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Date: Thu, 13 Aug 2020 08:39:44 +0200
From: Rasmus Villemoes <linux@...musvillemoes.dk>
To: Kees Cook <keescook@...omium.org>,
Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: "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] overflow: Add __must_check attribute to check_*() helpers
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@...musvillemoes.dk>
> Signed-off-by: Kees Cook <keescook@...omium.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
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