linux-kernel - [PATCH AUTOSEL 5.15 07/21] objtool,x86: Teach decode about LOOP* instructions

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Date:   Mon, 17 Oct 2022 20:09:26 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Sasha Levin <sashal@...nel.org>, jpoimboe@...nel.org,
        bp@...e.de, fenghua.yu@...el.com, tony.luck@...el.com
Subject: [PATCH AUTOSEL 5.15 07/21] objtool,x86: Teach decode about LOOP* instructions

From: Peter Zijlstra <peterz@...radead.org>

[ Upstream commit 7a7621dfa417aa3715d2a3bd1bdd6cf5018274d0 ]

When 'discussing' control flow Masami mentioned the LOOP* instructions
and I realized objtool doesn't decode them properly.

As it turns out, these instructions are somewhat inefficient and as
such unlikely to be emitted by the compiler (a few vmlinux.o checks
can't find a single one) so this isn't critical, but still, best to
decode them properly.

Reported-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/Yxhd4EMKyoFoH9y4@hirez.programming.kicks-ass.net
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 tools/objtool/arch/x86/decode.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index f62db0e006e9..16554496af9d 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -574,6 +574,12 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
 		*type = INSN_CONTEXT_SWITCH;
 		break;
 
+	case 0xe0: /* loopne */
+	case 0xe1: /* loope */
+	case 0xe2: /* loop */
+		*type = INSN_JUMP_CONDITIONAL;
+		break;
+
 	case 0xe8:
 		*type = INSN_CALL;
 		/*
-- 
2.35.1