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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAHk-=wgPCbZv0JgqoNWMOO+p=N772YW16xYk_pmb1GU7aeuPFA@mail.gmail.com>
Date: Sat, 26 Apr 2025 13:56:59 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Nathan Chancellor <nathan@...nel.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, 26 Apr 2025 at 13:05, Nathan Chancellor <nathan@...nel.org> wrote:
>
>     KBUILD_CFLAGS += -mllvm -trap-unreachable

Hmm. That certainly builds for me, but yeah, it generates new objtool
warnings, notably

   panic() missing __noreturn in .c/.h or NORETURN() in noreturns.h

and I *think* that is because that flag makes clang not honour our
*explicit* "this code is unreachable" annotations.

So now objtool complains about the fact that clang has generated some
random code that follows a call to 'panic()' even though objtool knows
that panic() cannot return.

And those explicit annotations definitely should be honored.

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.

             Linus