linux-hardening - Re: [PATCH 4/5] Makefile: Enable -Warray-bounds

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Date:   Wed, 2 Feb 2022 19:02:11 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Masahiro Yamada <masahiroy@...nel.org>,
        linux-kbuild@...r.kernel.org,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        clang-built-linux@...glegroups.com, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 4/5] Makefile: Enable -Warray-bounds

On Wed, Feb 02, 2022 at 03:21:49PM -0800, Guenter Roeck wrote:
> On 2/2/22 14:11, Kees Cook wrote:
> > On Wed, Feb 02, 2022 at 08:09:03AM -0800, Guenter Roeck wrote:
> > > On Wed, Aug 18, 2021 at 01:11:17AM -0700, Kees Cook wrote:
> > > > With the recent fixes for flexible arrays and expanded FORTIFY_SOURCE
> > > > coverage, it is now possible to enable -Warray-bounds. Since both
> > > > GCC and Clang include -Warray-bounds in -Wall, we just need to stop
> > > > disabling it.
> > > > 
> > > > Cc: Arnd Bergmann <arnd@...db.de>
> > > > Cc: Masahiro Yamada <masahiroy@...nel.org>
> > > > Cc: linux-kbuild@...r.kernel.org
> > > > Co-developed-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> > > > Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> > > > Signed-off-by: Kees Cook <keescook@...omium.org>
> > > > ---
> > > >   Makefile | 1 -
> > > >   1 file changed, 1 deletion(-)
> > > > 
> > > > diff --git a/Makefile b/Makefile
> > > > index a4aca01a4835..af22b83cede7 100644
> > > > --- a/Makefile
> > > > +++ b/Makefile
> > > > @@ -1072,7 +1072,6 @@ KBUILD_CFLAGS += $(call cc-disable-warning, stringop-truncation)
> > > >   # We'll want to enable this eventually, but it's not going away for 5.7 at least
> > > >   KBUILD_CFLAGS += $(call cc-disable-warning, zero-length-bounds)
> > > > -KBUILD_CFLAGS += -Wno-array-bounds
> > > >   KBUILD_CFLAGS += $(call cc-disable-warning, stringop-overflow)
> > > >   # Another good warning that we'll want to enable eventually
> > > 
> > > alpha:defconfig:
> > > 
> > > In function '__memset',
> > >      inlined from '__bad_pagetable' at arch/alpha/mm/init.c:79:2:
> > > ./arch/alpha/include/asm/string.h:37:32: error: '__builtin_memset' offset [0, 8191] is out of the bounds [0, 0] [-Werror=array-bounds]
> > >     37 |                         return __builtin_memset(s, c, n);
> > >        |                                ^~~~~~~~~~~~~~~~~~~~~~~~~
> > > In function '__memset',
> > >      inlined from '__bad_page' at arch/alpha/mm/init.c:86:2:
> > > ./arch/alpha/include/asm/string.h:37:32: error: '__builtin_memset' offset [0, 8191] is out of the bounds [0, 0] [-Werror=array-bounds]
> > >     37 |                         return __builtin_memset(s, c, n);
> > >        |                                ^~~~~~~~~~~~~~~~~~~~~~~~~
> > > In function '__memset',
> > >      inlined from 'paging_init' at arch/alpha/mm/init.c:256:2:
> > > ./arch/alpha/include/asm/string.h:37:32: error: '__builtin_memset' offset [0, 8191] is out of the bounds [0, 0] [-Werror=array-bounds]
> > >     37 |                         return __builtin_memset(s, c, n);
> > 
> > Ah! With Arnd and Nathan's help, I saw:
> > https://lore.kernel.org/all/20210912160149.2227137-3-linux@roeck-us.net/
> 
> Guilty as charged. Sorry, I didn't try to analyze the problem,
> or I might have noticed (and saved you some work).

No worries! It manifests as a completely different warning. It just
happens to come from the same internal diagnostics info, I think. :)

I was just delighted to see it already had a solution!

-Kees

-- 
Kees Cook