Re: [PATCH v2] ovl: fix dentry leak in ovl_get_redirect

[Liangyan <liangyan.peng@linux.alibaba.com>]

This is the race scenario based on call trace we captured which cause 
the dentry leak.


      CPU 0                                CPU 1
ovl_set_redirect                       lookup_fast
   ovl_get_redirect                       __d_lookup
     dget_dlock
       //no lock protection here            spin_lock(&dentry->d_lock)
       dentry->d_lockref.count++            dentry->d_lockref.count++


If we use dget_parent instead, we may have this race.


      CPU 0                                    CPU 1
ovl_set_redirect                           lookup_fast
   ovl_get_redirect                           __d_lookup
     dget_parent
       raw_seqcount_begin(&dentry->d_seq)      spin_lock(&dentry->d_lock)
       lockref_get_not_zero(&ret->d_lockref)   dentry->d_lockref.count++ 



On 20/12/21 下午8:11, Al Viro wrote:
> On Mon, Dec 21, 2020 at 07:14:44PM +0800, Joseph Qi wrote:
>> Hi Viro,
>>
>> On 12/21/20 2:26 PM, Al Viro wrote:
>>> On Sun, Dec 20, 2020 at 08:09:27PM +0800, Liangyan wrote:
>>>
>>>> +++ b/fs/overlayfs/dir.c
>>>> @@ -973,6 +973,7 @@ static char *ovl_get_redirect(struct dentry *dentry, bool abs_redirect)
>>>>   	for (d = dget(dentry); !IS_ROOT(d);) {
>>>>   		const char *name;
>>>>   		int thislen;
>>>> +		struct dentry *parent = NULL;
>>>>   
>>>>   		spin_lock(&d->d_lock);
>>>>   		name = ovl_dentry_get_redirect(d);
>>>> @@ -992,7 +993,22 @@ static char *ovl_get_redirect(struct dentry *dentry, bool abs_redirect)
>>>>   
>>>>   		buflen -= thislen;
>>>>   		memcpy(&buf[buflen], name, thislen);
>>>> -		tmp = dget_dlock(d->d_parent);
>>>> +		parent = d->d_parent;
>>>> +		if (unlikely(!spin_trylock(&parent->d_lock))) {
>>>> +			rcu_read_lock();
>>>> +			spin_unlock(&d->d_lock);
>>>> +again:
>>>> +			parent = READ_ONCE(d->d_parent);
>>>> +			spin_lock(&parent->d_lock);
>>>> +			if (unlikely(parent != d->d_parent)) {
>>>> +				spin_unlock(&parent->d_lock);
>>>> +				goto again;
>>>> +			}
>>>> +			rcu_read_unlock();
>>>> +			spin_lock_nested(&d->d_lock, DENTRY_D_LOCK_NESTED);
>>>> +		}
>>>> +		tmp = dget_dlock(parent);
>>>> +		spin_unlock(&parent->d_lock);
>>>>   		spin_unlock(&d->d_lock);
>>>
>>> Yecchhhh....  What's wrong with just doing
>>> 		spin_unlock(&d->d_lock);
>>> 		parent = dget_parent(d);
>>> 		dput(d);
>>> 		d = parent;
>>> instead of that?
>>>
>>
>> Now race happens on non-RCU path in lookup_fast(), I'm afraid d_seq can
>> not close the race window.
> 
> Explain, please.  What exactly are you observing?
>