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Date: Thu, 3 Feb 2022 12:58:17 -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 Subject: Re: [PATCH v6 1/4] Compiler Attributes: Add __pass_object_size for Clang 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 really don't like hiding the qualifier in the argument-attribute > macro like that. I'd rather we be more explicit about changing the > function interfaces in include/linux/fortify-string.h. It seemed redundant to have it separate when it would always be needed. In v5 I had the const in the BOS (now POS) macro, but realized that this was silly since it would always need to be that way for __pos. > For instance, in patch 4/4, let's take a look at this hunk: > > -__FORTIFY_INLINE char *strncpy(char *p, const char *q, __kernel_size_t size) > ... > +__FORTIFY_INLINE __diagnose_as(__builtin_strncpy, 1, 2, 3) > +char *strncpy(char * POS p, const char *q, __kernel_size_t size) > > manually expanded, this has changed the qualifiers on the type of the > first parameter from `char *` to `char * const`. Yup, that's expected, and I wanted to tie it strictly to the expansion of __pass_object_size since otherwise GCC would gain the "const" bit (which technically shouldn't matter, but this whole series has been nothing but corner case after corner case, and I didn't want to risk another one). > 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. :) -- Kees Cook
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