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

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.

Thanks,
Sid

[1] https://github.com/siddhesh/fortify-metrics
[2] https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html

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