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Date:   Thu, 22 Sep 2022 16:26:54 -0400
From:   Siddhesh Poyarekar <>
To:     Kees Cook <>,
Cc:     Nathan Chancellor <>,
        Nick Desaulniers <>,
        Arnd Bergmann <>, Juergen Gross <>,
        Boris Ostrovsky <>,
        Tom Rix <>, Miguel Ojeda <>,,
Subject: Re: [PATCH 0/4] fortify: Use __builtin_dynamic_object_size() when

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.



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