clang and drm issue: objtool warnings from clang build

[Bill Wendling]

On Sat, Apr 26, 2025 at 1:57 PM Linus Torvalds
<torvalds at linux-foundation.org> wrote:
>
> On Sat, 26 Apr 2025 at 13:05, Nathan Chancellor <nathan at kernel.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