Re: Dangerous code in cpumask_of_cpu?

[Johannes Weiner <hannes@saeurebad.de>]

Johannes Weiner <hannes@saeurebad.de> writes:

> Hi,
>
> 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 :)
>
> Oh, well you do access it outside scope, sorry.  Me sleepy.
>
> I guess because we dereference it immediately again, the location is not
> clobbered yet.  At least in my test case, gcc assembled it to code that
> puts the address in eax and derefences it immediately, before eax is
> reused:

Gee, just ignore this bs.  The address is in eax, not the value.

> static int *foo(void)
> {
>         int x = 42;
>         return &x;
> }
>
> int main(void)
> {
>         return *foo();
> }

However, this code seems to produce valid assembly with -O2.  gcc just
warns and fixes it up.

	Hannes