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Date:   Mon, 13 Sep 2021 09:20:26 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Pavel Machek <pavel@ucw.cz>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        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: <202109130918.D359A43@keescook>
References: <20210910234047.1019925-1-ndesaulniers@google.com>
 <20210913094947.GB12225@amd>
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On Mon, Sep 13, 2021 at 11:49:48AM +0200, Pavel Machek wrote:
> Hi!
> 
> > 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.
> 
> Please don't. gcc 4.9.2 is still in use here.
> 
> Plus, someone will want to prevent overflow in -stable, too, and you
> can't really raise gcc version there.

These changes won't go to prior stable kernels.

-- 
Kees Cook