Re: [PATCH v5 12/15] Compiler Attributes: add support for __nonstring (gcc >= 8)

[Nick Desaulniers <ndesaulniers@google.com>]

On Thu, Sep 20, 2018 at 10:23 AM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> From the GCC manual:
>
>   nonstring
>
>     The nonstring variable attribute specifies that an object or member
>     declaration with type array of char, signed char, or unsigned char,
>     or pointer to such a type is intended to store character arrays that
>     do not necessarily contain a terminating NUL. This is useful in detecting
>     uses of such arrays or pointers with functions that expect NUL-terminated
>     strings, and to avoid warnings when such an array or pointer is used as
>     an argument to a bounded string manipulation function such as strncpy.
>
>   https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html
>
> This attribute can be used for documentation purposes (i.e. replacing
> comments), but it is most helpful when the following warnings are enabled:
>
>   -Wstringop-overflow
>
>     Warn for calls to string manipulation functions such as memcpy and
>     strcpy that are determined to overflow the destination buffer.
>
>     [...]
>
>   -Wstringop-truncation
>
>     Warn for calls to bounded string manipulation functions such as
>     strncat, strncpy, and stpncpy that may either truncate the copied
>     string or leave the destination unchanged.
>
>     [...]
>
>     In situations where a character array is intended to store a sequence
>     of bytes with no terminating NUL such an array may be annotated with
>     attribute nonstring to avoid this warning. Such arrays, however,
>     are not suitable arguments to functions that expect NUL-terminated
>     strings. To help detect accidental misuses of such arrays GCC issues
>     warnings unless it can prove that the use is safe.
>
>   https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
>
> Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
> ---
>  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 f0f9fc398440..6b28c1b7310c 100644
> --- a/include/linux/compiler_attributes.h
> +++ b/include/linux/compiler_attributes.h
> @@ -34,6 +34,7 @@
>  # define __GCC4_has_attribute___externally_visible__  1
>  # define __GCC4_has_attribute___noclone__             1
>  # define __GCC4_has_attribute___optimize__            1
> +# define __GCC4_has_attribute___nonstring__           0
>  # define __GCC4_has_attribute___no_sanitize_address__ (__GNUC_MINOR__ >= 8)
>  #endif
>
> @@ -181,6 +182,19 @@
>   */
>  #define   noinline                      __attribute__((__noinline__))
>
> +/*
> + * Optional: only supported since gcc >= 8
> + * Optional: not supported by clang
> + * Optional: not supported by icc
> + *
> + *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-nonstring-variable-attribute
> + */
> +#if __has_attribute(__nonstring__)
> +# define __nonstring                    __attribute__((__nonstring__))
> +#else
> +# define __nonstring
> +#endif
> +
>  /*
>   *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-noreturn-function-attribute
>   * clang: https://clang.llvm.org/docs/AttributeReference.html#noreturn
> --
> 2.17.1
>

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

-- 
Thanks,
~Nick Desaulniers