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

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Message-ID: <20251117220953.4f7bccc7@pumpkin>
Date: Mon, 17 Nov 2025 22:09:53 +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 14:11:55 +0100
Alexandre Chartre <alexandre.chartre@...cle.com> wrote:

> 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.

Could you use NOP1*41 (etc) so that NOP5*4 is different from NOP1*20?
(I'm guessing you hand-decode the standard NOP sequences already?)

Hmm... you don't want to execute that many 0x90 bytes.
I think that case might have had a jump around them.

Do I remember something about the trailing nop being merged?
Perhaps that is the kernel patching code.
Something made me think objtool might (also) be doing it.

	David