[tip: objtool/core] objtool/x86: Fix NOP decode

tip-bot2 for Peter Zijlstra
2025-10-14T11:47:58Z

The following commit has been merged into the objtool/core branch of tip:

Commit-ID: 044f721ccd33103349eebbb960825584bc6d8e23
Gitweb: https://git.kernel.org/tip/044f721ccd33103349eebbb960825584bc6d8e23
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Wed, 24 Sep 2025 15:27:03 +02:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 14 Oct 2025 13:43:11 +02:00

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>
---
tools/objtool/arch/x86/decode.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index ef6e96d..204e2ad 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -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) {