Messages in this thread | | | Date | Tue, 08 Jul 2008 08:29:34 -0700 | From | Mike Travis <> | Subject | Re: Dangerous code in cpumask_of_cpu? |
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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.]
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.
Thanks, Mike
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