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Message-ID: <202008171235.816B3AD@keescook>
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
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