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

[Jeff Layton]

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@xxxxxxxx>
> > > ---
> > >  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 cleaning 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?

Thanks,
-- 
Jeff Layton <jlayton@xxxxxxxxxx>