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Date:   Fri, 10 Sep 2021 19:19:50 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Nathan Chancellor <nathan@kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Joe Perches <joe@perches.com>, Arnd Bergmann <arnd@kernel.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>, llvm@lists.linux.dev,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 00/10] raise minimum GCC version to 5.1
Message-ID: <202109101917.5BA95B87E0@keescook>
References: <20210910234047.1019925-1-ndesaulniers@google.com>
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On Fri, Sep 10, 2021 at 04:40:37PM -0700, Nick Desaulniers wrote:
> commit fad7cd3310db ("nbd: add the check to prevent overflow in
> __nbd_ioctl()")
> 
> raised an issue from the fallback helpers added in
> 
> commit f0907827a8a9 ("compiler.h: enable builtin overflow checkers and add fallback code")
> 
> Specifically, the helpers for checking whether the results of a
> multiplication overflowed (__unsigned_mul_overflow,
> __signed_add_overflow) use the division operator when
> !COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW. This is problematic for 64b
> operands on 32b hosts.
> 
> Also, because the macro is type agnostic, it is very difficult to write
> a similarly type generic macro that dispatches to one of:
> * div64_s64
> * div64_u64
> * div_s64
> * div_u64

Given that it's all compile-time type-aware goo, this isn't so bad. The
gist[1] you linked off the bug report is pretty close. Needs some
bikeshedding. ;)

> Raising the minimum supported versions allows us to remove all of the
> fallback helpers for !COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW, instead
> dispatching the compiler builtins.
> 
> arm64 has already raised the minimum supported GCC version to 5.1, do
> this for all targets now. See the link below for the previous
> discussion.

That said, I'd much prefer raising the minimum GCC -- no one appears
to actually be building on 4.9 -- there are close to 200 errors (ne?
warnings) on x86_64 allmodconfig there currently.

-Kees

[1] https://gist.github.com/nickdesaulniers/2479818f4983bbf2d688cebbab435863

-- 
Kees Cook