linux-kernel - Re: [PATCH] cpuset: fix cpuset_print_task_mems

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Message-ID: <51023454.6050501@huawei.com>
Date:	Fri, 25 Jan 2013 15:29:24 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	Tejun Heo <tj@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH] cpuset: fix cpuset_print_task_mems_allowed() vs rename()
 race

On 2013/1/25 15:26, Al Viro wrpte:
> On Fri, Jan 25, 2013 at 03:09:48PM +0800, Li Zefan wrote:
>> rename() will change dentry->d_name. The result of this race can
>> be worse than seeing partially rewritten name, but we might access
>> a stale pointer because rename() will re-allocate memory to hold
>> a longer name.
>>
>> It's safe in the protection of dentry->d_lock.
>>
>> Signed-off-by: Li Zefan <lizefan@...wei.com>
>> ---
>>  kernel/cpuset.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/kernel/cpuset.c b/kernel/cpuset.c
>> index 16be7c9..b2476c2 100644
>> --- a/kernel/cpuset.c
>> +++ b/kernel/cpuset.c
>> @@ -2606,8 +2606,12 @@ void cpuset_print_task_mems_allowed(struct task_struct *tsk)
>>  
>>  	dentry = task_cs(tsk)->css.cgroup->dentry;
>>  	spin_lock(&cpuset_buffer_lock);
>> +
>> +	spin_lock(&dentry->d_lock);
>>  	snprintf(cpuset_name, CPUSET_NAME_LEN,
>>  		 dentry ? (const char *)dentry->d_name.name : "/");
> 
> Ahem...  Can dentry actually be NULL here?  If not, this conditional
> is bogus; otherwise, spin_lock(&dentry->d_lock) is going to blow up...

oops, my fault...It's NULL if cpuset is not mounted.

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