linux-kernel - Re: [PATCH] objtool: Fix SYSCALL instruction handling and INSN_CONTEXT

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Message-ID: <rkxzcgnwmgvzycz2pndq7s6mwo6frocdpxwyangsamb7cjf2k2@ec67snehrq5f>
Date: Thu, 3 Apr 2025 12:20:54 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, 
	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH] objtool: Fix SYSCALL instruction handling and
 INSN_CONTEXT_SWITCH

On Thu, Apr 03, 2025 at 08:15:45PM +0100, Andrew Cooper wrote:
> On 03/04/2025 8:05 pm, Josh Poimboeuf wrote:
> > On Thu, Apr 03, 2025 at 07:57:42PM +0100, Andrew Cooper wrote:
> >> On 03/04/2025 7:48 pm, Josh Poimboeuf wrote:
> >>> diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
> >>> index 33d861c04ebd..628c2c8a0f6a 100644
> >>> --- a/tools/objtool/arch/x86/decode.c
> >>> +++ b/tools/objtool/arch/x86/decode.c
> >>> @@ -535,10 +535,9 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
> >>>  
> >>>  			insn->type = INSN_JUMP_CONDITIONAL;
> >>>  
> >>> -		} else if (op2 == 0x05 || op2 == 0x07 || op2 == 0x34 ||
> >>> -			   op2 == 0x35) {
> >>> +		} else if (op2 == 0x07) {
> >>>  
> >>> -			/* sysenter, sysret */
> >>> +			/* sysret */
> >>>  			insn->type = INSN_CONTEXT_SWITCH;
> >> Linux doesn't use SYSEXIT, but it's conceptually like SYSRET/ERETx so
> >> perhaps worth keeping the 0x35 here?
> > In theory yes, but objtool will never support x86-32.  Note I also
> > removed retf and jmpf, I'm thinking it's simpler to just stick to the
> > instructions we actually use.
> >
> 
> Perhaps, but they'll now become INSN_OTHER, won't they?
> 
> If they're instructions genuinely expected never to encounter, wouldn't
> it be better to make a hard error rather than to add another fallthrough
> case?

Fair enough.  Maybe I'll just leave them as INSN_CONTEXT_SWITCH, that'd
be easier than adding new assertions.

-- 
Josh