netfilter: qtaguid: initialize a local var to keep compiler happy.

There was a case that might have seemed like new_tag_stat was not
initialized and actually used.
Added comment explaining why it was impossible, and a BUG()
in case the logic gets changed.

Change-Id: I1eddd1b6f754c08a3bf89f7e9427e5dce1dfb081
Signed-off-by: JP Abgrall <[email protected]>

net/netfilter/xt_qtaguid.c

@@ -1265,7 +1265,7 @@ static void if_tag_stat_update(const char *ifname, uid_t uid,
 	struct data_counters *uid_tag_counters;
 	struct sock_tag *sock_tag_entry;
 	struct iface_stat *iface_entry;
-	struct tag_stat *new_tag_stat;
+	struct tag_stat *new_tag_stat = NULL;
 	MT_DEBUG("qtaguid: if_tag_stat_update(ifname=%s "
 		"uid=%u sk=%p dir=%d proto=%d bytes=%d)\n",
 		 ifname, uid, sk, direction, proto, bytes);
@@ -1330,8 +1330,19 @@ static void if_tag_stat_update(const char *ifname, uid_t uid,
 	}
 
 	if (acct_tag) {
+		/* Create the child {acct_tag, uid_tag} and hook up parent. */
 		new_tag_stat = create_if_tag_stat(iface_entry, tag);
 		new_tag_stat->parent_counters = uid_tag_counters;
+	} else {
+		/*
+		 * For new_tag_stat to be still NULL here would require:
+		 *  {0, uid_tag} exists
+		 *  and {acct_tag, uid_tag} doesn't exist
+		 *  AND acct_tag == 0.
+		 * Impossible. This reassures us that new_tag_stat
+		 * below will always be assigned.
+		 */
+		BUG_ON(!new_tag_stat);
 	}
 	tag_stat_update(new_tag_stat, direction, proto, bytes);
 	spin_unlock_bh(&iface_entry->tag_stat_list_lock);