Re: Dangerous code in cpumask_of_cpu?

[Andreas Schwab <schwab@suse.de>]

Johannes Weiner <hannes@saeurebad.de> writes:

> Hi,
>
> Rusty Russell <rusty@rustcorp.com.au> writes:
>
>> Hi Christoph/Mike,
>>
>>   Looked at cpumask_of_cpu as introduced in 
>> 9f0e8d0400d925c3acd5f4e01dbeb736e4011882 (x86: convert cpumask_of_cpu macro 
>> to allocated array), and I don't think it's safe:
>>
>>   #define cpumask_of_cpu(cpu)						\
>>   (*({								\
>> 	typeof(_unused_cpumask_arg_) m;					\
>> 	if (sizeof(m) == sizeof(unsigned long)) {			\
>> 		m.bits[0] = 1UL<<(cpu);					\
>> 	} else {							\
>> 		cpus_clear(m);						\
>> 		cpu_set((cpu), m);					\
>> 	}								\
>> 	&m;								\
>>   }))
>>
>> Referring to &m once out of scope is invalid, and I can't find any evidence 
>> that it's legal here.  In particular, the change 
>> b53e921ba1cff8453dc9a87a84052fa12d5b30bd (generic: reduce stack pressure in 
>> sched_affinity) which passes &m to other functions seems highly risky.
>>
>> I'm surprised this hasn't already hit us, but perhaps gcc isn't as clever as 
>> it could be?
>
> You don't refer to &m outside scope.  Look at the character below the
> first e of #define :)

The scope of m ends with the outmost braces, and the dereference is done
outside of it.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."