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

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Date:   Mon, 17 Aug 2020 11:33:10 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     dsterba@...e.cz, Kees Cook <keescook@...omium.org>,
        "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 17/08/2020 11.08, David Sterba wrote:
> On Sat, Aug 15, 2020 at 10:09:24AM -0700, Kees Cook wrote:
>>  
>> +/*
>> + * 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.

I wondered the same thing, but as I noted in a reply in the v1 thread,
that pattern is used in kernel/sched/, and the scheduler is a far more
critical path than anywhere these might be used, so if it's good enough
for kernel/sched/, it should be good enough here. I have no idea how one
could write a piece of non-trivial code to see if the hint actually
makes a difference.

> 
> 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);	\
> })))
> 

Well, maybe, but I'd be a little worried that the !! that unlikely()
slabs on its argument may count as a use of that argument, hence
nullifying the __must_check which is the main point - the unlikely just
being something we can add for free while touching this code. Haven't
tested it, though.

Rasmus