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* [RFC PATCH 01/11] bpf: context casting for tail call
@ 2019-05-20 23:50 Kris Van Hees
  0 siblings, 0 replies; 2+ messages in thread
From: Kris Van Hees @ 2019-05-20 23:50 UTC (permalink / raw)
  To: netdev, bpf, dtrace-devel, linux-kernel
  Cc: rostedt, mhiramat, acme, ast, daniel

Currently BPF programs are executed with a context that is provided by
code that initiates the execution.  Tracing tools that want to make use
of existing probes and events that allow BPF programs to be attached to
them are thus limited to the context information provided by the probe
or event source.  Often, more context is needed to allow tracing tools
the ablity to implement more complex constructs (e.g. more state-full
tracing).

This patch extends the tail-call mechanism to allow a BPF program of
one type to call a BPF program of another type.

BPF program types can specify two new operations in struct bpf_prog_ops:
- bool is_valid_tail_call(enum bpf_prog_type stype)
    This function is called from bpf_prog_array_valid_tail_call()
            which is called from bpf_check_tail_call()
            which is called from bpf_prog_select_runtime()
            which is called from bpf_prog_load() right after the
    verifier finishes processing the program.  It is called for every
    map of type BPF_MAP_TYPE_PROG_ARRAY, and is passed the type of the
    program that is being loaded and therefore will be the origin of
    tail calls.  It returns true if tail calls from the source BPF
    program type to the implementing program type are allowed.

- void *convert_ctx(enum bpf_prog_type stype, void *ctx)
    This function is called during the execution of a BPF tail-call.
    It returns a valid context for the implementing BPF program type,
    based on the passed context pointer (ctx) for BPF program type
    stype.

The program array holding BPF programs that you can tail-call into
continues to require that all programs are of the same type.  But when
a compatibility check is made in a program that performs a tail-call,
the is_valid_tail_call() function is called (if available) to allow
the target type to determine whether it can handle the conversion of
a context from the source type to the target type.  If the function is
not implemented by the program type, casting is denied.

During execution, the convert_ctx() function is called (if available)
to perform the conversion of the current context to the context that the
target type expects.  Since the program type of the executing BPF program
is not explicitly known during execution, the verifier inserts an
instruction right before the tail-call to assign the current BPF program
type to R4.

The interpreter calls convert_ctx() using the program type in R4 as
source program type, the program type associated with the program array
as target program type, and the context as provided in R1.

A helper (finalize_context) is added to allow tail called programs to
perform context setup based on information that is passed in from the
calling program by means of a map that is indexed by CPU id.  The actual
content of the map is defined by the BPF program type implementation
for the program type that is being called.

The bpf_prog_types array is now being exposed to the rest of the BPF
code (where before it was local to just the syscall handling) because
the is_valid_tail_call() and convert_ctx() operations need to be
accessible.

There is no noticeable effect on BPF program types that do not implement
this new feature.

A JIT implementation is not available yet in this first iteration.

v2: Fixed compilation when CONFIG_BPF_SYSCALL=n.
    Fixed casting issue on platforms with 32-bit pointers.

v3: Renamed the new program type operations to be more descriptive.
    Added finalize_context() helper.

Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
Reviewed-by: Nick Alcock <nick.alcock@oracle.com>
---
 include/linux/bpf.h                       |  3 +++
 include/uapi/linux/bpf.h                  | 11 ++++++++-
 kernel/bpf/core.c                         | 29 ++++++++++++++++++++++-
 kernel/bpf/syscall.c                      |  2 +-
 kernel/bpf/verifier.c                     | 16 +++++++++----
 tools/include/uapi/linux/bpf.h            | 11 ++++++++-
 tools/testing/selftests/bpf/bpf_helpers.h |  2 ++
 7 files changed, 66 insertions(+), 8 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 59631dd0777c..7a40a3cd7ff2 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -294,6 +294,8 @@ bpf_ctx_record_field_size(struct bpf_insn_access_aux *aux, u32 size)
 struct bpf_prog_ops {
 	int (*test_run)(struct bpf_prog *prog, const union bpf_attr *kattr,
 			union bpf_attr __user *uattr);
+	bool (*is_valid_tail_call)(enum bpf_prog_type stype);
+	void *(*convert_ctx)(enum bpf_prog_type stype, void *ctx);
 };
 
 struct bpf_verifier_ops {
@@ -571,6 +573,7 @@ extern const struct file_operations bpf_prog_fops;
 #undef BPF_PROG_TYPE
 #undef BPF_MAP_TYPE
 
