Re: [PATCH] reconnect_one(): fix a missing error code

NeilBrown
2017-06-14T21:54:57Z

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On Wed, Jun 14 2017, J. Bruce Fields wrote:

> On Wed, Jun 14, 2017 at 12:30:02PM +0300, Dan Carpenter wrote:
>> I found this bug by reviewing places where we do ERR_PTR(0) (which is
>> NULL).
>>
>> We used to return an error pointer if lookup_one_len() failed but we
>> moved this code into a helper function and accidentally removed that.
>> NULL is a valid return for this function but it's not what we intended.
>>
>> Fixes: bbf7a8a3562f ("exportfs: move most of reconnect_path to helper function")
>> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
>
> ACK. Agreed that the current code is wrong, and that this is the
> correct fix.
>
> What I don't quite understand yet is what the impact of the bug would
> be.
>

It is interesting that reconnect_path() handles the possibility of
reconnect_one() returning NULL, even though it will only do that if this
"bug" is triggered.
When that happens, the target_dir (a descendent of dentry) gets its
DCACHE_DISCONNECTED flag cleared.

The bug can presumably only be triggered by a race.
We look through a directory to find the name for an inode
(exportfs_get_name), then try to look up that name and it doesn't exist.

So presumably if you lose the race, some dentry will get
DCACHE_DISCONNECTED cleared, even though it is still disconnected.
This breaks a contract and can cause weirdness in dcache operations.

If the lookup_one_len_unlocked() fails, we should probably retry, at
least once. But if we do decide to give up, we shouldn't assume it all
worked.

So I suggest:
- the fix as provided by Dan, plus
- remove "if (!parent) break;" from reconnect_path(), plus
- maybe retry the get_name/lookup_one operation once if the first
attempt fails.

NeilBrown

> --b.
>
>>
>> diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
>> index 329a5d103846..451237745689 100644
>> --- a/fs/exportfs/expfs.c
>> +++ b/fs/exportfs/expfs.c
>> @@ -147,6 +147,7 @@ static struct dentry *reconnect_one(struct vfsmount *mnt,
>> tmp = lookup_one_len_unlocked(nbuf, parent, strlen(nbuf));
>> if (IS_ERR(tmp)) {
>> dprintk("%s: lookup failed: %d\n", __func__, PTR_ERR(tmp));
>> + err = PTR_ERR(tmp);
>> goto out_err;
>> }
>> if (tmp != dentry) {

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