linux-kernel - Re: [PATCH 0/4] fortify: Use __builtin_dynamic_object

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Message-ID: <e2a0debe-e99f-2259-1cb9-35193c387c82@gotplt.org>
Date:   Thu, 22 Sep 2022 16:26:54 -0400
From:   Siddhesh Poyarekar <siddhesh@...plt.org>
To:     Kees Cook <keescook@...omium.org>, linux-hardening@...r.kernel.org
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>, Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Tom Rix <trix@...hat.com>, Miguel Ojeda <ojeda@...nel.org>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH 0/4] fortify: Use __builtin_dynamic_object_size() when
 available

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