socket: Add assertions around naked refcount decrements

Sockets in a listen queue hold a reference to the parent listening
socket.  Several code paths release this reference manually when moving
a child socket out of the queue.

Replace comments about the expected post-decrement refcount value with
assertions.  Use refcount_load() instead of a plain load.  No functional
change intended.

MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D31974

1 files changed, 16 insertions, 10 deletions

diff --git a/sys/kern/uipc_socket.c b/sys/kern/uipc_socket.c
index f1a3b5acc827..9e898861f8f4 100644
--- a/sys/kern/uipc_socket.c
+++ b/sys/kern/uipc_socket.c
@@ -1120,11 +1120,12 @@ void
 sofree(struct socket *so)
 {
 	struct protosw *pr = so->so_proto;
+	bool last __diagused;
 
 	SOCK_LOCK_ASSERT(so);
 
-	if ((so->so_state & SS_NOFDREF) == 0 || so->so_count != 0 ||
-	    (so->so_state & SS_PROTOREF) || (so->so_qstate == SQ_COMP)) {
+	if ((so->so_state & (SS_NOFDREF | SS_PROTOREF)) != SS_NOFDREF ||
+	    refcount_load(&so->so_count) != 0 || so->so_qstate == SQ_COMP) {
 		SOCK_UNLOCK(so);
 		return;
 	}
@@ -1160,8 +1161,9 @@ sofree(struct socket *so)
 			    __func__, so, sol));
 			TAILQ_REMOVE(&sol->sol_incomp, so, so_list);
 			sol->sol_incqlen--;
-			/* This is guarenteed not to be the last. */
-			refcount_release(&sol->so_count);
+			last = refcount_release(&sol->so_count);
+			KASSERT(!last, ("%s: released last reference for %p",
+			    __func__, sol));
 			so->so_qstate = SQ_NONE;
 			so->so_listen = NULL;
 		} else
@@ -1169,7 +1171,7 @@ sofree(struct socket *so)
 			    ("%s: so %p not on (in)comp with so_listen",
 			    __func__, so));
 		sorele(sol);
-		KASSERT(so->so_count == 1,
+		KASSERT(refcount_load(&so->so_count) == 1,
 		    ("%s: so %p count %u", __func__, so, so->so_count));
 		so->so_count = 0;
 	}
@@ -1225,6 +1227,7 @@ soclose(struct socket *so)
 	struct accept_queue lqueue;
 	struct socket *sp, *tsp;
 	int error = 0;
+	bool last __diagused;
 
 	KASSERT(!(so->so_state & SS_NOFDREF), ("soclose: SS_NOFDREF on enter"));
 
@@ -1271,8 +1274,9 @@ drop:
 			sp->so_qstate = SQ_NONE;
 			sp->so_listen = NULL;
 			SOCK_UNLOCK(sp);
-			/* Guaranteed not to be the last. */
-			refcount_release(&so->so_count);
+			last = refcount_release(&so->so_count);
+			KASSERT(!last, ("%s: released last reference for %p",
+			    __func__, so));
 		}
 	}
 	KASSERT((so->so_state & SS_NOFDREF) == 0, ("soclose: NOFDREF"));
@@ -1284,7 +1288,7 @@ drop:
 			SOCK_UNLOCK(sp);
 			soabort(sp);
 		} else {
-			/* sp is now in sofree() */
+			/* See the handling of queued sockets in sofree(). */
 			SOCK_UNLOCK(sp);
 		}
 	}
@@ -4010,6 +4014,7 @@ soisconnecting(struct socket *so)
 void
 soisconnected(struct socket *so)
 {
+	bool last __diagused;
 
 	SOCK_LOCK(so);
 	so->so_state &= ~(SS_ISCONNECTING|SS_ISDISCONNECTING|SS_ISCONFIRMING);
@@ -4042,8 +4047,9 @@ soisconnected(struct socket *so)
 				sorele(head);
 				return;
 			}
-			/* Not the last one, as so holds a ref. */
-			refcount_release(&head->so_count);
+			last = refcount_release(&head->so_count);
+			KASSERT(!last, ("%s: released last reference for %p",
+			    __func__, head));
 		}
 again:
 		if ((so->so_options & SO_ACCEPTFILTER) == 0) {