linux-kernel - Re: [PATCH v2 28/62] objtool: Fix weak symbol hole detection for .cold functions

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Message-ID: <dqetdbtdhkipt5ighko7omvupe34zo76jky6digezrlra2madv@5hrau7wxbcvm>
Date: Wed, 4 Jun 2025 16:56:54 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, 
	Petr Mladek <pmladek@...e.com>, Miroslav Benes <mbenes@...e.cz>, 
	Joe Lawrence <joe.lawrence@...hat.com>, live-patching@...r.kernel.org, Song Liu <song@...nel.org>, 
	laokz <laokz@...mail.com>, Jiri Kosina <jikos@...nel.org>, 
	Marcos Paulo de Souza <mpdesouza@...e.com>, Weinan Liu <wnliu@...gle.com>, 
	Fazla Mehrab <a.mehrab@...edance.com>, Chen Zhongjin <chenzhongjin@...wei.com>, 
	Puranjay Mohan <puranjay@...nel.org>
Subject: Re: [PATCH v2 28/62] objtool: Fix weak symbol hole detection for
 .cold functions

On Mon, May 26, 2025 at 12:38:22PM +0200, Peter Zijlstra wrote:
> On Fri, May 09, 2025 at 01:16:52PM -0700, Josh Poimboeuf wrote:
> > When ignore_unreachable_insn() looks for weak function holes which jump
> > to their .cold functions, it assumes the parent function comes before
> > the corresponding .cold function in the symbol table.  That's not
> > necessarily the case with -ffunction-sections.
> > 
> > Mark all the holes beforehand (including .cold functions) so the
> > ordering of the discovery doesn't matter.
> 
> One of the things I have a 'todo' entry on, is rewriting all sections
> that reference any one of these instructions.
> 
> That is, things like fentry, alternatives, retpoline, static_call,
> jump_label.  Everything that can cause runtime code patching.
> 
> Once we are sure none of those sections will contain references to this
> dead code, we can go and wipe the actual code. Perhaps fill it with a
> UD1 instruction with some identifying immediate.

Yeah, that would be nice.

-- 
Josh