linux-kernel - Re: [PATCH 00/10] raise minimum GCC version to 5.1

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Date:   Mon, 13 Sep 2021 09:20:26 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Pavel Machek <pavel@....cz>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Nathan Chancellor <nathan@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Joe Perches <joe@...ches.com>, Arnd Bergmann <arnd@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/10] raise minimum GCC version to 5.1

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