af_unix: Detect dead SCC.

When iterating SCC, we call unix_vertex_dead() for each vertex
to check if the vertex is close()d and has no bridge to another
SCC.

If both conditions are true for every vertex in SCC, we can
execute garbage collection for all skb in the SCC.

The actual garbage collection is done in the following patch,
replacing the old implementation.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20240325202425.60930-14-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/unix/garbage.c

@@ -289,6 +289,39 @@ void unix_destroy_fpl(struct scm_fp_list *fpl)
 	unix_free_vertices(fpl);
 }
 
+static bool unix_vertex_dead(struct unix_vertex *vertex)
+{
+	struct unix_edge *edge;
+	struct unix_sock *u;
+	long total_ref;
+
+	list_for_each_entry(edge, &vertex->edges, vertex_entry) {
+		struct unix_vertex *next_vertex = unix_edge_successor(edge);
+
+		/* The vertex's fd can be received by a non-inflight socket. */
+		if (!next_vertex)
+			return false;
+
+		/* The vertex's fd can be received by an inflight socket in
+		 * another SCC.
+		 */
+		if (next_vertex->scc_index != vertex->scc_index)
+			return false;
+	}
+
+	/* No receiver exists out of the same SCC. */
+
+	edge = list_first_entry(&vertex->edges, typeof(*edge), vertex_entry);
+	u = edge->predecessor;
+	total_ref = file_count(u->sk.sk_socket->file);
+
+	/* If not close()d, total_ref > out_degree. */
+	if (total_ref != vertex->out_degree)
+		return false;
+
+	return true;
+}
+
 static bool unix_scc_cyclic(struct list_head *scc)
 {
 	struct unix_vertex *vertex;
@@ -377,6 +410,7 @@ static void __unix_walk_scc(struct unix_vertex *vertex, unsigned long *last_inde
 
 	if (vertex->index == vertex->scc_index) {
 		struct list_head scc;
+		bool scc_dead = true;
 
 		/* SCC finalised.
 		 *
@@ -391,6 +425,9 @@ static void __unix_walk_scc(struct unix_vertex *vertex, unsigned long *last_inde
 
 			/* Mark vertex as off-stack. */
 			vertex->index = unix_vertex_grouped_index;
+
+			if (scc_dead)
+				scc_dead = unix_vertex_dead(vertex);
 		}
 
 		if (!unix_graph_maybe_cyclic)
@@ -431,13 +468,18 @@ static void unix_walk_scc_fast(void)
 	while (!list_empty(&unix_unvisited_vertices)) {
 		struct unix_vertex *vertex;
 		struct list_head scc;
+		bool scc_dead = true;
 
 		vertex = list_first_entry(&unix_unvisited_vertices, typeof(*vertex), entry);
 		list_add(&scc, &vertex->scc_entry);
 
-		list_for_each_entry_reverse(vertex, &scc, scc_entry)
+		list_for_each_entry_reverse(vertex, &scc, scc_entry) {
 			list_move_tail(&vertex->entry, &unix_visited_vertices);
 
+			if (scc_dead)
+				scc_dead = unix_vertex_dead(vertex);
+		}
+
 		list_del(&scc);
 	}