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

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Message-ID: <20250526103822.GL24938@noisy.programming.kicks-ass.net>
Date: Mon, 26 May 2025 12:38:22 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.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 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.