Re: [PATCH] ceph: don't return -ESTALE if there's still an open file

[Luis Henriques <lhenriques@suse.com>]

On Fri, May 15, 2020 at 09:42:24AM +0300, Amir Goldstein wrote:
> +CC: fstests
> 
> On Thu, May 14, 2020 at 4:15 PM Jeff Layton <jlayton@kernel.org> wrote:
> >
> > On Thu, 2020-05-14 at 13:48 +0100, Luis Henriques wrote:
> > > On Thu, May 14, 2020 at 08:10:09AM -0400, Jeff Layton wrote:
> > > > On Thu, 2020-05-14 at 12:14 +0100, Luis Henriques wrote:
> > > > > Similarly to commit 03f219041fdb ("ceph: check i_nlink while converting
> > > > > a file handle to dentry"), this fixes another corner case with
> > > > > name_to_handle_at/open_by_handle_at.  The issue has been detected by
> > > > > xfstest generic/467, when doing:
> > > > >
> > > > >  - name_to_handle_at("/cephfs/myfile")
> > > > >  - open("/cephfs/myfile")
> > > > >  - unlink("/cephfs/myfile")
> > > > >  - open_by_handle_at()
> > > > >
> > > > > The call to open_by_handle_at should not fail because the file still
> > > > > exists and we do have a valid handle to it.
> > > > >
> > > > > Signed-off-by: Luis Henriques <lhenriques@suse.com>
> > > > > ---
> > > > >  fs/ceph/export.c | 13 +++++++++++--
> > > > >  1 file changed, 11 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/fs/ceph/export.c b/fs/ceph/export.c
> > > > > index 79dc06881e78..8556df9d94d0 100644
> > > > > --- a/fs/ceph/export.c
> > > > > +++ b/fs/ceph/export.c
> > > > > @@ -171,12 +171,21 @@ struct inode *ceph_lookup_inode(struct super_block *sb, u64 ino)
> > > > >
> > > > >  static struct dentry *__fh_to_dentry(struct super_block *sb, u64 ino)
> > > > >  {
> > > > > + struct ceph_inode_info *ci;
> > > > >   struct inode *inode = __lookup_inode(sb, ino);
> > > > > +
> > > > >   if (IS_ERR(inode))
> > > > >           return ERR_CAST(inode);
> > > > >   if (inode->i_nlink == 0) {
> > > > > -         iput(inode);
> > > > > -         return ERR_PTR(-ESTALE);
> > > > > +         bool is_open;
> > > > > +         ci = ceph_inode(inode);
> > > > > +         spin_lock(&ci->i_ceph_lock);
> > > > > +         is_open = __ceph_is_file_opened(ci);
> > > > > +         spin_unlock(&ci->i_ceph_lock);
> > > > > +         if (!is_open) {
> > > > > +                 iput(inode);
> > > > > +                 return ERR_PTR(-ESTALE);
> > > > > +         }
> > > > >   }
> > > > >   return d_obtain_alias(inode);
> > > > >  }
> > > >
> > > > Thanks Luis. Out of curiousity, is there any reason we shouldn't ignore
> > > > the i_nlink value here? Does anything obviously break if we do?
> > >
> > > Yes, the scenario described in commit 03f219041fdb is still valid, which
> > > is basically the same but without the extra open(2):
> > >
> > >   - name_to_handle_at("/cephfs/myfile")
> > >   - unlink("/cephfs/myfile")
> > >   - open_by_handle_at()
> > >
> >
> > Ok, I guess we end up doing some delayed cleanup, and that allows the
> > inode to be found in that situation.
> >
> > > The open_by_handle_at man page isn't really clear about these 2 scenarios,
> > > but generic/426 will fail if -ESTALE isn't returned.  Want me to add a
> > > comment to the code, describing these 2 scenarios?
> > >
> >
> > (cc'ing Amir since he added this test)
> >
> > I don't think there is any hard requirement that open_by_handle_at
> > should fail in that situation. It generally does for most filesystems
> > due to the way they handle cl794798fa xfsqa: test open_by_handle() on unlinked and freed inode clusters
> eaning up unlinked inodes, but I don't
> > think it's technically illegal to allow the inode to still be found. If
> > the caller cares about whether it has been unlinked it can always test
> > i_nlink itself.
> >
> > Amir, is this required for some reason that I'm not aware of?
> 
> Hi Jeff,
> 
> The origin of this test is in fstests commit:
> 794798fa xfsqa: test open_by_handle() on unlinked and freed inode clusters
> 
> It was introduced to catch an xfs bug, so this behavior is the expectation
> of xfs filesystem, but note that it is not a general expectation to fail
> open_by_handle() after unlink(), it is an expectation to fail open_by_handle()
> after unlink() + sync() + drop_caches.

Yes, sorry I should have mentioned the sync+drop_caches in the
description.

> I have later converted the test to generic, because I needed to check the
> same expectation for overlayfs use case, which is:
> The original inode is always there (in lower layer), unlink creates a whiteout
> mark and open_by_handle should treat that as ESTALE, otherwise the
> unlinked files would be accessible to nfs clients forever.
> 
> In overlayfs, we handle the open file case by returning a dentry only
> in case the inode with deletion mark in question is already in inode cache,
> but we take care not to populate inode cache with the check.
> It is easier, because we do not need to get inode into cache for checking
> the delete marker.
> 
> Maybe you could instead check in __fh_to_dentry():
> 
>     if (inode->i_nlink == 0 && atomic_read(&inode->i_count) == 1)) {
>         iput(inode);
>         return ERR_PTR(-ESTALE);
>     }
> 
> The above is untested, so I don't know if it's enough to pass generic/426.

Yes, I can confirm that this also fixes the issue -- both tests pass.
__ceph_is_file_opened() uses some internal counters per inode, incremented
each time a file is open in a specific mode.  The problem is that these
counters require some extra locking (maybe they should be atomic_t?), so
you're suggestion is probably better.

> Note that generic/467 also checks the same behavior for rmdir().

Yeah, but the only test-case failing with cephfs is the one described
above (i.e. "open_by_handle -dkr ...").

> If you decide that ceph does not need to comply to this behavior,
> then we probably need to whitelist/blocklist the filesystems that
> want to test this behavior, which will be a shame.

Unless Jeff has any objection, I'm happy sending v2, simplifying the patch
to use your simpler solution (and mentioning sync+drop_caches in the
commit message).

Cheers,
--
Luis