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

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Date:   Fri, 10 Sep 2021 15:02:14 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     sfr@...b.auug.org.au
Cc:     axboe@...nel.dk, josef@...icpanda.com, libaokun1@...wei.com,
        linux-kernel@...r.kernel.org, linux-next@...r.kernel.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        llvm@...ts.linux.dev, Arnd Bergmann <arnd@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>
Subject: Re: linux-next: build failure while building Linus' tree

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
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