S
 	$(MAKE) -f $(BASEDIR)/Rules.mk $(@D)/.$(@F).1.o
-	$(LD) $(LDFLAGS) -T xen.lds -N prelink.o \
+	$(LD) $(LDFLAGS) -T xen.lds -N prelink.o --build-id=sha1 \
 	    $(@D)/.$(@F).1.o -o $@
 	rm -f $(@D)/.$(@F).[0-9]*

diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index 6553cff..2176782 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -67,6 +67,11 @@ SECTIONS
        *(.rodata.*)
   } :text

+  .note.gnu.build-id : {
+      __note_gnu_build_id_start = .;
+      *(.note.gnu.build-id)
+  } :text
+
   . = ALIGN(SMP_CACHE_BYTES);
   .data.read_mostly : {
        /* Exception table */
diff --git a/xen/common/kernel.c b/xen/common/kernel.c
index e9d41b6..9814585 100644
--- a/xen/common/kernel.c
+++ b/xen/common/kernel.c
@@ -6,9 +6,11 @@

 #include <xen/init.h>
 #include <xen/lib.h>
+#include <xen/elf.h>
 #include <xen/errno.h>
 #include <xen/version.h>
 #include <xen/sched.h>
+#include <xen/types.h>
 #include <xen/paging.h>
 #include <xen/nmi.h>
 #include <xen/guest_access.h>
@@ -227,6 +229,10 @@ void __init do_initcalls(void)
  * Simple hypercalls.
  */

+#define NT_GNU_BUILD_ID 3
+
+extern char * __note_gnu_build_id_start;  /* defined in linker script */
+
 DO(xen_version)(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
 {
     switch ( cmd )
@@ -360,11 +366,30 @@ DO(xen_version)(int cmd,
XEN_GUEST_HANDLE_PARAM(void) arg)

     case XENVER_build_id:
     {
-        xen_build_id_t build_id;
+        xen_build_id_t ascii_id;
+        Elf_Note * n = (Elf_Note *)&__note_gnu_build_id_start;
+        char * binary_id;
+        int i;
+
+        memset(ascii_id, 0, sizeof(ascii_id));
+
+        /* check if we really have a build-id */
+        if ( NT_GNU_BUILD_ID != n->type )
+            return 0;
+
+        /* sanity check, name should be "GNU" for ld-generated build-id */
+        if ( 0 != strncmp(ELFNOTE_NAME(n), "GNU", n->namesz))
+            return 0;
+
+        binary_id = (char *)ELFNOTE_DESC(n);
+
+        /* convert to printable format */
+        for (i = 0; i < n->descsz && (i + 1) * 2 <
sizeof(xen_build_id_t); i++)
+        {
+            snprintf(&ascii_id[i * 2], 3, "%02hhx", binary_id[i]);
+        }

-        memset(build_id, 0, sizeof(build_id));
-        safe_strcpy(build_id, xen_build_id());
-        if ( copy_to_guest(arg, build_id, ARRAY_SIZE(build_id)) )
+        if ( copy_to_guest(arg, ascii_id, ARRAY_SIZE(ascii_id)) )
             return -EFAULT;
         return 0;
     }
diff --git a/xen/common/version.c b/xen/common/version.c
index 5c3dbb0..b152e27 100644
--- a/xen/common/version.c
+++ b/xen/common/version.c
@@ -55,8 +55,3 @@ const char *xen_banner(void)
 {
     return XEN_BANNER;
 }
-
-const char *xen_build_id(void)
-{
-    return XEN_BUILD_ID;
-}
diff --git a/xen/include/xen/compile.h.in b/xen/include/xen/compile.h.in
index 939685e..440ecb2 100644
--- a/xen/include/xen/compile.h.in
+++ b/xen/include/xen/compile.h.in
@@ -10,5 +10,4 @@
 #define XEN_EXTRAVERSION	"@@extraversion@@"

 #define XEN_CHANGESET		"@@changeset@@"
-#define XEN_BUILD_ID        "@@changeset@@"
 #define XEN_BANNER		\
-- 
2.5.0

Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger
Ust-ID: DE289237879
Eingetragen am Amtsgericht Charlottenburg HRB 149173 B

(no subject)

[Martin Pohlack]

Sending again without MUA-mangled patch.
Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger
Ust-ID: DE289237879
Eingetragen am Amtsgericht Charlottenburg HRB 149173 B

[PATCH] xsplice: Use ld-embedded build-ids

[Martin Pohlack]

Todo:
  * Should be moved to sysctl to only allow Dom0 access
  * Maybe convert to binary transport to userland instead of printable form

