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

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Date:   Wed, 7 Sep 2022 11:40:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
        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" <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...e.de>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH] objtool,x86: Teach decode about LOOP* instructions

On Wed, Sep 07, 2022 at 09:06:12AM +0000, David Laight wrote:
> From: Peter Zijlstra
> > Sent: 07 September 2022 10:01
> > 
> > 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.
> 
> Do they ever appear in the kernel?

No; that is, not on any of the random vmlinux.o images I checked this
morning.

Still, best to properly decode them anyway.