linux-hardening - Re: [PATCH] Compiler Attributes: Introduce __access

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Date:   Fri, 23 Sep 2022 19:17:31 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Siddhesh Poyarekar <siddhesh@...plt.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] Compiler Attributes: Introduce __access_*() function
 attribute

On Fri, Sep 23, 2022 at 04:54:24PM -0700, Kees Cook wrote:
> Added in GCC 10.1, the "access" function attribute to mark pointer
> arguments for how they are expected to be accessed in a given function.
> Both their access type (read/write, read-only, or write-only) and bounds
> are specified. While it is legal to provide only the pointer argument
> position and access type, design the kernel macros to require also the
> bounds (element count) argument position: if a function has no bounds
> argument, refactor the code to include one.
> 
> These can be used multiple times. For example:
> 
> __access_wo(2, 3) __access_ro(4, 5)
> int copy_something(struct context *ctx, u32 *dst, size_t dst_count,
> 		   u8 *src, int src_len);
> 
> (And if "dst" will also be read, it could use __access_rw(2, 3) instead.)
> 
> These can inform the compile-time diagnostics of GCC including
> -Warray-bounds, -Wstringop-overflow, etc, and can affect
> __builtin_dynamic_object_size() results.
> 
> Cc: Miguel Ojeda <ojeda@...nel.org>
> Cc: Siddhesh Poyarekar <siddhesh@...plt.org>
> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> Cc: Nathan Chancellor <nathan@...nel.org>
> Cc: Tom Rix <trix@...hat.com>
> Cc: llvm@...ts.linux.dev
> Signed-off-by: Kees Cook <keescook@...omium.org>

Acked-by: Gustavo A. R. Silva <gustavoars@...nel.org>

--
Gustavo

> ---
>  include/linux/compiler_attributes.h | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
> index 9a9907fad6fd..6f3d40f7ee5e 100644
> --- a/include/linux/compiler_attributes.h
> +++ b/include/linux/compiler_attributes.h
> @@ -20,6 +20,22 @@
>   * Provide links to the documentation of each supported compiler, if it exists.
>   */
>  
> +/*
> + * Optional: only supported since gcc >= 10
> + * Optional: not supported by Clang
> + *
> + *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-access-function-attribute
> + */
> +#if __has_attribute(__access__)
> +#define __access_rw(ptr, count)	__attribute__((__access__(read_write, ptr, count)))
> +#define __access_ro(ptr, count)	__attribute__((__access__(read_only,  ptr, count)))
> +#define __access_wo(ptr, count)	__attribute__((__access__(write_only, ptr, count)))
> +#else
> +#define __access_rw(ptr, count)
> +#define __access_ro(ptr, count)
> +#define __access_wo(ptr, count)
> +#endif
> +
>  /*
>   *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-alias-function-attribute
>   */
> -- 
> 2.34.1
>