* use ld to actually embed the build ID
* convert to textual representation in hypervisor and report in
  printable form

Signed-off-by: Martin Pohlack <mpohlack@amazon.de>
---
 xen/arch/x86/Makefile        |  4 ++--
 xen/arch/x86/xen.lds.S       |  5 +++++
 xen/common/kernel.c          | 33 +++++++++++++++++++++++++++++----
 xen/common/version.c         |  5 -----
 xen/include/xen/compile.h.in |  1 -
 5 files changed, 36 insertions(+), 12 deletions(-)

diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile
index 5f24951..f724bd8 100644
--- a/xen/arch/x86/Makefile
+++ b/xen/arch/x86/Makefile
@@ -108,11 +108,11 @@ $(TARGET)-syms: prelink.o xen.lds $(BASEDIR)/common/symbols-dummy.o
 	    $(BASEDIR)/common/symbols-dummy.o -o $(@D)/.$(@F).0
 	$(NM) -n $(@D)/.$(@F).0 | $(BASEDIR)/tools/symbols >$(@D)/.$(@F).0.S
 	$(MAKE) -f $(BASEDIR)/Rules.mk $(@D)/.$(@F).0.o
-	$(LD) $(LDFLAGS) -T xen.lds -N prelink.o \
+	$(LD) $(LDFLAGS) -T xen.lds -N prelink.o --build-id=sha1 \
 	    $(@D)/.$(@F).0.o -o $(@D)/.$(@F).1
 	$(NM) -n $(@D)/.$(@F).1 | $(BASEDIR)/tools/symbols >$(@D)/.$(@F).1.S
 	$(MAKE) -f $(BASEDIR)/Rules.mk $(@D)/.$(@F).1.o
-	$(LD) $(LDFLAGS) -T xen.lds -N prelink.o \
+	$(LD) $(LDFLAGS) -T xen.lds -N prelink.o --build-id=sha1 \
 	    $(@D)/.$(@F).1.o -o $@
 	rm -f $(@D)/.$(@F).[0-9]*
 
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index 6553cff..2176782 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -67,6 +67,11 @@ SECTIONS
        *(.rodata.*)
   } :text
 
+  .note.gnu.build-id : {
+      __note_gnu_build_id_start = .;
+      *(.note.gnu.build-id)
+  } :text
+
   . = ALIGN(SMP_CACHE_BYTES);
   .data.read_mostly : {
        /* Exception table */
diff --git a/xen/common/kernel.c b/xen/common/kernel.c
index e9d41b6..9814585 100644
--- a/xen/common/kernel.c
+++ b/xen/common/kernel.c
@@ -6,9 +6,11 @@
 
 #include <xen/init.h>
 #include <xen/lib.h>
+#include <xen/elf.h>
 #include <xen/errno.h>
 #include <xen/version.h>
 #include <xen/sched.h>
+#include <xen/types.h>
 #include <xen/paging.h>
 #include <xen/nmi.h>
 #include <xen/guest_access.h>
@@ -227,6 +229,10 @@ void __init do_initcalls(void)
  * Simple hypercalls.
  */
 
+#define NT_GNU_BUILD_ID 3
+
+extern char * __note_gnu_build_id_start;  /* defined in linker script */
+
 DO(xen_version)(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
 {
     switch ( cmd )
@@ -360,11 +366,30 @@ DO(xen_version)(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
 
     case XENVER_build_id:
     {
-        xen_build_id_t build_id;
+        xen_build_id_t ascii_id;
+        Elf_Note * n = (Elf_Note *)&__note_gnu_build_id_start;
+        char * binary_id;
+        int i;
+
+        memset(ascii_id, 0, sizeof(ascii_id));
+
+        /* check if we really have a build-id */
+        if ( NT_GNU_BUILD_ID != n->type )
+            return 0;
+
+        /* sanity check, name should be "GNU" for ld-generated build-id */
+        if ( 0 != strncmp(ELFNOTE_NAME(n), "GNU", n->namesz))
+            return 0;
+
+        binary_id = (char *)ELFNOTE_DESC(n);
+
+        /* convert to printable format */
+        for (i = 0; i < n->descsz && (i + 1) * 2 < sizeof(xen_build_id_t); i++)
+        {
+            snprintf(&ascii_id[i * 2], 3, "%02hhx", binary_id[i]);
+        }
 
