Re: linux-next: build warning after merge of the akpm tree

[Marco Elver]

On Mon, 7 Dec 2020 at 13:38, 'Dmitry Vyukov' via kasan-dev
<kasan-dev@xxxxxxxxxxxxxxxx> wrote:
> On Mon, Dec 7, 2020 at 1:08 PM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> > > > Hi all,
> > > >
> > > > After merging the akpm tree, today's linux-next build (powerpc
> > > > allyesconfig) produced warnings like this:
> > > >
> > > > kernel/kcov.c:296:14: warning: conflicting types for built-in function '__sanitizer_cov_trace_switch'; expected 'void(long unsigned int,  void *)' [-Wbuiltin-declaration-mismatch]
> > > >   296 | void notrace __sanitizer_cov_trace_switch(u64 val, u64 *cases)
> > > >       |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > >
> > > Odd.  clang wants that signature, according to
> > > https://clang.llvm.org/docs/SanitizerCoverage.html.  But gcc seems to
> > > want a different signature.  Beats me - best I can do is to cc various
> > > likely culprits ;)
> > >
> > > Which gcc version?  Did you recently update gcc?
> > >
> > > > ld: warning: orphan section `.data..Lubsan_data177' from `arch/powerpc/oprofile/op_model_pa6t.o' being placed in section `.data..Lubsan_data177'
> > > >
> > > > (lots of these latter ones)
> > > >
> > > > I don't know what produced these, but it is in the akpm-current or
> > > > akpm trees.
> >
> > I can reproduce this in x86_64 build as well but only if I enable
> > UBSAN as well. There were some recent UBSAN changes by Kees, so maybe
> > that's what affected the warning.
> > Though, the warning itself looks legit and unrelated to UBSAN. In
> > fact, if the compiler expects long and we accept u64, it may be broken
> > on 32-bit arches...
>
> No, I think it works, the argument should be uint64.
>
> I think both gcc and clang signatures are correct and both want
> uint64_t. The question is just how uint64_t is defined :) The old
> printf joke that one can't write portable format specifier for
> uint64_t.
>
> What I know so far:
> clang 11 does not produce this warning even with obviously wrong
> signatures (e.g. short).
> I wasn't able to trigger it with gcc on 32-bits at all. KCOV is not
> supported on i386 and on arm I got no warnings even with obviously
> wrong signatures (e.g. short).
> Using "(unsigned long val, void *cases)" fixes the warning on x86_64.
>
> I am still puzzled why gcc considers this as a builtin because we
> don't enable -fsanitizer-coverage on this file. I am also puzzled how
> UBSAN affects things.

It might be some check-for-builtins check gone wrong if it enables any
one of the sanitizers. That would be confirmed if it works with

UBSAN_SANITIZE_kcov.o := n

> We could change the signature to long, but it feels wrong/dangerous
> because the variable should really be 64-bits (long is broken on
> 32-bits).
> Or we could introduce a typedef that is long on 64-bits and 'long
> long' on 32-bits.