linux-kernel - Re: [PATCH v4 00/28] objtool: Function validation tracing

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Message-ID: <20251117094210.3c3e4f40@pumpkin>
Date: Mon, 17 Nov 2025 09:42:10 +0000
From: David Laight <david.laight.linux@...il.com>
To: Alexandre Chartre <alexandre.chartre@...cle.com>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>, linux-kernel@...r.kernel.org,
 mingo@...nel.org, peterz@...radead.org
Subject: Re: [PATCH v4 00/28] objtool: Function validation tracing

On Mon, 17 Nov 2025 08:50:45 +0100
Alexandre Chartre <alexandre.chartre@...cle.com> wrote:

> On 11/14/25 22:34, Josh Poimboeuf wrote:
...
> David raises the issue that a side-by-side display requires a large window.
> 
> The compact display could be like this:
> 
> Alternative with single instruction:
> 
>    bb8:  do_one_initcall+0x1a8    <alternative.bb8>
>                                   = callq  *0x0(%rip)        # 0xbbe <pv_ops+0xf8>    (if default)
>                                   = sti                                               (if !X86_FEATURE_XENPV)
>                                   = callq  BUG_func                                   (if +X86_FEATURE_ALWAYS)
> 
> Alternative with multiple instructions:
> 
>    82e7:  __switch_to_asm+0x27    <alternative.82e7>
>                                   = DEFAULT
>    82e7:  __switch_to_asm+0x27    | jmp    0x8312 <__switch_to_asm+0x52>
>                                   |
>                                   = !X86_FEATURE_ALWAYS
>    82e7:  __switch_to_asm+0x27    | NOP1
>    82e8:  __switch_to_asm+0x28    | NOP1
>    82e9:  __switch_to_asm+0x29    | callq  0x82ef <__switch_to_asm+0x2f>
>    82ee:  __switch_to_asm+0x2e    | int3
>    82ef:  __switch_to_asm+0x2f    | add    $0x8,%rsp
>    82f3:  __switch_to_asm+0x33    | lfence
>                                   |
>                                   = X86_FEATURE_RSB_CTXSW
>    82e7:  __switch_to_asm+0x27    | mov    $0x10,%r12
>    82ee:  __switch_to_asm+0x2e    | callq  0x82f4 <__switch_to_asm+0x34>
>    82f3:  __switch_to_asm+0x33    | int3
>    82f4:  __switch_to_asm+0x34    | callq  0x82fa <__switch_to_asm+0x3a>
>    82f9:  __switch_to_asm+0x39    | int3
>    82fa:  __switch_to_asm+0x3a    | add    $0x10,%rsp
>    82fe:  __switch_to_asm+0x3e    | dec    %r12
>    8301:  __switch_to_asm+0x41    | jne    0x82ee <__switch_to_asm+0x2e>
>    8303:  __switch_to_asm+0x43    | lfence
>    8306:  __switch_to_asm+0x46    | movq   $0xffffffffffffffff,%gs:0x0(%rip)        # 0x20b <__x86_call_depth>

That does looks better.
Although I think there ought to be some indication of the 31 NOP bytes
at the end of the middle alternative.

I'd also decode those callq as 'callq .+6' - not sure what other people think?
It is rather specific to that code.

	David