-        memset(build_id, 0, sizeof(build_id));
-        safe_strcpy(build_id, xen_build_id());
-        if ( copy_to_guest(arg, build_id, ARRAY_SIZE(build_id)) )
+        if ( copy_to_guest(arg, ascii_id, ARRAY_SIZE(ascii_id)) )
             return -EFAULT;
         return 0;
     }
diff --git a/xen/common/version.c b/xen/common/version.c
index 5c3dbb0..b152e27 100644
--- a/xen/common/version.c
+++ b/xen/common/version.c
@@ -55,8 +55,3 @@ const char *xen_banner(void)
 {
     return XEN_BANNER;
 }
-
-const char *xen_build_id(void)
-{
-    return XEN_BUILD_ID;
-}
diff --git a/xen/include/xen/compile.h.in b/xen/include/xen/compile.h.in
index 939685e..440ecb2 100644
--- a/xen/include/xen/compile.h.in
+++ b/xen/include/xen/compile.h.in
@@ -10,5 +10,4 @@
 #define XEN_EXTRAVERSION	"@@extraversion@@"
 
 #define XEN_CHANGESET		"@@changeset@@"
-#define XEN_BUILD_ID        "@@changeset@@"
 #define XEN_BANNER		\
-- 
2.5.0

Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger
Ust-ID: DE289237879
Eingetragen am Amtsgericht Charlottenburg HRB 149173 B

Re: [PATCH] xsplice: Use ld-embedded build-ids

[Jan Beulich]

>>> On 05.08.15 at 16:09, <mpohlack@amazon.de> wrote:
> Todo:
>   * Should be moved to sysctl to only allow Dom0 access

Because of?

>   * Maybe convert to binary transport to userland instead of printable form

Indeed.

> @@ -360,11 +366,30 @@ DO(xen_version)(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
>  
>      case XENVER_build_id:
>      {
> -        xen_build_id_t build_id;
> +        xen_build_id_t ascii_id;
> +        Elf_Note * n = (Elf_Note *)&__note_gnu_build_id_start;
> +        char * binary_id;
> +        int i;
> +
> +        memset(ascii_id, 0, sizeof(ascii_id));
> +
> +        /* check if we really have a build-id */
> +        if ( NT_GNU_BUILD_ID != n->type )
> +            return 0;

This needs to signal an error.

> +
> +        /* sanity check, name should be "GNU" for ld-generated build-id */
> +        if ( 0 != strncmp(ELFNOTE_NAME(n), "GNU", n->namesz))
> +            return 0;

Same here.

> +        binary_id = (char *)ELFNOTE_DESC(n);
> +
> +        /* convert to printable format */
> +        for (i = 0; i < n->descsz && (i + 1) * 2 < sizeof(xen_build_id_t); i++)
> +        {
> +            snprintf(&ascii_id[i * 2], 3, "%02hhx", binary_id[i]);
> +        }

No need for the braces, and no need for the"hh" modifier.

Jan

Re: [PATCH] xsplice: Use ld-embedded build-ids

[Martin Pohlack]

On 11.08.2015 16:12, Jan Beulich wrote:
>>>> On 05.08.15 at 16:09, <mpohlack@amazon.de> wrote:
>> Todo:
>>   * Should be moved to sysctl to only allow Dom0 access
> 
> Because of?

The discussion in this thread:

[Xen-devel] [RFC PATCH v3.1 2/2] xsplice: Add hook for build_id

was:
----------------------------------------------------------------------
>> Martin Pohlack:
>> We should not expose the build_id to normal guests, but only to Dom0.
>>
>> A build_id uniquely identifies a specific build and I don't see how that
>> information would be required from DomU.  It might actually help an
>> attacker to build his return-oriented programming exploit against a
>> specific build.
>>
>> The normal version numbers should be enough to know about capabilities
>> and API.
>
> Andrew Cooper:
> 
> It will need its own XSM hook, but need not be strictly limited to just
> dom0.
----------------------------------------------------------------------

>>   * Maybe convert to binary transport to userland instead of printable form
> 
> Indeed.
> 
>> @@ -360,11 +366,30 @@ DO(xen_version)(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
>>  
>>      case XENVER_build_id:
>>      {
>> -        xen_build_id_t build_id;
>> +        xen_build_id_t ascii_id;
>> +        Elf_Note * n = (Elf_Note *)&__note_gnu_build_id_start;
>> +        char * binary_id;
>> +        int i;
>> +
>> +        memset(ascii_id, 0, sizeof(ascii_id));
>> +
>> +        /* check if we really have a build-id */
>> +        if ( NT_GNU_BUILD_ID != n->type )
>> +            return 0;
> 
> This needs to signal an error.

