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Message-ID: <202209212032.5F392F42@keescook> Date: Wed, 21 Sep 2022 20:33:52 -0700 From: Kees Cook <keescook@...omium.org> To: Siddhesh Poyarekar <siddhesh@...plt.org> Cc: linux-hardening@...r.kernel.org, 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 Wed, Sep 21, 2022 at 07:43:17AM -0400, Siddhesh Poyarekar wrote: > 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. So far I've not seen any measurable performance difference, but I just may not be creative enough yet. So far, the tunable is building a kernel with or without FORTIFY_SOURCE and UBSAN_BOUNDS. :) -- Kees Cook
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