linux-kernel - [tip: objtool/core] objtool/x86: Fix NOP decode

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Message-ID: <176044247632.709179.2414903530940509412.tip-bot2@tip-bot2>
Date: Tue, 14 Oct 2025 11:47:56 -0000
From: "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: objtool/core] objtool/x86: Fix NOP decode

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@...radead.org>
AuthorDate:    Wed, 24 Sep 2025 15:27:03 +02:00
Committer:     Peter Zijlstra <peterz@...radead.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@...radead.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) {