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.
Thanks,
Joseph
