Fix AF_UNIX listen() w/ zero backlog.

In https://github.com/google/gvisor/commit/f075522849fa a check to increase zero
to a minimum backlog length was removed from sys_socket.go to bring it in parity
with linux and then in tcp/endpoint.go we bump backlog by 1. But this broke
calling listen on a AF_UNIX socket w/ a zero backlog as in linux it does allow 1
connection even with a zero backlog.

This was caught by a php runtime test socket_abstract_path.phpt.

PiperOrigin-RevId: 369974744

2 files changed, 3 insertions, 7 deletions

diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index 50f72bf38..3a7b2d166 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -2398,10 +2398,6 @@ func (e *endpoint) shutdownLocked(flags tcpip.ShutdownFlags) tcpip.Error {
 // Listen puts the endpoint in "listen" mode, which allows it to accept
 // new connections.
 func (e *endpoint) Listen(backlog int) tcpip.Error {
-	// Accept one more than the configured listen backlog to keep in parity with
-	// Linux. Ref, because of missing equality check here:
-	// https://github.com/torvalds/linux/blob/7acac4b3196/include/net/sock.h#L937
-	backlog++
 	err := e.listen(backlog)
 	if err != nil {
 		if !err.IgnoreStats() {
diff --git a/pkg/tcpip/transport/tcp/tcp_test.go b/pkg/tcpip/transport/tcp/tcp_test.go
index 9f29a48fb..3750b0691 100644
--- a/pkg/tcpip/transport/tcp/tcp_test.go
+++ b/pkg/tcpip/transport/tcp/tcp_test.go
@@ -5736,7 +5736,7 @@ func TestListenSynRcvdQueueFull(t *testing.T) {
 	}
 
 	// Test acceptance.
-	if err := c.EP.Listen(0); err != nil {
+	if err := c.EP.Listen(1); err != nil {
 		t.Fatalf("Listen failed: %s", err)
 	}
 
@@ -5837,7 +5837,7 @@ func TestListenBacklogFullSynCookieInUse(t *testing.T) {
 	}
 
 	// Test for SynCookies usage after filling up the backlog.
-	if err := c.EP.Listen(0); err != nil {
+	if err := c.EP.Listen(1); err != nil {
 		t.Fatalf("Listen failed: %s", err)
 	}
 
@@ -6120,7 +6120,7 @@ func TestPassiveFailedConnectionAttemptIncrement(t *testing.T) {
 	if err := c.EP.Bind(tcpip.FullAddress{Addr: context.StackAddr, Port: context.StackPort}); err != nil {
 		t.Fatalf("Bind failed: %s", err)
 	}
-	if err := c.EP.Listen(0); err != nil {
+	if err := c.EP.Listen(1); err != nil {
 		t.Fatalf("Listen failed: %s", err)
 	}