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

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Message-ID: <3673c975-dc51-47ea-940b-8f09afa213a5@oracle.com>
Date: Mon, 17 Nov 2025 14:11:55 +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 13:37, David Laight wrote:
> On Mon, 17 Nov 2025 10:47:06 +0100
> Alexandre Chartre <alexandre.chartre@...cle.com> wrote:
> 
>> On 11/17/25 10:42, David Laight wrote:
> ...
>>> 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)
> 
> That is the sort of thing I was thinking of.
> Perhaps the actual opcodes on one line - eg: NOP5; NOP5; NOP5; NOP1

That might not always be very compact. For example __switch_to_asm() has 41 NOP1.
I will use NOP<n> for now, and we can improve later.

  
>>> 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.
> 
> The 'problem' is that mostly you want the branch target - except when it is small.
> Then you don't need both 'address' and 'symbol+offset', and it is quicker to find
> the target by looking at the branch distance.
> I'm not sure how you'd please everyone :-)
> 
> I'm sure one of the disassemblers ends up giving you the target address in a form
> that isn't on the instruction line!
> I've definitely counted opcode bytes to find the target.
> 

I will investigate this for a later patch. Maybe have both the distance and the
destination (could be an option?).

Thanks,

alex.