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Subject: Re: [PATCH] overflow: Add __must_check attribute to check_*() helpers
To:     Kees Cook <keescook@chromium.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Leon Romanovsky <leon@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com
References: <202008121450.405E4A3@keescook>
From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <f7b6ad2f-4b35-1ca8-0137-05b27a0eb574@rasmusvillemoes.dk>
Date:   Thu, 13 Aug 2020 08:39:44 +0200
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On 12/08/2020 23.51, Kees Cook wrote:
> Since the destination variable of the check_*_overflow() helpers will
> contain a wrapped value on failure, it would be best to make sure callers
> really did check the return result of the helper. Adjust the macros to use
> a bool-wrapping static inline that is marked with __must_check. This means
> the macros can continue to have their type-agnostic behavior while gaining
> the function attribute (that cannot be applied directly to macros).
> 
> Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  include/linux/overflow.h | 51 +++++++++++++++++++++++-----------------
>  1 file changed, 30 insertions(+), 21 deletions(-)
> 
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 93fcef105061..ef7d538c2d08 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -43,6 +43,16 @@
>  #define is_non_negative(a) ((a) > 0 || (a) == 0)
>  #define is_negative(a) (!(is_non_negative(a)))
>  
> +/*
> + * Allows to effectively us apply __must_check to a macro so we can have

word ordering?

> + * both the type-agnostic benefits of the macros while also being able to
> + * enforce that the return value is, in fact, checked.
> + */
> +static inline bool __must_check __must_check_bool(bool condition)
> +{
> +	return unlikely(condition);
> +}
> +
>  #ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
>  /*
>   * For simplicity and code hygiene, the fallback code below insists on
> @@ -52,32 +62,32 @@
>   * alias for __builtin_add_overflow, but add type checks similar to
>   * below.
>   */
> -#define check_add_overflow(a, b, d) ({		\
> +#define check_add_overflow(a, b, d) __must_check_bool(({	\
>  	typeof(a) __a = (a);			\
>  	typeof(b) __b = (b);			\
>  	typeof(d) __d = (d);			\
>  	(void) (&__a == &__b);			\
>  	(void) (&__a == __d);			\
>  	__builtin_add_overflow(__a, __b, __d);	\
> -})
> +}))

Sorry, I meant to send this before your cooking was done but forgot
about it again. Not a big deal, but it occurred to me it might be better
to rename the existing check_*_overflow to __check_*_overflow (in both
branches of the COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW), and then

#define check_*_overflow(a, b, d)
__must_check_bool(__check_*_overflow(a, b, d))

Mostly because it gives less whitespace churn, but it might also be
handy to have the dunder versions available (if nothing else then
perhaps in some test code).

But as I said, no biggie, I'm fine either way. Now I'm just curious if
0-day is going to find some warning introduced by this :)

Rasmus