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

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Message-ID: <20250426232344.GA1395819@ax162>
Date: Sat, 26 Apr 2025 19:23:44 -0400
From: Nathan Chancellor <nathan@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: 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>,
	Bill Wendling <morbo@...gle.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 Sat, Apr 26, 2025 at 01:56:59PM -0700, Linus Torvalds wrote:
> IOW, there's a *big* difference between "the programmer told me this
> is unreachable, so I won't generate code past this point" and "I have
> decided this is undefined behavior, so now I won't generate code past
> this point".
> 
> So what I'm asking for is absolutely not "trap on unreachable". That's
> wrong and just plain stupid.
> 
> I'm asking for "trap on UD instead of *assuming* it's unreachable".
> 
> Because clearly that code *can* be reached, it's just doing something undefined.
> 
> See? Big big difference.

Ah yes, that is a big yet subtle difference that I had not considered,
my bad for missing that. I was only thinking about the implicitly
inserted __builtin_unreachable() from potential UB, not the explicitly
added ones from the developers.

I suspect that it would not be easy to split that distinction in LLVM
but since I am not a compiler person, I will see if this has come up
before and talk to people otherwise. I know there has been work in LLVM
to try and stop undefined behavior from destroying control flow with
things like the freeze instruction but I am not sure that would help us
in this situation. Pardon my ignorance though, isn't something like this
basically just '-fsanitize=undefined -fsanitize-trap=all'?

Cheers,
Nathan