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SubjectRe: [fuse-devel] [PATCH] fuse: fix occasional dentry leak when readdirplus is used
On 07/16/2013 06:39 AM, Niels de Vos wrote:
> On Mon, Jul 15, 2013 at 04:08:22PM -0400, Brian Foster wrote:
>> On 07/15/2013 08:59 AM, Niels de Vos wrote:
...
>
>>> ---
>>> fs/fuse/dir.c | 4 +++-
>>> 1 files changed, 3 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
>>> index 0eda527..da67a15 100644
>>> --- a/fs/fuse/dir.c
>>> +++ b/fs/fuse/dir.c
>>> @@ -1246,7 +1246,9 @@ static int fuse_direntplus_link(struct file *file,
>>> if (err)
>>> goto out;
>>> dput(dentry);
>>> - dentry = NULL;
>>> + } else if (dentry) {
>>> + /* this dentry does not have a d_inode, just drop it */
>>> + dput(dentry);
>>> }
>>
>> I'm not really familiar with the dcache code, but is it appropriate to
>> also d_invalidate() the dentry in this case (as the previous code block
>> does)? Perhaps Miklos or somebody more familiar with dcache can confirm...
>
> I do not *think* d_invalidate() is needed. The vmcores I have seem where
> this BUG() happened, only have dentry->d_flags = 0x18 which translates
> to (DCACHE_OP_DELETE | DCACHE_OP_PRUNE) and d_subdirs as an empty list.
> d_invalidate() only calls __d_drop(), which only does something when the
> dentry is hashed.
>

I ran a little experiment to invoke a d_lookup() after the
fuse_direntplus_link() function has done its work in this particular
case. I do receive the newly allocated dentry.

Subsequently, I hacked __d_lookup_rcu() to continue iterating the list
after finding an entry to check for duplicates and print a message. What
I see is that after the dput() (via a subsequent lookup resulting from a
getxattr call), the negative dentry is still hashed and on the list (my
printk() kicks in after the existing d_unhashed() check). The flags for
both dentries are
(DCACHE_OP_REVALIDATE|DCACHE_REFERENCED|DCACHE_RCUACCESS). I'm not aware
of the steps involved in the original reproducer, but the simple
multi-mount test leads to this state. Running a d_invalidate() clears it
up. So perhaps it's required in some cases and not all.

That said, some of those flags seem to simply specify whether a dentry
op handler is defined for the particular operation or not.

> I am not sure if a dentry can be hashed, but still does not have a valid
> non-NULL d_inode. If that is the case, d_invalidate() should indeed be
> called.
>

I'm not sure why it would need to have a valid inode. A dentry with a
NULL inode is valid, no? I think the question is whether the above state
(multiple, hashed dentries) can be valid for whatever reason. It
certainly looks suspicious.

Brian

> Thanks,
> Niels
>
>> Brian
>>
>>>
>>> dentry = d_alloc(parent, &name);
>>>
>>
>



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