linux-kernel - [PATCH] objtool,x86: Teach decode about LOOP* instructions

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Message-ID: <Yxhd4EMKyoFoH9y4@hirez.programming.kicks-ass.net>
Date:   Wed, 7 Sep 2022 11:01:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        bpf <bpf@...r.kernel.org>, linux-kernel@...r.kernel.org,
        Borislav Petkov <bp@...e.de>,
        Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org
Subject: [PATCH] objtool,x86: Teach decode about LOOP* instructions

On Wed, Sep 07, 2022 at 09:06:45AM +0200, Peter Zijlstra wrote:
> On Wed, Sep 07, 2022 at 09:55:21AM +0900, Masami Hiramatsu (Google) wrote:
> 
> > +/* Return the jump target address or 0 */
> > +static inline unsigned long insn_get_branch_addr(struct insn *insn)
> > +{
> > +	switch (insn->opcode.bytes[0]) {
> > +	case 0xe0:	/* loopne */
> > +	case 0xe1:	/* loope */
> > +	case 0xe2:	/* loop */
> 
> Oh cute, objtool doesn't know about those, let me go add them.

---
Subject: objtool,x86: Teach decode about LOOP* instructions

With kprobes also needing to follow control flow; it was found that
objtool is missing the branches from the LOOP* instructions.

Reported-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.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 c260006106be..1c253b4b7ce0 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -635,6 +635,12 @@ int arch_decode_instruction(struct objtool_file *file, 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;
 		/*