linux-kernel - Re: clang and drm issue: objtool warnings from clang build

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Message-ID: <20250429091742.GF4198@noisy.programming.kicks-ass.net>
Date: Tue, 29 Apr 2025 11:17:42 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Bill Wendling <morbo@...gle.com>, Nathan Chancellor <nathan@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Harry Wentland <harry.wentland@....com>,
	Leo Li <sunpeng.li@....com>,
	Alex Deucher <alexander.deucher@....com>,
	Christian König <christian.koenig@....com>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Justin Stitt <justinstitt@...gle.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	llvm@...ts.linux.dev
Subject: Re: clang and drm issue: objtool warnings from clang build

On Mon, Apr 28, 2025 at 12:34:27PM -0700, Linus Torvalds wrote:

> And yes, this really is a very potential and real security issue. In
> the kernel I don't think we have this ever happening, partly because a
> lot of configurations use gcc which afaik doesn't have this particular
> horrendous model of UD.

I see more and more people use clang, in a large part because of Rust.

Anyway, I've seen clang pull this stop-codegen-on-UB trick before (link
was upstream in the thread) and yes, it is horrific crap. At the time I
proposed emitting at the very least a UD2 instruction rather than just
straight up stopping code gen, but I think your proposal for a code-gen
knob to just not do this 'optimisation' at all is much better.