linux-kernel - Re: [PATCH] sched/core: Clear the root_domain cpumasks in init

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Date:	Fri, 4 Dec 2015 16:28:28 +0800
From:	Xunlei Pang <xlpang@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH] sched/core: Clear the root_domain cpumasks in
 init_rootdomain()

Hi Ingo,

On 12/04/2015 at 04:09 PM, Ingo Molnar wrote:
> * Xunlei Pang <xlpang@...hat.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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