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

[Kees Cook]

On Wed, Sep 21, 2022 at 07:43:17AM -0400, Siddhesh Poyarekar wrote:
> On 2022-09-20 15:22, Kees Cook wrote:
> > Since the commits starting with c37495d6254c ("slab: add __alloc_size
> > attributes for better bounds checking"), the compilers have runtime
> > allocation size hints available in some places. This was immediately
> > available to CONFIG_UBSAN_BOUNDS, but CONFIG_FORTIFY_SOURCE needed
> > updating to explicitly make use the hints via the associated
> > __builtin_dynamic_object_size() helper. Detect and use the builtin when
> > it is available, increasing the accuracy of the mitigation. When runtime
> > sizes are not available, __builtin_dynamic_object_size() falls back to
> > __builtin_object_size(), leaving the existing bounds checking unchanged.
> 
> I don't know yet what the overhead is for __builtin_dynamic_object_size vs
> __builtin_object_size, were you able to measure it somehow for the kernel?
> If there's a significant tradeoff, it may make sense to provide a user
> override.

So far I've not seen any measurable performance difference, but I just
may not be creative enough yet.

So far, the tunable is building a kernel with or without FORTIFY_SOURCE
and UBSAN_BOUNDS. :)

-- 
Kees Cook