Re: Dangerous code in cpumask_of_cpu?

[Johannes Weiner <hannes@saeurebad.de>]

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 :)

But then, this code should probably just evaluate to m without this
obscure *(&m) construct.

	Hannes