Re: [patch 9/9] mm/vmalloc: add __alloc_size attributes for better bounds checking

From: Joe Perches <joe@perches.com>
To: Kees Cook <keescook@chromium.org>, linux-kernel@vger.kernel.org
Cc: Andrew Morton <akpm@linux-foundation.org>,
	apw@canonical.com, Christoph Lameter <cl@linux.com>,
	Daniel Micay <danielmicay@gmail.com>,
	Dennis Zhou <dennis@kernel.org>,
	dwaipayanray1@gmail.com, Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Linux-MM <linux-mm@kvack.org>,
	Lukas Bulwahn <lukas.bulwahn@gmail.com>,
	mm-commits@vger.kernel.org, Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Miguel Ojeda <ojeda@kernel.org>,
	Pekka Enberg <penberg@kernel.org>,
	David Rientjes <rientjes@google.com>, Tejun Heo <tj@kernel.org>,
	Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [patch 9/9] mm/vmalloc: add __alloc_size attributes for better bounds checking
Date: Tue, 21 Sep 2021 16:45:44 -0700	[thread overview]
Message-ID: <af3c775a1515f97c8dbe6a6651bd6e4b6986e8cd.camel@perches.com> (raw)
In-Reply-To: <202109211630.2D00627@keescook>

On Tue, 2021-09-21 at 16:37 -0700, Kees Cook wrote:
> On Fri, Sep 10, 2021 at 10:23:48AM -0700, Linus Torvalds wrote:
> > On Thu, Sep 9, 2021 at 8:10 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> > > 
> > > +__alloc_size(1)
> > >  extern void *vmalloc(unsigned long size);
> > [...]
> > 
> > All of these are added in the wrong place - inconsistent with the very
> > compiler documentation the patches add.
> > 
> > The function attributes are generally added _after_ the function,
> > although admittedly we've been quite confused here before.
> > 
> > But the very compiler documentation you point to in the patch that
> > adds these macros gives that as the examples both for gcc and clang:
> > 
> > + *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-alloc_005fsize-function-attribute
> > + * clang: https://clang.llvm.org/docs/AttributeReference.html#alloc-size
> > 
> > and honestly I think that is the preferred format because this is
> > about the *function*, not about the return type.
> > 
> > Do both placements work? Yes.
> 
> I'm cleaning this up now, and have discovered that the reason for the
> before-function placement is consistency with static inlines. If I do this:
> 
> static __always_inline void * kmalloc(size_t size, gfp_t flags) __alloc_size(1)
> {
> 	...
> }
> 
> GCC is very angry:
> 
> ./include/linux/slab.h:519:1: error: attributes should be specified before the declarator in a function definition
>   519 | static __always_inline void *kmalloc_large(size_t size, gfp_t flags) __alloc_size(1)
>       | ^~~~~~
> 
> It's happy if I treat it as a "return type attribute" in the ordering,
> though:
> 
> static __always_inline void * __alloc_size(1) kmalloc(size_t size, gfp_t flags)
> 
> I'll do that unless you have a preference for somewhere else...

_please_ put it before the return type on a separate line.

[__attributes]
[static inline const] <return type> function(<args...>)