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Date:   Wed, 21 Sep 2022 07:43:17 -0400
From:   Siddhesh Poyarekar <siddhesh@...plt.org>
To:     Kees Cook <keescook@...omium.org>, linux-hardening@...r.kernel.org
Cc:     Miguel Ojeda <ojeda@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>, llvm@...ts.linux.dev,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] fortify: Use __builtin_dynamic_object_size() when
 available

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

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