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Date: Mon, 17 Aug 2020 11:08:54 +0200
From: David Sterba <dsterba@...e.cz>
To: Kees Cook <keescook@...omium.org>
Cc: Rasmus Villemoes <linux@...musvillemoes.dk>,
"Gustavo A. R. Silva" <gustavoars@...nel.org>,
Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>,
Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v2] overflow: Add __must_check attribute to check_*()
helpers
On Sat, Aug 15, 2020 at 10:09:24AM -0700, 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@...musvillemoes.dk>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> v2:
> - de-generalized __must_check_overflow() from being named "bool" (willy)
> - fix comment typos (rasmus)
> v1: https://lore.kernel.org/lkml/202008121450.405E4A3@keescook
> ---
> include/linux/overflow.h | 39 ++++++++++++++++++++++++---------------
> 1 file changed, 24 insertions(+), 15 deletions(-)
>
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 93fcef105061..f1c4e7b56bd9 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 for effectively applying __must_check to a macro so we can have
> + * 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_overflow(bool overflow)
> +{
> + return unlikely(overflow);
How does the 'unlikely' hint propagate through return? It is in a static
inline so compiler has complete information in order to use it, but I'm
curious if it actually does.
> +}
> +
> #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_overflow(({ \
> typeof(a) __a = (a); \
> typeof(b) __b = (b); \
> typeof(d) __d = (d); \
> (void) (&__a == &__b); \
> (void) (&__a == __d); \
> __builtin_add_overflow(__a, __b, __d); \
> -})
> +}))
In case the hint gets dropped, the fix would probably be
#define check_add_overflow(a, b, d) unlikely(__must_check_overflow(({ \
typeof(a) __a = (a); \
typeof(b) __b = (b); \
typeof(d) __d = (d); \
(void) (&__a == &__b); \
(void) (&__a == __d); \
__builtin_add_overflow(__a, __b, __d); \
})))
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