Yes, ENOSYS, (or ENOENT, ENODATA)?

>> +
>> +        /* sanity check, name should be "GNU" for ld-generated build-id */
>> +        if ( 0 != strncmp(ELFNOTE_NAME(n), "GNU", n->namesz))
>> +            return 0;
> 
> Same here.
> 
>> +        binary_id = (char *)ELFNOTE_DESC(n);
>> +
>> +        /* convert to printable format */
>> +        for (i = 0; i < n->descsz && (i + 1) * 2 < sizeof(xen_build_id_t); i++)
>> +        {
>> +            snprintf(&ascii_id[i * 2], 3, "%02hhx", binary_id[i]);
>> +        }
> 
> No need for the braces, and no need for the"hh" modifier.
> 
> Jan
> 

Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger
Ust-ID: DE289237879
Eingetragen am Amtsgericht Charlottenburg HRB 149173 B

Re: [PATCH] xsplice: Use ld-embedded build-ids

[Jan Beulich]

>>> On 14.08.15 at 14:59, <mpohlack@amazon.com> wrote:
> On 11.08.2015 16:12, Jan Beulich wrote:
>>>>> On 05.08.15 at 16:09, <mpohlack@amazon.de> wrote:
>>> Todo:
>>>   * Should be moved to sysctl to only allow Dom0 access
>> 
>> Because of?
> 
> The discussion in this thread:
> 
> [Xen-devel] [RFC PATCH v3.1 2/2] xsplice: Add hook for build_id
> 
> was:
> ----------------------------------------------------------------------
>>> Martin Pohlack:
>>> We should not expose the build_id to normal guests, but only to Dom0.
>>>
>>> A build_id uniquely identifies a specific build and I don't see how that
>>> information would be required from DomU.  It might actually help an
>>> attacker to build his return-oriented programming exploit against a
>>> specific build.
>>>
>>> The normal version numbers should be enough to know about capabilities
>>> and API.
>>
>> Andrew Cooper:
>> 
>> It will need its own XSM hook, but need not be strictly limited to just
>> dom0.
> ----------------------------------------------------------------------

So I'm confused - I asked "why Dom0 only" and then you point me to
Andrew saying it doesn't need to be Dom0 only?

>>> @@ -360,11 +366,30 @@ DO(xen_version)(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
>>>  
>>>      case XENVER_build_id:
>>>      {
>>> -        xen_build_id_t build_id;
>>> +        xen_build_id_t ascii_id;
>>> +        Elf_Note * n = (Elf_Note *)&__note_gnu_build_id_start;
>>> +        char * binary_id;
>>> +        int i;
>>> +
>>> +        memset(ascii_id, 0, sizeof(ascii_id));
>>> +
>>> +        /* check if we really have a build-id */
>>> +        if ( NT_GNU_BUILD_ID != n->type )
>>> +            return 0;
>> 
>> This needs to signal an error.
> 
> Yes, ENOSYS, (or ENOENT, ENODATA)?

Definitely not ENOSYS. ENODATA or EOPNOTSUPP.

Jan

Re: [PATCH] xsplice: Use ld-embedded build-ids

[Martin Pohlack]

On 14.08.2015 15:54, Jan Beulich wrote:
>>>> On 14.08.15 at 14:59, <mpohlack@amazon.com> wrote:
>> On 11.08.2015 16:12, Jan Beulich wrote:
>>>>>> On 05.08.15 at 16:09, <mpohlack@amazon.de> wrote:
>>>> Todo:
>>>>   * Should be moved to sysctl to only allow Dom0 access
>>>
>>> Because of?
>>
>> The discussion in this thread:
>>
>> [Xen-devel] [RFC PATCH v3.1 2/2] xsplice: Add hook for build_id
>>
>> was:
>> ----------------------------------------------------------------------
>>>> Martin Pohlack:
>>>> We should not expose the build_id to normal guests, but only to Dom0.
>>>>
>>>> A build_id uniquely identifies a specific build and I don't see how that
>>>> information would be required from DomU.  It might actually help an
>>>> attacker to build his return-oriented programming exploit against a
>>>> specific build.
>>>>
>>>> The normal version numbers should be enough to know about capabilities
>>>> and API.
>>>
>>> Andrew Cooper:
>>>
>>> It will need its own XSM hook, but need not be strictly limited to just
>>> dom0.
>> ----------------------------------------------------------------------
> 
> So I'm confused - I asked "why Dom0 only" and then you point me to
> Andrew saying it doesn't need to be Dom0 only?

Sorry about that, my (not expressed) thinking was that we should
restrict that to Dom0 for the XSM-disabled case.

