linux-kernel - Re: linux-next: build failure while building Linus' tree

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Date:   Fri, 10 Sep 2021 15:17:13 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     axboe@...nel.dk, josef@...icpanda.com, libaokun1@...wei.com,
        linux-kernel@...r.kernel.org, linux-next@...r.kernel.org,
        llvm@...ts.linux.dev, Arnd Bergmann <arnd@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>, sfr@...b.auug.org.au
Subject: Re: linux-next: build failure while building Linus' tree

On Fri, Sep 10, 2021 at 3:02 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Thu, Sep 9, 2021 at 3:50 PM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> >
> > + Rasmus
> >
> > This was introduced in
> > commit f0907827a8a91 ("compiler.h: enable builtin overflow checkers and add
> > fallback code")
> > which added division using the `/` operator, which is problematic when checking
> > for overflows of 64b operands on 32b targets.
> >
> > We'll probably need helpers from linux/math64.h and some combination of
> > __builtin_choose_expr/__builtin_types_compatible_p.
> >
> > That will help us fix another compiler bug for older clang releases, too.
> > https://github.com/ClangBuiltLinux/linux/issues/1438.
>
> Ok, I have something hacked up that I think will work:
> https://gist.github.com/nickdesaulniers/2479818f4983bbf2d688cebbab435863

hmm...playing around with adding some static asserts to the above, I
don't think it's quite right, specifically:
#define div_64(dividend, divisor) ({            \
  typeof(dividend) z;

might declare z with the expected sign, but not necessarily the
correct width when the dividend is 32b but the divisor is 64b.  Feels
a bit like trying to encode the C type promotion rules in a macro...

> This incomplete diff is a little hacked up to reproduce the issue with
> a known-bad revision of clang that demonstrates a similar issue to GCC
> 4.9.  You can ignore the movement of check_mul_overflow and friends in
> include/linux/overflow.h.
>
> I think I'm going to break that up into 2 or 3 patches:
> 1. move is_signed_type from include/linux/overflow.h to perhaps
> include/linux/typecheck.h.
> 2. add div64_x64, div_x64, and div_64 to include/linux/math64.h, use
> them in include/linux/overflow.h to fix GCC 4.9
> 3. move multiply fallbacks out of
> COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW for clang < 14.
> --
> Thanks,
> ~Nick Desaulniers



-- 
Thanks,
~Nick Desaulniers