[PATCH v2.1 6.5/70] x86/kexec: Annotate embedded data with ELF metadata

From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Xen-devel <xen-devel@lists.xenproject.org>
Cc: "Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Jan Beulich" <JBeulich@suse.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>, "Wei Liu" <wl@xen.org>
Subject: [PATCH v2.1 6.5/70] x86/kexec: Annotate embedded data with ELF metadata
Date: Thu, 17 Feb 2022 10:01:58 +0000	[thread overview]
Message-ID: <20220217100158.22402-1-andrew.cooper3@citrix.com> (raw)
In-Reply-To: <20220214125127.17985-1-andrew.cooper3@citrix.com>

Scanning for embedded endbranch instructions involves parsing the .text
disassembly.  Data in the kexec trampoline has no ELF metadata, so objdump
treats it as instructions and tries to disassemble.  Convert:

  ffff82d040396108 <compatibility_mode_far>:
  ffff82d040396108:       00 00                   add    %al,(%rax)
  ffff82d04039610a:       00 00                   add    %al,(%rax)
  ffff82d04039610c:       10 00                   adc    %al,(%rax)

  ffff82d04039610e <compat_mode_gdt_desc>:
  ffff82d04039610e:       17                      (bad)
          ...

  ffff82d040396118 <compat_mode_gdt>:
          ...
  ffff82d040396120:       ff                      (bad)
  ffff82d040396121:       ff 00                   incl   (%rax)
  ffff82d040396123:       00 00                   add    %al,(%rax)
  ffff82d040396125:       93                      xchg   %eax,%ebx
  ffff82d040396126:       cf                      iret
  ffff82d040396127:       00 ff                   add    %bh,%bh
  ffff82d040396129:       ff 00                   incl   (%rax)
  ffff82d04039612b:       00 00                   add    %al,(%rax)
  ffff82d04039612d:       9b                      fwait
  ffff82d04039612e:       cf                      iret
          ...

  ffff82d040396130 <compat_mode_idt>:
          ...

  ffff82d0403961b6 <kexec_reloc_size>:
  ffff82d0403961b6:       b6 01                   mov    $0x1,%dh
          ...

to:

  ffff82d040396108 <compatibility_mode_far>:
  ffff82d040396108:       00 00 00 00 10 00                               ......

  ffff82d04039610e <compat_mode_gdt_desc>:
  ffff82d04039610e:       17 00 00 00 00 00 00 00 00 00                   ..........

  ffff82d040396118 <compat_mode_gdt>:
          ...
  ffff82d040396120:       ff ff 00 00 00 93 cf 00 ff ff 00 00 00 9b cf 00 ................

  ffff82d040396130 <compat_mode_idt>:
  ffff82d040396130:       00 00 00 00 00 00                               ......

  ffff82d040396136 <reloc_stack>:
          ...

Most data just gains type and size metadata.  The reloc_stack label is the
wrong end of the data block to have a size, so move it to the lowest address
and introduce .Lreloc_stack_base as a replacement.

While kexec_reloc_size could gain metadata, it's use in the linker
assertion (while correct) is deeply confusing to follow.  Drop it entirely,
using a linker symbol instead to denote the end of the trampoline.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Wei Liu <wl@xen.org>

The remainder of the 32bit code has mode-invariant lengths, so disassembles
safely as 64bit.  The only differences come from 32/64bit implicit register
sizes.

v2.1:
 * New
---
 xen/arch/x86/include/asm/machine_kexec.h |  2 +-
 xen/arch/x86/machine_kexec.c             |  2 +-
 xen/arch/x86/x86_64/kexec_reloc.S        | 22 +++++++++++++++++-----
 xen/arch/x86/xen.lds.S                   |  3 ++-
 4 files changed, 21 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/include/asm/machine_kexec.h b/xen/arch/x86/include/asm/machine_kexec.h
index ba0d469d077b..d4880818c1d9 100644
--- a/xen/arch/x86/include/asm/machine_kexec.h
+++ b/xen/arch/x86/include/asm/machine_kexec.h
@@ -9,7 +9,7 @@ extern void kexec_reloc(unsigned long reloc_code, unsigned long reloc_pt,
                         unsigned long ind_maddr, unsigned long entry_maddr,
                         unsigned long flags);
 