+extern const struct bpf_prog_ops * const bpf_prog_types[];
 extern const struct bpf_prog_ops bpf_offload_prog_ops;
 extern const struct bpf_verifier_ops tc_cls_act_analyzer_ops;
 extern const struct bpf_verifier_ops xdp_analyzer_ops;
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 63e0cf66f01a..61abe6b56948 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2672,6 +2672,14 @@ union bpf_attr {
  *		0 on success.
  *
  *		**-ENOENT** if the bpf-local-storage cannot be found.
+ *
+ * int bpf_finalize_context(void *ctx, struct bpf_map *map)
+ *	Description
+ *		Perform any final context setup after a tail call took
+ *		place from another BPF program type into a program of
+ *		the implementing program type.
+ *	Return
+ *		0 on success, or a negative error in case of failure.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -2782,7 +2790,8 @@ union bpf_attr {
 	FN(strtol),			\
 	FN(strtoul),			\
 	FN(sk_storage_get),		\
-	FN(sk_storage_delete),
+	FN(sk_storage_delete),		\
+	FN(finalize_context),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 242a643af82f..225b1be766b0 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1456,10 +1456,12 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
 		CONT;
 
 	JMP_TAIL_CALL: {
+		void *ctx = (void *) (unsigned long) BPF_R1;
 		struct bpf_map *map = (struct bpf_map *) (unsigned long) BPF_R2;
 		struct bpf_array *array = container_of(map, struct bpf_array, map);
 		struct bpf_prog *prog;
 		u32 index = BPF_R3;
+		u32 type = BPF_R4;
 
 		if (unlikely(index >= array->map.max_entries))
 			goto out;
@@ -1471,6 +1473,13 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
 		prog = READ_ONCE(array->ptrs[index]);
 		if (!prog)
 			goto out;
+		if (prog->aux->ops->convert_ctx) {
+			ctx = prog->aux->ops->convert_ctx(type, ctx);
+			if (!ctx)
+				goto out;
+
+			BPF_R1 = (u64) (uintptr_t) ctx;
+		}
 
 		/* ARG1 at this point is guaranteed to point to CTX from
 		 * the verifier side due to the fact that the tail call is
@@ -1667,6 +1676,23 @@ bool bpf_prog_array_compatible(struct bpf_array *array,
 	       array->owner_jited == fp->jited;
 }
 
+bool bpf_prog_array_valid_tail_call(struct bpf_array *array,
+				    const struct bpf_prog *fp)
+{
+#ifdef CONFIG_BPF_SYSCALL
+	const struct bpf_prog_ops *ops;
+
+	if (array->owner_jited != fp->jited)
+		return false;
+
+	ops = bpf_prog_types[array->owner_prog_type];
+	if (ops->is_valid_tail_call)
+		return ops->is_valid_tail_call(fp->type);
+#endif
+
+	return false;
+}
+
 static int bpf_check_tail_call(const struct bpf_prog *fp)
 {
 	struct bpf_prog_aux *aux = fp->aux;
@@ -1680,7 +1706,8 @@ static int bpf_check_tail_call(const struct bpf_prog *fp)
 			continue;
 
 		array = container_of(map, struct bpf_array, map);
-		if (!bpf_prog_array_compatible(array, fp))
+		if (!bpf_prog_array_compatible(array, fp) &&
+		    !bpf_prog_array_valid_tail_call(array, fp))
 			return -EINVAL;
 	}
 
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index ad3ccf82f31d..f76fd30ad372 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -1179,7 +1179,7 @@ static int map_freeze(const union bpf_attr *attr)
 	return err;
 }
 
-static const struct bpf_prog_ops * const bpf_prog_types[] = {
+const struct bpf_prog_ops * const bpf_prog_types[] = {
 #define BPF_PROG_TYPE(_id, _name) \
 	[_id] = & _name ## _prog_ops,
 #define BPF_MAP_TYPE(_id, _ops)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 95f9354495ad..f9e5536fd1af 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7982,9 +7982,10 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
 			insn->imm = 0;
 			insn->code = BPF_JMP | BPF_TAIL_CALL;
 
+			cnt = 0;
 			aux = &env->insn_aux_data[i + delta];
 			if (!bpf_map_ptr_unpriv(aux))
-				continue;
+				goto privileged;
 
