linux-kernel - Re: [PATCH v2 42/62] kbuild,x86: Fix module permissions for __jump_table and __bug

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Message-ID: <20250526110634.GO24938@noisy.programming.kicks-ass.net>
Date: Mon, 26 May 2025 13:06:34 +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>,
	Masahiro Yamada <masahiroy@...nel.org>
Subject: Re: [PATCH v2 42/62] kbuild,x86: Fix module permissions for
 __jump_table and __bug_table

On Fri, May 09, 2025 at 01:17:06PM -0700, Josh Poimboeuf wrote:
> An upcoming patch will add the SHF_MERGE flag to x86 __jump_table and
> __bug_table so their entry sizes can be defined in inline asm.
> 
> However, those sections have SHF_WRITE, which the Clang linker (lld)
> explicitly forbids combining with SHF_MERGE.
> 
> Those sections are modified at runtime and must remain writable.  While
> SHF_WRITE is ignored by vmlinux, it's still needed for modules.
> 
> To work around the linker interference, remove SHF_WRITE during
> compilation and restore it after linking the module.

*groan*

This and the following patches marking a whole bunch of sections M,
seems to suggest you're going to rely on sh_entsize actually working.

There was an ld.lld bug, and IIRC you need to enforce llvm-20 or later
if you want this to be so.