[PATCH v4 net-next 08/15] af_unix: Fix up unix_edge.successor for embryo socket.

[Kuniyuki Iwashima <kuniyu@amazon.com>]

To garbage collect inflight AF_UNIX sockets, we must define the
cyclic reference appropriately.  This is a bit tricky if the loop
consists of embryo sockets.

Suppose that the fd of AF_UNIX socket A is passed to D and the fd B
to C and that C and D are embryo sockets of A and B, respectively.
It may appear that there are two separate graphs, A (-> D) and
B (-> C), but this is not correct.

     A --. .-- B
          X
     C <-' `-> D

Now, D holds A's refcount, and C has B's refcount, so unix_release()
will never be called for A and B when we close() them.  However, no
one can call close() for D and C to free skbs holding refcounts of A
and B because C/D is in A/B's receive queue, which should have been
purged by unix_release() for A and B.

So, here's another type of cyclic reference.  When a fd of an AF_UNIX
socket is passed to an embryo socket, the reference is indirectly held
by its parent listening socket.

  .-> A                            .-> B
  |   `- sk_receive_queue          |   `- sk_receive_queue
  |      `- skb                    |      `- skb
  |         `- sk == C             |         `- sk == D
  |            `- sk_receive_queue |           `- sk_receive_queue
  |               `- skb +---------'               `- skb +-.
  |                                                         |
  `---------------------------------------------------------'

Technically, the graph must be denoted as A <-> B instead of A (-> D)
and B (-> C) to find such a cyclic reference without touching each
socket's receive queue.

  .-> A --. .-- B <-.
  |        X        |  ==  A <-> B
  `-- C <-' `-> D --'

We apply this fixup during GC by fetching the real successor by
unix_edge_successor().

When we call accept(), we clear unix_sock.listener under unix_gc_lock
not to confuse GC.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
---
 include/net/af_unix.h |  1 +
 net/unix/af_unix.c    |  2 +-
 net/unix/garbage.c    | 20 +++++++++++++++++++-
 3 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index dc7469191195..414463803b7e 100644
--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h
@@ -24,6 +24,7 @@ void unix_inflight(struct user_struct *user, struct file *fp);
 void unix_notinflight(struct user_struct *user, struct file *fp);
 void unix_add_edges(struct scm_fp_list *fpl, struct unix_sock *receiver);
 void unix_del_edges(struct scm_fp_list *fpl);
+void unix_update_edges(struct unix_sock *receiver);
 int unix_prepare_fpl(struct scm_fp_list *fpl);
 void unix_destroy_fpl(struct scm_fp_list *fpl);
 void unix_gc(void);
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index af74e7ebc35a..ae77e2dc0dae 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1721,7 +1721,7 @@ static int unix_accept(struct socket *sock, struct socket *newsock, int flags,
 	}
 
 	tsk = skb->sk;
-	unix_sk(tsk)->listener = NULL;
+	unix_update_edges(unix_sk(tsk));
 	skb_free_datagram(sk, skb);
 	wake_up_interruptible(&unix_sk(sk)->peer_wait);
 
diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index 33aadaa35346..40a40028261b 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -101,6 +101,17 @@ struct unix_sock *unix_get_socket(struct file *filp)
 	return NULL;
 }
 
+static struct unix_vertex *unix_edge_successor(struct unix_edge *edge)
+{
+	/* If an embryo socket has a fd, the listener has
+	 * indirectly holds the fd's refcnt.
+	 */
+	if (edge->successor->listener)
+		return unix_sk(edge->successor->listener)->vertex;
+
+	return edge->successor->vertex;
+}
+
 static LIST_HEAD(unix_unvisited_vertices);
 
 enum unix_vertex_index {
@@ -209,6 +220,13 @@ void unix_del_edges(struct scm_fp_list *fpl)
 	fpl->inflight = false;
 }
 
+void unix_update_edges(struct unix_sock *receiver)
+{
+	spin_lock(&unix_gc_lock);
+	receiver->listener = NULL;
+	spin_unlock(&unix_gc_lock);
+}
+
 int unix_prepare_fpl(struct scm_fp_list *fpl)
 {
 	struct unix_vertex *vertex;
@@ -268,7 +286,7 @@ static void __unix_walk_scc(struct unix_vertex *vertex)
 
 	/* Explore neighbour vertices (receivers of the current vertex's fd). */
 	list_for_each_entry(edge, &vertex->edges, vertex_entry) {
-		struct unix_vertex *next_vertex = edge->successor->vertex;
+		struct unix_vertex *next_vertex = unix_edge_successor(edge);
 
 		if (!next_vertex)
 			continue;
-- 
2.30.2