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

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Message-ID: <CAGG=3QWKQ++Yhh0w8zJobs=C4M7F01doAw9t2h=f2W9wxSG3Mw@mail.gmail.com>
Date: Mon, 28 Apr 2025 10:53:34 -0700
From: Bill Wendling <morbo@...gle.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: 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 Sat, Apr 26, 2025 at 1:57 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> 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.
>
There's also this flag:

  -mllvm -no-trap-after-noreturn

Here's the documentation for both flags:

static cl::opt<bool>
    EnableTrapUnreachable("trap-unreachable", cl::Hidden,
                          cl::desc("Enable generating trap for unreachable"));

static cl::opt<bool> EnableNoTrapAfterNoreturn(
    "no-trap-after-noreturn", cl::Hidden,
    cl::desc("Do not emit a trap instruction for 'unreachable' IR instructions "
             "after noreturn calls, even if --trap-unreachable is set."));


Could you share how you configured your build?

-bw

> 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