-extern unsigned int kexec_reloc_size;
+extern const char kexec_reloc_end[];
 
 #endif
 
diff --git a/xen/arch/x86/machine_kexec.c b/xen/arch/x86/machine_kexec.c
index 08ec9fd43b1d..751a9efcaf6a 100644
--- a/xen/arch/x86/machine_kexec.c
+++ b/xen/arch/x86/machine_kexec.c
@@ -117,7 +117,7 @@ int machine_kexec_load(struct kexec_image *image)
     }
 
     code_page = __map_domain_page(image->control_code_page);
-    memcpy(code_page, kexec_reloc, kexec_reloc_size);
+    memcpy(code_page, kexec_reloc, kexec_reloc_end - (char *)kexec_reloc);
     unmap_domain_page(code_page);
 
     /*
diff --git a/xen/arch/x86/x86_64/kexec_reloc.S b/xen/arch/x86/x86_64/kexec_reloc.S
index d488d127cfb9..05bf8810cee6 100644
--- a/xen/arch/x86/x86_64/kexec_reloc.S
+++ b/xen/arch/x86/x86_64/kexec_reloc.S
@@ -34,7 +34,7 @@ ENTRY(kexec_reloc)
         movq    %rcx, %rbp
 
         /* Setup stack. */
-        leaq    (reloc_stack - kexec_reloc)(%rdi), %rsp
+        leaq    (.Lreloc_stack_base - kexec_reloc)(%rdi), %rsp
 
         /* Load reloc page table. */
         movq    %rsi, %cr3
@@ -175,10 +175,16 @@ compatibility_mode_far:
         .long 0x00000000             /* set in call_32_bit above */
         .word 0x0010
 
+        .type compatibility_mode_far, @object
+        .size compatibility_mode_far, . - compatibility_mode_far
+
 compat_mode_gdt_desc:
         .word .Lcompat_mode_gdt_end - compat_mode_gdt -1
         .quad 0x0000000000000000     /* set in call_32_bit above */
 
+        .type compat_mode_gdt_desc, @object
+        .size compat_mode_gdt_desc, . - compat_mode_gdt_desc
+
         .align 8
 compat_mode_gdt:
         .quad 0x0000000000000000     /* null                              */
@@ -186,16 +192,22 @@ compat_mode_gdt:
         .quad 0x00cf9b000000ffff     /* 0x0010 ring 0 code, compatibility */
 .Lcompat_mode_gdt_end:
 
+        .type compat_mode_gdt, @object
+        .size compat_mode_gdt, . - compat_mode_gdt
+
 compat_mode_idt:
         .word 0                      /* limit */
         .long 0                      /* base */
 
+        .type compat_mode_idt, @object
+        .size compat_mode_idt, . - compat_mode_idt
+
         /*
          * 16 words of stack are more than enough.
          */
-        .fill 16,8,0
 reloc_stack:
+        .fill 16,8,0
+.Lreloc_stack_base:
 
-        .globl kexec_reloc_size
-kexec_reloc_size:
-        .long . - kexec_reloc
+        .type reloc_stack, @object
+        .size reloc_stack, . - reloc_stack
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index c399178ac123..13fc7ee008c1 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -87,6 +87,7 @@ SECTIONS
        *(.text.unlikely)
        *(.fixup)
        *(.text.kexec)
+       kexec_reloc_end = .;
        *(.gnu.warning)
        _etext = .;             /* End of text section */
   } PHDR(text) = 0x9090
@@ -433,7 +434,7 @@ ASSERT(__2M_rwdata_end <= XEN_VIRT_END - XEN_VIRT_START + __XEN_VIRT_START -
        "Xen image overlaps stubs area")
 
 #ifdef CONFIG_KEXEC
-ASSERT(kexec_reloc_size - kexec_reloc <= PAGE_SIZE, "kexec_reloc is too large")
+ASSERT(kexec_reloc_end - kexec_reloc <= PAGE_SIZE, "kexec_reloc is too large")
 #endif
 
 /* The Multiboot setup paths relies on this to simplify superpage PTE creation. */
-- 
2.11.0


