clang and drm issue: objtool warnings from clang build

[Linus Torvalds]

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.

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