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objtool/x86: Fix NOP decode 

For x86_64 the kernel consistently uses 2 instructions for all NOPs:

  90       - NOP
  0f 1f /0 - NOPL

Notably:

 - REP NOP is PAUSE, not a NOP instruction.

 - 0f {0c...0f} is reserved space,
   except for 0f 0d /1, which is PREFETCHW, not a NOP.

 - 0f {19,1c...1f} is reserved space,
   except for 0f 1f /0, which is NOPL.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
This commit is contained in:
Peter Zijlstra 2025-09-24 15:27:03 +02:00
parent 76e1851a1b
commit 044f721ccd
1 changed files with 11 additions and 4 deletions
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15
tools/objtool/arch/x86/decode.c
View File

@ -494,6 +494,12 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
break;
case 0x90:
if (rex_b) /* XCHG %r8, %rax */
break;
if (prefix == 0xf3) /* REP NOP := PAUSE */
break;
insn->type = INSN_NOP;
break;
@ -547,13 +553,14 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
} else if (op2 == 0x0b || op2 == 0xb9) {
/* ud2 */
/* ud2, ud1 */
insn->type = INSN_BUG;
} else if (op2 == 0x0d || op2 == 0x1f) {
} else if (op2 == 0x1f) {
/* nopl/nopw */
insn->type = INSN_NOP;
/* 0f 1f /0 := NOPL */
if (modrm_reg == 0)
insn->type = INSN_NOP;
} else if (op2 == 0x1e) {