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Date:   Thu, 4 May 2023 12:37:37 -0600
From:   "Gustavo A. R. Silva" <gustavoars@kernel.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     Miguel Ojeda <ojeda@kernel.org>, Bill Wendling <morbo@google.com>,
        Qing Zhao <qing.zhao@oracle.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Tom Rix <trix@redhat.com>, llvm@lists.linux.dev,
        linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH] Compiler Attributes: Add __counted_by macro
Message-ID: <ZFP7cfQpb60wfLd+@work>
References: <20230504181636.never.222-kees@kernel.org>
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On Thu, May 04, 2023 at 11:16:40AM -0700, Kees Cook wrote:
> In an effort to annotate all flexible array members with their run-time
> size information, the "element_count" attribute is being introduced by
> Clang[1] and GCC[2] in future releases. This annotation will provide
> the CONFIG_UBSAN_BOUNDS and CONFIG_FORTIFY_SOURCE features the ability
> to perform run-time bounds checking on otherwise unknown-size flexible
> arrays.

It's happening! :D

> 
> Even though the attribute is under development, we can start the
> annotation process in the kernel. This requires defining a macro for
> it, even if we have to change the name of the actual attribute later.
> Since it is likely that this attribute may change its name to "counted_by"
> in the future (to better align with a future total bytes "sized_by"
> attribute), name the wrapper macro "__counted_by", which also reads more
> clearly (and concisely) in structure definitions.
> 
> [1] https://reviews.llvm.org/D148381
> [2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
> 
> Cc: Miguel Ojeda <ojeda@kernel.org>
> Cc: Bill Wendling <morbo@google.com>
> Cc: Qing Zhao <qing.zhao@oracle.com>
> Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Nathan Chancellor <nathan@kernel.org>
> Cc: Tom Rix <trix@redhat.com>
> Cc: llvm@lists.linux.dev
> Signed-off-by: Kees Cook <keescook@chromium.org>

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Thanks!
--
Gustavo

> ---
>  include/linux/compiler_attributes.h | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
> index e659cb6fded3..9d63fe2024d5 100644
> --- a/include/linux/compiler_attributes.h
> +++ b/include/linux/compiler_attributes.h
> @@ -123,6 +123,18 @@
>  # define __designated_init
>  #endif
>  
> +/*
> + * Optional: future support coming in clang 17 and gcc 14
> + *
> + *   gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
> + * clang: https://reviews.llvm.org/D148381
> + */
> +#if __has_attribute(__element_count__)
> +# define __counted_by(member)		__attribute__((__element_count__(member)))
> +#else
> +# define __counted_by(member)
> +#endif
> +
>  /*
>   * Optional: only supported since clang >= 14.0
>   *
> -- 
> 2.34.1
>