>>>> @@ -360,11 +366,30 @@ DO(xen_version)(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
>>>>  
>>>>      case XENVER_build_id:
>>>>      {
>>>> -        xen_build_id_t build_id;
>>>> +        xen_build_id_t ascii_id;
>>>> +        Elf_Note * n = (Elf_Note *)&__note_gnu_build_id_start;
>>>> +        char * binary_id;
>>>> +        int i;
>>>> +
>>>> +        memset(ascii_id, 0, sizeof(ascii_id));
>>>> +
>>>> +        /* check if we really have a build-id */
>>>> +        if ( NT_GNU_BUILD_ID != n->type )
>>>> +            return 0;
>>>
>>> This needs to signal an error.
>>
>> Yes, ENOSYS, (or ENOENT, ENODATA)?
> 
> Definitely not ENOSYS. ENODATA or EOPNOTSUPP.
> 
> Jan
> 

Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger
Ust-ID: DE289237879
Eingetragen am Amtsgericht Charlottenburg HRB 149173 B

Re: [PATCH] xsplice: Use ld-embedded build-ids

[Konrad Rzeszutek Wilk]

On Fri, Aug 14, 2015 at 9:57 AM, Martin Pohlack <mpohlack@amazon.com> wrote:
> On 14.08.2015 15:54, Jan Beulich wrote:
>>>>> On 14.08.15 at 14:59, <mpohlack@amazon.com> wrote:
>>> On 11.08.2015 16:12, Jan Beulich wrote:
>>>>>>> On 05.08.15 at 16:09, <mpohlack@amazon.de> wrote:
>>>>> Todo:
>>>>>   * Should be moved to sysctl to only allow Dom0 access
>>>>
>>>> Because of?
>>>
>>> The discussion in this thread:
>>>
>>> [Xen-devel] [RFC PATCH v3.1 2/2] xsplice: Add hook for build_id
>>>
>>> was:
>>> ----------------------------------------------------------------------
>>>>> Martin Pohlack:
>>>>> We should not expose the build_id to normal guests, but only to Dom0.
>>>>>
>>>>> A build_id uniquely identifies a specific build and I don't see how that
>>>>> information would be required from DomU.  It might actually help an
>>>>> attacker to build his return-oriented programming exploit against a
>>>>> specific build.
>>>>>
>>>>> The normal version numbers should be enough to know about capabilities
>>>>> and API.
>>>>
>>>> Andrew Cooper:
>>>>
>>>> It will need its own XSM hook, but need not be strictly limited to just
>>>> dom0.
>>> ----------------------------------------------------------------------
>>
>> So I'm confused - I asked "why Dom0 only" and then you point me to
>> Andrew saying it doesn't need to be Dom0 only?
>
> Sorry about that, my (not expressed) thinking was that we should
> restrict that to Dom0 for the XSM-disabled case.
>

That may make this more complex.

If we want to restrict it to this we may as well just stick this in sysctl
and have it be part of the xsplice ops.

Let me do that.

Re: [RFC PATCH v3.1 2/2] xsplice: Add hook for build_id

[Jan Beulich]

>>> On 27.07.15 at 21:20, <konrad@kernel.org> wrote:
> --- a/xen/include/xen/compile.h.in
> +++ b/xen/include/xen/compile.h.in
> @@ -10,4 +10,5 @@
>  #define XEN_EXTRAVERSION	"@@extraversion@@"
>  
>  #define XEN_CHANGESET		"@@changeset@@"
> +#define XEN_BUILD_ID        "@@changeset@@"

How can the changset be a valid / sufficient build ID (even if maybe
this is intended to only be a default / fallback)? Wasn't this meant
to specifically account for rebuilds (with, say, a compiler slightly
updated from the original one, and hence possibly producing
slightly different code)?

Jan

Hotpatch construction and __LINE__ (was: [RFC PATCH v3.1] xSplice design.)

[Martin Pohlack]

Hi,

Another high-level point to think about is how we want to handle inlined
__LINE__ references.  This problem is related to hotpatch construction
and potentially has influence on the design of the hotpatching
infrastructure in Xen.

Let me try to explain the problem:

We have file1.c with functions f1 and f2 (in that order).  f2 contains a
BUG() (or WARN()) macro and at that point embeds the source line number
into the generated code for f2.

Now we want to hotpatch f1 and the hotpatch source-code patch adds 2
lines to f1 and as a consequence shifts out f2 by two lines.  The newly
constructed file1.o will now contain differences in both binary
functions f1 (because we actually changed it with the applied patch) and
f2 (because the contained BUG macro embeds the new line number).