 			/* instead of changing every JIT dealing with tail_call
 			 * emit two extra insns:
@@ -7999,13 +8000,20 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
 
 			map_ptr = BPF_MAP_PTR(aux->map_state);
 			insn_buf[0] = BPF_JMP_IMM(BPF_JGE, BPF_REG_3,
-						  map_ptr->max_entries, 2);
+						  map_ptr->max_entries, 3);
 			insn_buf[1] = BPF_ALU32_IMM(BPF_AND, BPF_REG_3,
 						    container_of(map_ptr,
 								 struct bpf_array,
 								 map)->index_mask);
-			insn_buf[2] = *insn;
-			cnt = 3;
+			cnt = 2;
+
+privileged:
+			/* store the BPF program type of the current program in
+			 * R4 so it is known in case this tail call requires
+			 * casting the context to a different program type
+			 */
+			insn_buf[cnt++] = BPF_MOV64_IMM(BPF_REG_4, prog->type);
+			insn_buf[cnt++] = *insn;
 			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
 			if (!new_prog)
 				return -ENOMEM;
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 63e0cf66f01a..61abe6b56948 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -2672,6 +2672,14 @@ union bpf_attr {
  *		0 on success.
  *
  *		**-ENOENT** if the bpf-local-storage cannot be found.
+ *
+ * int bpf_finalize_context(void *ctx, struct bpf_map *map)
+ *	Description
+ *		Perform any final context setup after a tail call took
+ *		place from another BPF program type into a program of
+ *		the implementing program type.
+ *	Return
+ *		0 on success, or a negative error in case of failure.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -2782,7 +2790,8 @@ union bpf_attr {
 	FN(strtol),			\
 	FN(strtoul),			\
 	FN(sk_storage_get),		\
-	FN(sk_storage_delete),
+	FN(sk_storage_delete),		\
+	FN(finalize_context),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call
diff --git a/tools/testing/selftests/bpf/bpf_helpers.h b/tools/testing/selftests/bpf/bpf_helpers.h
index 6e80b66d7fb1..d98a62b3b56c 100644
--- a/tools/testing/selftests/bpf/bpf_helpers.h
+++ b/tools/testing/selftests/bpf/bpf_helpers.h
@@ -216,6 +216,8 @@ static void *(*bpf_sk_storage_get)(void *map, struct bpf_sock *sk,
 	(void *) BPF_FUNC_sk_storage_get;
 static int (*bpf_sk_storage_delete)(void *map, struct bpf_sock *sk) =
 	(void *)BPF_FUNC_sk_storage_delete;
+static int (*bpf_finalize_context)(void *ctx, void *map) =
+	(void *) BPF_FUNC_finalize_context;
 
 /* llvm builtin functions that eBPF C program may use to
  * emit BPF_LD_ABS and BPF_LD_IND instructions
-- 
2.20.1


^ permalink raw reply	[flat|nested] 2+ messages in thread
* [RFC PATCH 00/11] bpf, trace, dtrace: DTrace BPF program type implementation and sample use
@ 2019-05-20 23:47 Kris Van Hees
  2019-05-21 20:39 ` [RFC PATCH 01/11] bpf: context casting for tail call Kris Van Hees
  0 siblings, 1 reply; 2+ messages in thread
From: Kris Van Hees @ 2019-05-20 23:47 UTC (permalink / raw)
  To: netdev, bpf, dtrace-devel, linux-kernel
  Cc: rostedt, mhiramat, acme, ast, daniel

This patch set is also available, applied to bpf-next, at the following URL:

	https://github.com/oracle/dtrace-linux-kernel/tree/dtrace-bpf

The patches in this set are part of an larger effort to re-implement DTrace
based on existing Linux kernel features wherever possible.  This allows
existing DTrace scripts to run without modification on Linux and to write
new scripts using a tracing tool that people may already be familiar with.
This set of patches is posted as an RFC.  I am soliciting feedback on the
patches, especially because they cross boundaries between tracing and BPF.
Some of the features might be combined with existing more specialized forms
of similar functionality, and perhaps some functionality could be moved to
other parts of the code.

This set of patches provides the initial core to make it possible to execute
DTrace BPF programs as probe actions, triggered from existing probes in the
kernel (right now just kprobe, but more will be added in followup patches).
The DTrace BPF programs run in a specific DTrace context that is independent
from the probe-specific BPF program context because DTrace actions are
implemented based on a general probe concept (an abstraction of the various
specific probe types).

It also provides a mechanism to store probe data in output buffers directly
from BPF programs, using direct store instructions.  Finally, it provides a
simple sample userspace tool to load programs, collect data, and print out the
data.  This little tool is currently hardcoded to process a single test case,
to show how the BPF program is to be constructed and to show how to retrieve
data from the output buffers.

The work presented here would not be possible without the effort many people
have put into tracing features on Linux.  Especially BPF is instrumental in
being able to do this project because it provides a safe and fast virtual
execution engine that can be leveraged to execute probe actions in a more
elegant manner.  The perf_event ring-buffer output mechanism has also proven
to be very beneficial to starting a re-implementation of DTrace on Linux,
especially because it avoids needing to add yet another buffer implementation
to the kernel.  It really helped with being able to re-use functionality.

