Re: [PATCH 0/4] fortify: Use __builtin_dynamic_object_size() when available

[Kees Cook <keescook@chromium.org>]

On Thu, Sep 22, 2022 at 04:26:54PM -0400, Siddhesh Poyarekar wrote:
> On 2022-09-20 15:21, Kees Cook wrote:
> > Hi,
> > 
> > This adjusts CONFIG_FORTIFY_SOURCE's coverage to include greater runtime
> > size checking from GCC and Clang's __builtin_dynamic_object_size(), which
> > the compilers can track either via code flow or from __alloc_size() hints.
> > 
> 
> FTR, I ran a linux build using gcc with allyesconfig and fortify-metrics[1]
> to get a sense of how much object size coverage would improve with
> __builtin_dynamic_object_size.  With a total of 3,877 __builtin_object_size
> calls, about 11.37% succeed in getting a result that is not (size_t)-1.  If
> they were replaced by __builtin_dynamic_object_size as this patch proposes,
> the success rate improves to 16.25%, which is a ~1.4x improvement.

Thanks for check that! Yeah, a 40% increase in coverage is nice. :0

> This is a decent improvement by itself but it can be amplified further by
> adding __attribute__((access (...)))[2] to function prototypes and
> definitions, especially for functions that take in buffers and their sizes
> as arguments since __builtin_dynamic_object_size in gcc is capable of
> recognizing that and using it for object size determination (and hence to
> fortify calls) within those functions.

Yeah, this could be another interest set of additions. It seems like it
might be more "coder friendly" if, in the future that has the
__element_count__ attribute, it could be used in function parameters
too, like:

If we had:

int do_something(struct context *ctx, u32 *data, int count)

this seems less easy to read to me:

int __access(read_write, 2, 3) do_something(struct context *ctx, u32 *data, int count)

as this seems more readable to me, though I guess the access-mode
information is lost:

int do_something(struct context *ctx, u32 * __element_count(count) data, int count)

But yes, this would be excellent to start adding!

-Kees

-- 
Kees Cook