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Date:   Tue, 8 Feb 2022 15:00:25 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Nathan Chancellor <nathan@...nel.org>, llvm@...ts.linux.dev,
        Miguel Ojeda <ojeda@...nel.org>,
        George Burgess IV <gbiv@...gle.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v7 2/8] Compiler Attributes: Add __pass_object_size for Clang

On Tue, Feb 8, 2022 at 2:53 PM 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).
>
> This attribute has an additional benefit that it can be used even on
> non-inline functions to gain argument size information.
>
> [1] https://github.com/llvm/llvm-project/issues/53516

Thanks for the patch!
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

>
> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> Cc: Nathan Chancellor <nathan@...nel.org>
> Cc: llvm@...ts.linux.dev
> Reviewed-by: Miguel Ojeda <ojeda@...nel.org>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  include/linux/compiler_attributes.h | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
> index 37e260020221..d0c503772061 100644
> --- a/include/linux/compiler_attributes.h
> +++ b/include/linux/compiler_attributes.h
> @@ -263,6 +263,20 @@
>   */
>  #define __packed                        __attribute__((__packed__))
>
> +/*
> + * Note: the "type" argument should match any __builtin_object_size(p, type) usage.
> + *
> + * Optional: not supported by gcc.
> + * Optional: not supported by icc.
> + *
> + * clang: https://clang.llvm.org/docs/AttributeReference.html#pass-object-size-pass-dynamic-object-size
> + */
> +#if __has_attribute(__pass_object_size__)
> +# define __pass_object_size(type)      __attribute__((__pass_object_size__(type)))
> +#else
> +# define __pass_object_size(type)
> +#endif
> +
>  /*
>   *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-pure-function-attribute
>   */
> --
> 2.30.2
>
>


-- 
Thanks,
~Nick Desaulniers

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