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

[Siddhesh Poyarekar]

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.

Thanks,
Sid