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: 70431bfd82 ("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>

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;

fs/smb/client/fscache.c

@@ -43,12 +43,23 @@ int cifs_fscache_get_super_cookie(struct cifs_tcon *tcon)
 	char *key;
 	int ret = -ENOMEM;
 
+	if (tcon->fscache_acquired)
+		return 0;
+
+	mutex_lock(&tcon->fscache_lock);
+	if (tcon->fscache_acquired) {
+		mutex_unlock(&tcon->fscache_lock);
+		return 0;
+	}
+	tcon->fscache_acquired = true;
+
 	tcon->fscache = NULL;
 	switch (sa->sa_family) {
 	case AF_INET:
 	case AF_INET6:
 		break;
 	default:
+		mutex_unlock(&tcon->fscache_lock);
 		cifs_dbg(VFS, "Unknown network family '%d'\n", sa->sa_family);
 		return -EINVAL;
 	}
@@ -57,6 +68,7 @@ int cifs_fscache_get_super_cookie(struct cifs_tcon *tcon)
 
 	sharename = extract_sharename(tcon->tree_name);
 	if (IS_ERR(sharename)) {
+		mutex_unlock(&tcon->fscache_lock);
 		cifs_dbg(FYI, "%s: couldn't extract sharename\n", __func__);
 		return PTR_ERR(sharename);
 	}
@@ -90,6 +102,7 @@ out_2:
 	kfree(key);
 out:
 	kfree(sharename);
+	mutex_unlock(&tcon->fscache_lock);
 	return ret;
 }
 

fs/smb/client/misc.c

@@ -139,6 +139,9 @@ tcon_info_alloc(bool dir_leases_enabled)
 	atomic_set(&ret_buf->num_local_opens, 0);
 	atomic_set(&ret_buf->num_remote_opens, 0);
 	ret_buf->stats_from_time = ktime_get_real_seconds();
+#ifdef CONFIG_CIFS_FSCACHE
+	mutex_init(&ret_buf->fscache_lock);
+#endif
 
 	return ret_buf;
 }