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

[Dmitry Vyukov]

On Mon, Dec 7, 2020 at 1:52 PM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> 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

Yes, it "fixes" the warning.
Initially I thought it's not a good solution because we want to detect
UBSAN bugs in KCOV. But on second thought, if UBSAN detects a bug in
KCOV, it may lead to infinite recursion. We already disable all other
sanitizers on KCOV for this reason, so it's reasonable to disable
UBSAN as well. And as a side effect it "resolves" the warning as well.
I mailed:
https://lore.kernel.org/lkml/20201209100152.2492072-1-dvyukov@xxxxxxxxxx/T/#u

Thanks

> > 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.