Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <9d80e0a3-3fcc-0676-4529-79743f418557@gotplt.org>
Date:   Wed, 21 Sep 2022 07:43:17 -0400
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.12.0
Subject: Re: [PATCH 4/4] fortify: Use __builtin_dynamic_object_size() when
 available
Content-Language: en-US
To:     Kees Cook <keescook@chromium.org>, linux-hardening@vger.kernel.org
Cc:     Miguel Ojeda <ojeda@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Tom Rix <trix@redhat.com>, llvm@lists.linux.dev,
        Juergen Gross <jgross@suse.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        linux-kernel@vger.kernel.org
References: <20220920192202.190793-1-keescook@chromium.org>
 <20220920192202.190793-5-keescook@chromium.org>
From:   Siddhesh Poyarekar <siddhesh@gotplt.org>
In-Reply-To: <20220920192202.190793-5-keescook@chromium.org>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

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