afs: Detect cell aliases 2 - Cells with no root volumes

Implement the second phase of cell alias detection.  This part handles
alias detection for cells that don't have root.cell volumes and so we have
to find some other volume or fileserver to query.

We take the first volume from each such cell and attempt to look it up in
the new cell.  If found, we compare the records, if they are the same, we
judge the cell names to be aliases.
Signed-off-by: David Howells <dhowells@redhat.com>

fs/afs/vl_alias.c

@@ -180,6 +180,94 @@ static int afs_compare_cell_roots(struct afs_cell *cell)
 	return 1;
 }
 
+/*
+ * Query the new cell for a volume from a cell we're already using.
+ */
+static int afs_query_for_alias_one(struct afs_cell *cell, struct key *key,
+				   struct afs_cell *p)
+{
+	struct afs_volume *volume, *pvol = NULL;
+	int ret;
+
+	/* Arbitrarily pick the first volume in the list. */
+	read_lock(&p->proc_lock);
+	if (!list_empty(&p->proc_volumes))
+		pvol = afs_get_volume(list_first_entry(&p->proc_volumes,
+						       struct afs_volume, proc_link));
+	read_unlock(&p->proc_lock);
+	if (!pvol)
+		return 0;
+
+	_enter("%s:%s", cell->name, pvol->name);
+
+	/* And see if it's in the new cell. */
+	volume = afs_sample_volume(cell, key, pvol->name, pvol->name_len);
+	if (IS_ERR(volume)) {
+		afs_put_volume(cell->net, pvol);
+		if (PTR_ERR(volume) != -ENOMEDIUM)
+			return PTR_ERR(volume);
+		/* That volume is not in the new cell, so not an alias */
+		return 0;
+	}
+
+	/* The new cell has a like-named volume also - compare volume ID,
+	 * server and address lists.
+	 */
+	ret = 0;
+	if (pvol->vid == volume->vid) {
+		rcu_read_lock();
+		if (afs_compare_volume_slists(volume, pvol))
+			ret = 1;
+		rcu_read_unlock();
+	}
+
+	afs_put_volume(cell->net, volume);
+	afs_put_volume(cell->net, pvol);
+	return ret;
+}
+
+/*
+ * Query the new cell for volumes we know exist in cells we're already using.
+ */
+static int afs_query_for_alias(struct afs_cell *cell, struct key *key)
+{
+	struct afs_cell *p;
+
+	_enter("%s", cell->name);
+
+	if (mutex_lock_interruptible(&cell->net->proc_cells_lock) < 0)
+		return -ERESTARTSYS;
+
+	hlist_for_each_entry(p, &cell->net->proc_cells, proc_link) {
+		if (p == cell || p->alias_of)
+			continue;
+		if (list_empty(&p->proc_volumes))
+			continue;
+		if (p->root_volume)
+			continue; /* Ignore cells that have a root.cell volume. */
+		afs_get_cell(p);
+		mutex_unlock(&cell->net->proc_cells_lock);
+
+		if (afs_query_for_alias_one(cell, key, p) != 0)
+			goto is_alias;
+
+		if (mutex_lock_interruptible(&cell->net->proc_cells_lock) < 0) {
+			afs_put_cell(cell->net, p);
+			return -ERESTARTSYS;
+		}
+
+		afs_put_cell(cell->net, p);
+	}
+
+	mutex_unlock(&cell->net->proc_cells_lock);
+	_leave(" = 0");
+	return 0;
+
+is_alias:
+	cell->alias_of = p; /* Transfer our ref */
+	return 1;
+}
+
 static int afs_do_cell_detect_alias(struct afs_cell *cell, struct key *key)
 {
 	struct afs_volume *root_volume;
@@ -199,7 +287,7 @@ static int afs_do_cell_detect_alias(struct afs_cell *cell, struct key *key)
 	/* Okay, this cell doesn't have an root.cell volume.  We need to
 	 * locate some other random volume and use that to check.
 	 */
-	return -ENOMEDIUM;
+	return afs_query_for_alias(cell, key);
 }
 
 /*