cifs: Fix reacquisition of volume cookie on still-live connection

During mount, cifs_mount_get_tcon() gets a tcon resource connection record
and then attaches an fscache volume cookie to it.  However, it does this
irrespective of whether or not the tcon returned from cifs_get_tcon() is a
new record or one that's already in use.  This leads to a warning about a
volume cookie collision and a leaked volume cookie because tcon->fscache
gets reset.

Fix this be adding a mutex and a "we've already tried this" flag and only
doing it once for the lifetime of the tcon.

[!] Note: Looking at cifs_mount_get_tcon(), a more general solution may
actually be required.  Reacquiring the volume cookie isn't the only thing
that function does: it also partially reinitialises the tcon record without
any locking - which may cause live filesystem ops already using the tcon
through a previous mount to malfunction.

This can be reproduced simply by something like:

    mount //example.com/test /xfstest.test -o user=shares,pass=xxx,fsc
    mount //example.com/test /mnt -o user=shares,pass=xxx,fsc

Fixes: 70431bfd825d ("cifs: Support fscache indexing rewrite")
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
cc: Shyam Prasad N <sprasad@microsoft.com>
cc: linux-cifs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/fs/smb/client/cifsglob.h b/fs/smb/client/cifsglob.h
index d6669ce4ae87..fc09d1c0ee07 100644
--- a/fs/smb/client/cifsglob.h
+++ b/fs/smb/client/cifsglob.h
@@ -1276,7 +1276,9 @@ struct cifs_tcon {
 	__u32 max_cached_dirs;
 #ifdef CONFIG_CIFS_FSCACHE
 	u64 resource_id;		/* server resource id */
+	bool fscache_acquired;		/* T if we've tried acquiring a cookie */
 	struct fscache_volume *fscache;	/* cookie for share */
+	struct mutex fscache_lock;	/* Prevent regetting a cookie */
 #endif
 	struct list_head pending_opens;	/* list of incomplete opens */
 	struct cached_fids *cfids;