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

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Message-ID: <81693206-9002-4669-ab74-fda3d31c25bb@oracle.com>
Date: Mon, 17 Nov 2025 10:47:06 +0100
From: Alexandre Chartre <alexandre.chartre@...cle.com>
To: David Laight <david.laight.linux@...il.com>
Cc: alexandre.chartre@...cle.com, 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 11/17/25 10:42, David Laight wrote:
> 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 am now compacting the code by removing all trailing NOPs. I should probably
improve that with printing the actual number of NOPs, for example NOP31 (or nop31)

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

This is done by libopcodes. I will need to check if there is an option to display
the branch distance instead of the branch target.

Thanks,

alex.