linux-kernel - Re: [PATCH v2] overflow: Add __must_check attribute to check

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Date:   Mon, 17 Aug 2020 12:36:51 -0700
From:   Kees Cook <keescook@...omium.org>
To:     dsterba@...e.cz, 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 Mon, Aug 17, 2020 at 11:08:54AM +0200, David Sterba wrote:
> On Sat, Aug 15, 2020 at 10:09:24AM -0700, Kees Cook wrote:
> > +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.

It may not -- it depends on how the compiler decides to deal with it. :)

> 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);	\
> })))

Unfortunately not, as the unlikely() ends up eating the __must_check
attribute. :(

-- 
Kees Cook