Re: [PATCH v2 1/7] Compiler Attributes: Add __alloc_size() for better bounds checking

From: Kees Cook <keescook@chromium.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, Miguel Ojeda <ojeda@kernel.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	clang-built-linux@googlegroups.com, Joe Perches <joe@perches.com>,
	Andy Whitcroft <apw@canonical.com>,
	Dwaipayan Ray <dwaipayanray1@gmail.com>,
	Lukas Bulwahn <lukas.bulwahn@gmail.com>,
	Christoph Lameter <cl@linux.com>,
	Pekka Enberg <penberg@kernel.org>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Daniel Micay <danielmicay@gmail.com>,
	Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Michal Marek <michal.lkml@markovi.net>,
	linux-mm@kvack.org, linux-kbuild@vger.kernel.org,
	linux-hardening@vger.kernel.org
Subject: Re: [PATCH v2 1/7] Compiler Attributes: Add __alloc_size() for better bounds checking
Date: Thu, 19 Aug 2021 00:06:13 -0700	[thread overview]
Message-ID: <202108190003.37FF2F1D7@keescook> (raw)
In-Reply-To: <20210818161912.f14722707e06de1f046e948d@linux-foundation.org>

On Wed, Aug 18, 2021 at 04:19:12PM -0700, Andrew Morton wrote:
> On Wed, 18 Aug 2021 14:40:15 -0700 Kees Cook <keescook@chromium.org> wrote:
> 
> > GCC and Clang can use the "alloc_size" attribute to better inform the
> > results of __builtin_object_size() (for compile-time constant values).
> > Clang can additionally use alloc_size to inform the results of
> > __builtin_dynamic_object_size() (for run-time values).
> > 
> > Because GCC sees the frequent use of struct_size() as an allocator size
> > argument, and notices it can return SIZE_MAX (the overflow indication),
> > it complains about these call sites may overflow (since SIZE_MAX is
> > greater than the default -Walloc-size-larger-than=PTRDIFF_MAX). This
> > isn't helpful since we already know a SIZE_MAX will be caught at run-time
> > (this was an intentional design). Instead, just disable this check as
> > it is both a false positive and redundant. (Clang does not have this
> > warning option.)
> > 
> > ...
> >
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -1078,9 +1078,13 @@ KBUILD_CFLAGS += $(call cc-disable-warning, stringop-overflow)
> >  # Another good warning that we'll want to enable eventually
> >  KBUILD_CFLAGS += $(call cc-disable-warning, restrict)
> >  
> > -# Enabled with W=2, disabled by default as noisy
> >  ifdef CONFIG_CC_IS_GCC
> > +# Enabled with W=2, disabled by default as noisy
> >  KBUILD_CFLAGS += -Wno-maybe-uninitialized
> > +
> > +# The allocators already balk at large sizes, so silence the compiler
> > +# warnings for bounds checks involving those possible values.
> > +KBUILD_CFLAGS += -Wno-alloc-size-larger-than
> >  endif
> >  
> >  # disable invalid "can't wrap" optimizations for signed / pointers
> 
> Makefile has changed.  I did this:
> 
> --- a/Makefile~compiler-attributes-add-__alloc_size-for-better-bounds-checking
> +++ a/Makefile
> @@ -1003,6 +1003,12 @@ KBUILD_CFLAGS += $(call cc-disable-warni
>  # Enabled with W=2, disabled by default as noisy
>  KBUILD_CFLAGS += $(call cc-disable-warning, maybe-uninitialized)
>  
> +ifdef CONFIG_CC_IS_GCC
> +# The allocators already balk at large sizes, so silence the compiler
> +# warnings for bounds checks involving those possible values.
> +KBUILD_CFLAGS += -Wno-alloc-size-larger-than
> +endif
> +
>  # disable invalid "can't wrap" optimizations for signed / pointers
>  KBUILD_CFLAGS	+= -fno-strict-overflow

Oh, er, where did "Makefile: remove stale cc-option checks" go? Ah, I
see now:
https://lore.kernel.org/mm-commits/20210814215814.W_qqW%25akpm@linux-foundation.org/T/#u

Looks like I just happened to pick the wrong linux-next. ;)

Thanks for the fix-up!

-- 
Kees Cook