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Date: Thu, 3 Feb 2022 16:29:22 -0800 From: Kees Cook <keescook@...omium.org> To: Nick Desaulniers <ndesaulniers@...gle.com> Cc: George Burgess IV <gbiv@...gle.com>, Miguel Ojeda <ojeda@...nel.org>, Nathan Chancellor <nathan@...nel.org>, llvm@...ts.linux.dev, linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org, Sami Tolvanen <samitolvanen@...gle.com> Subject: Re: [PATCH v6 1/4] Compiler Attributes: Add __pass_object_size for Clang On Thu, Feb 03, 2022 at 02:01:06PM -0800, Nick Desaulniers wrote: > On Thu, Feb 3, 2022 at 12:58 PM Kees Cook <keescook@...omium.org> wrote: > > > > On Thu, Feb 03, 2022 at 12:18:24PM -0800, Nick Desaulniers wrote: > > > On Thu, Feb 3, 2022 at 9:33 AM Kees Cook <keescook@...omium.org> wrote: > > > > > > > > In order to gain greater visibility to type information when using > > > > __builtin_object_size(), Clang has a function attribute "pass_object_size" > > > > that will make size information available for marked arguments in > > > > a function by way of implicit additional function arguments that are > > > > then wired up the __builtin_object_size(). > > > > > > > > This is needed to implement FORTIFY_SOURCE in Clang, as a workaround > > > > to Clang's __builtin_object_size() having limited visibility[1] into types > > > > across function calls (even inlines). > > > > > > > > Since any usage must also be const, include it in the macro. > > > > > > I think it might be helpful to quote the docs > > > (https://clang.llvm.org/docs/AttributeReference.html#pass-object-size-pass-dynamic-object-size) > > > > > > >> Additionally, any parameter that pass_object_size is applied to must be marked const at its function’s definition. > > > > > > One thing that's concerning to me is though: > > > > > > >> It is an error to take the address of a function with pass_object_size on any of its parameters. > > > > > > Surely the kernel has indirect calls to some of these functions > > > somewhere? Is that just an issue for C++ name-mangling perhaps? > > > > AFAIU, this shouldn't be a problem for any of these. Nothing is trying > > to take memcpy, memset, etc by address. The macro-ified version of this > > change proved that out. :) > > I thought Sami had found a location where memcpy was invoked > indirectly as part of his kcfi work? Maybe I'm misremembering. > > https://github.com/samitolvanen/linux/commit/46a777fb35784a8c6daf13d67de8bfb5148adc2a#diff-a27660992abdf360d01deac6364db31836d0d98b5d9573b7fc10a6785a669975R16 Hm, I've had memcpy as a macro for a while now, so dunno! That's not a sensible thing to call indirectly. :) -- Kees Cook
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