Re: [PATCH] fortify: Do not cast to "unsigned char"

[Nick Desaulniers]

On Tue, Oct 25, 2022 at 4:17 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> Do not cast to "unsigned char", as this needlessly creates type problems
> when attempting builds without -Wno-pointer-sign[1]. The intent of the
> cast is to drop possible "const" types.
>
> [1] https://lore.kernel.org/lkml/CAHk-=wgz3Uba8w7kdXhsqR1qvfemYL+OFQdefJnkeqXG8qZ_pA@xxxxxxxxxxxxxx/
>
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Fixes: 3009f891bb9f ("fortify: Allow strlen() and strnlen() to pass compile-time known lengths")
> Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Cc: linux-hardening@xxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  include/linux/fortify-string.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
> index f3dd5d1a6a25..09a032f6ce6b 100644
> --- a/include/linux/fortify-string.h
> +++ b/include/linux/fortify-string.h
> @@ -18,7 +18,7 @@ void __write_overflow_field(size_t avail, size_t wanted) __compiletime_warning("
>
>  #define __compiletime_strlen(p)                                        \
>  ({                                                             \
> -       unsigned char *__p = (unsigned char *)(p);              \
> +       char *__p = (char *)(p);                                \

If the intent of __p is to avoid repeated application of side effects
from the evaluation of the macro parameter p, this could also be:

__auto_type __p = (p);

I think this is nice because it doesn't strip potential const
qualifiers from the macro parameter.

>         size_t __ret = SIZE_MAX;                                \
>         size_t __p_size = __member_size(p);                     \
>         if (__p_size != SIZE_MAX &&                             \
> --
> 2.34.1
>


-- 
Thanks,
~Nick Desaulniers