Without additional information, an algorithm comparing file1.o before
and after hotpatch application will determine both functions to be
changed and will have to include both into the binary hotpatch.

Options:

1. Transform source code patches for hotpatches to be line-neutral for
   each chunk.  This can be done in almost all cases with either
   reformatting of the source code or by introducing artificial
   preprocessor "#line n" directives to adjust for the introduced
   differences.

   This approach is low-tech and simple.  Potentially generated
   backtraces and existing debug information refers to the original
   build and does not reflect hotpatching state except for actually
   hotpatched functions but should be mostly correct.

2. Ignoring the problem and living with artificially large hotpatches
   that unnecessarily patch many functions.

   This approach might lead to some very large hotpatches depending on
   content of specific source file.  It may also trigger pulling in
   functions into the hotpatch that cannot reasonable be hotpatched due
   to limitations of a hotpatching framework (init-sections, parts of
   the hotpatching framework itself, ...) and may thereby prevent us
   from patching a specific problem.

   The decision between 1. and 2. can be made on a patch--by-patch
   basis.

3. Introducing an indirection table for storing line numbers and
   treating that specially for binary diffing.  I believe Linux follows
   this approach, but the details escape me ATM.

   We might either use this indirection table for runtime use and patch
   that with each hotpatch (similarly to exception tables) or we might
   purely use it when building hotpatches to ignore functions that only
   differ at exactly the location where a line-number is embedded.

Similar considerations are true to a lesser extent for __FILE__, but I
would argue that file renaming should be done outside of hotpatches.

Martin

On 27.07.2015 21:20, Konrad Rzeszutek Wilk wrote:
> Hey!
> 
> Since v3 [http://lists.xen.org/archives/html/xen-devel/2015-07/msg00990.html]
>  - Nailed down the comments, ingested them in.
>  - Wrote and tested some code.
> RFC v2 [http://lists.xen.org/archives/html/xen-devel/2015-05/msg02142.html]
>  - Ingested every review comment in.
> 
> 
> The patches for the code are a shell - there is no patching done at all and
> it is very much just to test out the design and hypercalls. The hard parts
> are yet to come :-)
> 
> At the Seattle LinuxCon/Xen Summit I will be presenting about xSplice and
> referring to this URL. There is also an slot for brainstorming to talk
> in details about things we disagree - and there is ample time to talk
> during dinner. Martin who has been heavily reviewing the design will be there
> and I hope other folks will be there as well to shape the design and
> how we want this to work.
> 
> The big outstanding issues are how we want to handle preemption. That
> is the problem of making an hypercall and waiting for the hypervisor
> to do its job (and the VCPU is blocked). In the past some XSAs have come
> out to resolve this and I would very much like this to have it addressed at start.
> 
> I think the other issues that have been raised should also be discussed
> naturally, but the above is crucial (at least for me). I've attached the
> patches on how I thought the preemption part could be solved by having an 'worker'
> in hypervisor acting on the commands - and we just poll on the status to see
> what the hypervisor has done so far.
> 
> Lastly, I also plan to add an Wiki to outline the dependency implementation
> parts that so far bubbled up - I figured Wiki would be better as some folks
> could put their name behind it.
> 
> Now please excuse the roughness of the patch and this giant one huge having
> everything in it. It ought to be split in three at least: hypervisor, toolstacks
> (libxc and libxl) - that is to be done later.
> 
>  docs/misc/xsplice.h           |   80 +++
>  docs/misc/xsplice.markdown    | 1230 +++++++++++++++++++++++++++++++++++++++++
>  docs/misc/xsplice_test.c      |   78 +++
>  tools/libxc/include/xenctrl.h |   16 +
>  tools/libxc/xc_misc.c         |  183 ++++++
>  tools/libxc/xc_private.c      |    3 +
>  tools/misc/Makefile           |    4 +
>  tools/misc/xen-xsplice.c      |  385 +++++++++++++
>  xen/common/Makefile           |    1 +
>  xen/common/kernel.c           |   11 +
>  xen/common/keyhandler.c       |    8 +-
>  xen/common/sysctl.c           |    5 +
>  xen/common/version.c          |    5 +
>  xen/common/xsplice.c          |  405 ++++++++++++++
>  xen/include/public/sysctl.h   |   66 +++
>  xen/include/public/version.h  |    4 +
>  xen/include/xen/compile.h.in  |    1 +
>  xen/include/xen/version.h     |    1 +
>  xen/include/xen/xsplice.h     |    9 +
>  19 files changed, 2494 insertions(+), 1 deletion(-)
> 
> Konrad Rzeszutek Wilk (2):
>       xsplice: rfc.v3.1
>       xsplice: Add hook for build_id
> 

Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger
Ust-ID: DE289237879
Eingetragen am Amtsgericht Charlottenburg HRB 149173 B

Re: Hotpatch construction and __LINE__

[Andrew Cooper]

On 05/08/15 09:55, Martin Pohlack wrote:
> Hi,
>
> Another high-level point to think about is how we want to handle inlined
> __LINE__ references.  This problem is related to hotpatch construction
> and potentially has influence on the design of the hotpatching
> infrastructure in Xen.
>
> Let me try to explain the problem:
>
> We have file1.c with functions f1 and f2 (in that order).  f2 contains a
> BUG() (or WARN()) macro and at that point embeds the source line number
> into the generated code for f2.
>
> Now we want to hotpatch f1 and the hotpatch source-code patch adds 2
> lines to f1 and as a consequence shifts out f2 by two lines.  The newly
> constructed file1.o will now contain differences in both binary
> functions f1 (because we actually changed it with the applied patch) and
> f2 (because the contained BUG macro embeds the new line number).
>
> Without additional information, an algorithm comparing file1.o before
> and after hotpatch application will determine both functions to be
> changed and will have to include both into the binary hotpatch.
>
> Options:
>
> 1. Transform source code patches for hotpatches to be line-neutral for
>    each chunk.  This can be done in almost all cases with either
>    reformatting of the source code or by introducing artificial
>    preprocessor "#line n" directives to adjust for the introduced
>    differences.
>
>    This approach is low-tech and simple.  Potentially generated
>    backtraces and existing debug information refers to the original
>    build and does not reflect hotpatching state except for actually
>    hotpatched functions but should be mostly correct.
>
> 2. Ignoring the problem and living with artificially large hotpatches
>    that unnecessarily patch many functions.
>
>    This approach might lead to some very large hotpatches depending on
>    content of specific source file.  It may also trigger pulling in
>    functions into the hotpatch that cannot reasonable be hotpatched due
>    to limitations of a hotpatching framework (init-sections, parts of
>    the hotpatching framework itself, ...) and may thereby prevent us
>    from patching a specific problem.
>
>    The decision between 1. and 2. can be made on a patch--by-patch
>    basis.
>
> 3. Introducing an indirection table for storing line numbers and
>    treating that specially for binary diffing.  I believe Linux follows
>    this approach, but the details escape me ATM.
>
>    We might either use this indirection table for runtime use and patch
>    that with each hotpatch (similarly to exception tables) or we might
>    purely use it when building hotpatches to ignore functions that only
>    differ at exactly the location where a line-number is embedded.
>
> Similar considerations are true to a lesser extent for __FILE__, but I
> would argue that file renaming should be done outside of hotpatches.

Looking at `strings xen-syms`, we have very few embedded
__FILE__/__LINE__'s in other strings, meaning that most come from %s/%d
formatting.

bug/warn/assert frames have their references in the exception table, so
should be reasonably self contained to fix up.

Regular printk()s are definitely more problematic however.  Their
references will typically be in %rsi/%rdx and shifting any #line
reference across the 256 boundary will change the encoding size of
setting the print up.

Jan had a plan to make Xen read its own DWARF symbol table rather than
using the current cludge we have where we partially link Xen, extract
the public symbol table, rewrite symbol-offsets.c and relink it onto the
end.

Perhaps there is an opportunity to piggy-back onto that and have all
file/line references coming from the DWARF information.  I suspect this
will be similar to what Linux does, as it already has support for using
its own DWARF/STABS information.

Personally, I think #2 should be avoided wherever possible.  The bigger
the hotpatch, the greater the opportunity for something to go wrong.

Any panic/stack trace should be able to identify exactly which Xen is
running, including some hotpatch status.  Hopatches will be[1] small
targeted fixes, so I don't think it is unreasonable to expect someone
interpreting a stack trace to need to adjust file/line references by the
hotpatch delta.

~Andrew

[1] woe betide anyone attempting to use hotpatches for general software
updates.

Re: Hotpatch construction and __LINE__

[Jan Beulich]

>>> On 05.08.15 at 15:25, <andrew.cooper3@citrix.com> wrote:
> Jan had a plan to make Xen read its own DWARF symbol table rather than
> using the current cludge we have where we partially link Xen, extract
> the public symbol table, rewrite symbol-offsets.c and relink it onto the
> end.

