Re: Dangerous code in cpumask_of_cpu?

[Rusty Russell <rusty@rustcorp.com.au>]

On Wednesday 09 July 2008 01:29:34 Mike Travis wrote:
> Johannes Weiner wrote:
> > 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
>
> IIRC, the problem was I needed an lvalue and it seems that the *(&m) was
> the way I was able to coerce gcc into producing it.  That's not to say
> there may be a better way however... ;-)  [Btw, I picked up this technique
> in the (original) per_cpu() macro.]

Yes, but I could do that because it wasn't referring to a temporary variable.

> Note the lvalue isn't used for changing the cpumask value, but for sending
> it to functions like set_cpus_allowed_ptr() to avoid pushing the 512 bytes
> of a 4096 cpus cpumask onto the stack.  So it becomes &(*(&m)))  ... ;-) 
> But I thought I checked the assembly for different config options and it
> looked ok.

Yeah, the problem is that a future gcc will cause horrible and subtle 
breakage.

I think we are going to want a get_cpumask()/put_cpumask() pattern for this.

Rusty.