The patch set provides the following patches:

    1. bpf: context casting for tail call

	This patch adds the ability to tail-call into a BPF program of a
	different type than the one initiating the call.  It provides two
	program type specific operations: is_valid_tail_call (to validate
	whether the tail-call between the source type and target type is
	allowed) and convert_ctx (to create a context for the target type
	based on the context of the source type).  It also provides a
	bpf_finalize_context() helper function prototype.  BPF program types
	should implement this helper to perform any final context setup that
	may need to be done within the execution context of the program type.
	This helper is typically invoked as the first statement in an eBPF
	program that can be tail-called from another type.

    2. bpf: add BPF_PROG_TYPE_DTRACE

	This patch adds BPF_PROG_TYPE_DTRACE as a new BPF program type, without
	actually providing an implementation.  The actual implementation is
	added in patch 4 (see below).  We do it this way because the
	implementation is being added to the tracing subsystem as a component
	that I would be happy to maintain (if merged) whereas the declaration
	of the program type must be in the bpf subsystem.  Since the two
	subsystems are maintained by different people, we split the
	implementing patches across maintainer boundaries while ensuring that
	the kernel remains buildable between patches.

    3. bpf: export proto for bpf_perf_event_output helper

	This patch make a prototype available for the bpf_perf_event_output
	helper so that program types outside of the base tracing eBPF code can
	make use of it.

    4. trace: initial implementation of DTrace based on kernel facilities

	This patch provides the most basic implementation of the DTrace
	execution core based on eBPF and other kernel facilities.  This
	version only supports kprobes.  It makes use of the cross-program-type
	tail-call support adding with patch 1 (see above).

    5. trace: update Kconfig and Makefile to include DTrace

	This patch adds DTrace to the kernel config system and it ensures that
	if CONFIG_DTRACE is set, the implementation of the DTrace core is
	compiled into the kernel.

    6. dtrace: tiny userspace tool to exercise DTrace support features

	This patch provides a tiny userspace DTrace consumer as a
	proof-of-concept and to test the DTrace eBPF program type and its use
	by the DTrace core.

    7. bpf: implement writable buffers in contexts

	This patch adds the ability to specify writable buffers in an eBPF
	program type context.  The public context declaration should provide
	<buf> and <buf>_end members (<buf> can be any valid identifier) for
	each buffer.  The is_valid_access() function for the program type
	should force the register type of read access to <buf> as
	PTR_TO_BUFFER whereas reading <buf>_end should yield register type
	PTR_TO_BUFFER_END.  The functionality is nearly identical to
	PTR_TO_PACKET and PTR_TO_PACKET_END.  Contexts can have multiple
	writable buffers, distinguished from one another by a new buf_id
	member in the bpf_reg_state struct.  For every writable buffer, both
	<buf> and <buf>_end must provide the same buf_id value (using
	offset(context, <buf>) is a good and convenient choice). 

    8. perf: add perf_output_begin_forward_in_page

	This patch introduces a new function to commence the process of
	writing data to a perf_event ring-buffer.  This variant enforces the
	requirement that the data to be written cannot cross a page boundary.
	It will fill the remainder of the current page with zeros and allocate
	space for the data in the next page if the remainder of the current
	page is too small.  This is necessary to allow eBPF program to write
	to the buffer space directly with statements like: buf[offset] = value.

    9. bpf: mark helpers explicitly whether they may change the context

	This patch changes the way BPF determines whether a helper may change
	the content of the context (i.e. if it does, any range information
	related to pointers in the context must be invalidated).  The original
	implementation contained a hard-coded list of helpers that change the
	context.  The new implementation adds a new field to the helper proto
	struct (ctx_update, default false).

    10. bpf: add bpf_buffer_reserve and bpf_buffer_commit helpers

	This patch adds two new helpers: bpf_buffer_reserve (to set up a
	specific buffer in the context as writable space of a given size) and
	bpf_buffer_commit (to finalize the data written to the buffer prepared
	with bpf_buffer_reserve).

    11. dtrace: make use of writable buffers in BPF

	This patch updates the initial implementation of the DTrace core and
	the proof-of-concept utility to make use of the writable-buffer support
	and the bpf_buffer_reserve and bpf_buffer_commit helpers.

(More detailed descriptions can be found in the individual commit messages.)

The road ahead is roughly as follows:

    - Adding support for DTrace specific probe meta data to be available to
      DTrace BPF programs
    - Adding support for other probe types
    - Adding support for probe arguments
    - Adding support for the DTrace probe naming mechanism to map DTrace
      style probe names to the actual defined probes in the kernel
    - Adding support for DTrace features that currently do not exist in
      the kernel as existing functionality
    - Rework the existing dtrace utility to make use of the new implementation
    - Keep adding features to the DTrace system

	Cheers,
	Kris

^ permalink raw reply	[flat|nested] 2+ messages in thread

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