Either I mis-expressed this, or you mis-remember: I'm in no way
intending to use DWARF information. What I want to use is the
_full_ ELF / COFF symbol table instead of nm output (which has
quite a bit of interesting information discarded).

> Perhaps there is an opportunity to piggy-back onto that and have all
> file/line references coming from the DWARF information.  I suspect this
> will be similar to what Linux does, as it already has support for using
> its own DWARF/STABS information.

I'm curious where you found this as I'm unaware of such
functionality. Recently they even decided to discard all
.eh_frame / .dwarf_frame generating annotations the
compiler can't itself generate.

Jan

Re: Hotpatch construction and __LINE__

[Andrew Cooper]

On 12/08/15 09:09, Jan Beulich wrote:
>>>> On 05.08.15 at 15:25, <andrew.cooper3@citrix.com> wrote:
>> Jan had a plan to make Xen read its own DWARF symbol table rather than
>> using the current cludge we have where we partially link Xen, extract
>> the public symbol table, rewrite symbol-offsets.c and relink it onto the
>> end.
> Either I mis-expressed this, or you mis-remember: I'm in no way
> intending to use DWARF information. What I want to use is the
> _full_ ELF / COFF symbol table instead of nm output (which has
> quite a bit of interesting information discarded).

I misremembered.  Sorry.

>
>> Perhaps there is an opportunity to piggy-back onto that and have all
>> file/line references coming from the DWARF information.  I suspect this
>> will be similar to what Linux does, as it already has support for using
>> its own DWARF/STABS information.
> I'm curious where you found this as I'm unaware of such
> functionality.

The x86 linker script includes the information (given appropriate
CONFIG_ options), and the perf subsystem appears to have code to
interpret itself.

~Andrew

> Recently they even decided to discard all
> .eh_frame / .dwarf_frame generating annotations the
> compiler can't itself generate.
>
> Jan
>

Re: Hotpatch construction and __LINE__

[Ross Lagerwall]

On 08/05/2015 09:55 AM, Martin Pohlack wrote:
> Hi,
>
> Another high-level point to think about is how we want to handle inlined
> __LINE__ references.  This problem is related to hotpatch construction
> and potentially has influence on the design of the hotpatching
> infrastructure in Xen.
>
> Let me try to explain the problem:
>
> We have file1.c with functions f1 and f2 (in that order).  f2 contains a
> BUG() (or WARN()) macro and at that point embeds the source line number
> into the generated code for f2.
>
> Now we want to hotpatch f1 and the hotpatch source-code patch adds 2
> lines to f1 and as a consequence shifts out f2 by two lines.  The newly
> constructed file1.o will now contain differences in both binary
> functions f1 (because we actually changed it with the applied patch) and
> f2 (because the contained BUG macro embeds the new line number).
>
> Without additional information, an algorithm comparing file1.o before
> and after hotpatch application will determine both functions to be
> changed and will have to include both into the binary hotpatch.
>
> Options:
>
> 1. Transform source code patches for hotpatches to be line-neutral for
>     each chunk.  This can be done in almost all cases with either
>     reformatting of the source code or by introducing artificial
>     preprocessor "#line n" directives to adjust for the introduced
>     differences.
>
>     This approach is low-tech and simple.  Potentially generated
>     backtraces and existing debug information refers to the original
>     build and does not reflect hotpatching state except for actually
>     hotpatched functions but should be mostly correct.
>
> 2. Ignoring the problem and living with artificially large hotpatches
>     that unnecessarily patch many functions.
>
>     This approach might lead to some very large hotpatches depending on
>     content of specific source file.  It may also trigger pulling in
>     functions into the hotpatch that cannot reasonable be hotpatched due
>     to limitations of a hotpatching framework (init-sections, parts of
>     the hotpatching framework itself, ...) and may thereby prevent us
>     from patching a specific problem.
>
>     The decision between 1. and 2. can be made on a patch--by-patch
>     basis.
>
> 3. Introducing an indirection table for storing line numbers and
>     treating that specially for binary diffing.  I believe Linux follows
>     this approach, but the details escape me ATM.
>
>     We might either use this indirection table for runtime use and patch
>     that with each hotpatch (similarly to exception tables) or we might
>     purely use it when building hotpatches to ignore functions that only
>     differ at exactly the location where a line-number is embedded.
>

For BUG, ASSERT, and WARN, the line number is stored in the bug frame 
table and as currently implemented, the build tool ignores changes to 
the bug frame table if there's no corresponding functional change. Which 
mostly fixes this issue.

-- 
Ross Lagerwall