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Message-ID: <202209221714.1D792FE6@keescook> Date: Thu, 22 Sep 2022 17:20:51 -0700 From: Kees Cook <keescook@...omium.org> To: Siddhesh Poyarekar <siddhesh@...plt.org> Cc: linux-hardening@...r.kernel.org, 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 Thu, Sep 22, 2022 at 04:26:54PM -0400, Siddhesh Poyarekar wrote: > 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. Thanks for check that! Yeah, a 40% increase in coverage is nice. :0 > 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. Yeah, this could be another interest set of additions. It seems like it might be more "coder friendly" if, in the future that has the __element_count__ attribute, it could be used in function parameters too, like: If we had: int do_something(struct context *ctx, u32 *data, int count) this seems less easy to read to me: int __access(read_write, 2, 3) do_something(struct context *ctx, u32 *data, int count) as this seems more readable to me, though I guess the access-mode information is lost: int do_something(struct context *ctx, u32 * __element_count(count) data, int count) But yes, this would be excellent to start adding! -Kees -- Kees Cook
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