Re: [PATCH] sched/core: Clear the root_domain cpumasks in init_rootdomain()

Xunlei Pang
2015-12-04T08:28:37Z

Hi Ingo,

On 12/04/2015 at 04:09 PM, Ingo Molnar wrote:
> * Xunlei Pang <xlpang@redhat.com> wrote:
>
>>> Hm, is the alloc_cpumask_var() done in alloc_sched_domains() safe?
>> Until now, I haven't found any other similar issues, but I will check further.
>>
>>> At least the usage pattern in init_sched_domains() looks unsafe:
>>>
>>> doms_cur = alloc_sched_domains(ndoms_cur);
>>> if (!doms_cur)
>>> doms_cur = &fallback_doms;
>>> cpumask_andnot(doms_cur[0], cpu_map, cpu_isolated_map);
> So is this pattern in init_sched_domains() correct, for OFFSTACK=y?
>
> It looks wrong to me, as alloc_sched_domains() allocates an uninitialized cpumask
> via alloc_cpumask_var() and returns it:
>
> cpumask_var_t *alloc_sched_domains(unsigned int ndoms)
> {
> int i;
> cpumask_var_t *doms;
>
> doms = kmalloc(sizeof(*doms) * ndoms, GFP_KERNEL);
> if (!doms)
> return NULL;
> for (i = 0; i < ndoms; i++) {
> if (!alloc_cpumask_var(&doms[i], GFP_KERNEL)) {
> free_sched_domains(doms, i);
> return NULL;
> }
> }
> return doms;
> }
>
> and then this code:
>
>>> cpumask_andnot(doms_cur[0], cpu_map, cpu_isolated_map);
> uses it without first clearing it.
>
> So is this another such bug, or am I missing something?

Yeah, I noticed that as well. But fortunately cpumask_andnot(),
cpumask_and() and the like will clear doms_cur[] indirectly, also
cpu_isolated_map, cpu_active_mask, etc doesn't contain any
garbage bits. I also checked the use of it by cpuset, no extra such
bug found by me so far.

Regards,
Xunlei

>
> Thanks,
>
> Ingo
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