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

6 files changed, 71 insertions, 15 deletions

diff --git a/pkg/sentry/socket/unix/transport/connectioned.go b/pkg/sentry/socket/unix/transport/connectioned.go
index 408dfb08d..33f9aeb06 100644
--- a/pkg/sentry/socket/unix/transport/connectioned.go
+++ b/pkg/sentry/socket/unix/transport/connectioned.go
@@ -346,11 +346,11 @@ func (e *connectionedEndpoint) BidirectionalConnect(ctx context.Context, ce Conn
 
 		return nil
 	default:
-		// Busy; return ECONNREFUSED per spec.
+		// Busy; return EAGAIN per spec.
 		ne.Close(ctx)
 		e.Unlock()
 		ce.Unlock()
-		return syserr.ErrConnectionRefused
+		return syserr.ErrTryAgain
 	}
 }
 
diff --git a/pkg/sentry/syscalls/linux/sys_socket.go b/pkg/sentry/syscalls/linux/sys_socket.go
index eff251cec..5e9e940df 100644
--- a/pkg/sentry/syscalls/linux/sys_socket.go
+++ b/pkg/sentry/syscalls/linux/sys_socket.go
@@ -383,12 +383,19 @@ func Listen(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscal
 		// Linux treats incoming backlog as uint with a limit defined by
 		// sysctl_somaxconn.
 		// https://github.com/torvalds/linux/blob/7acac4b3196/net/socket.c#L1666
-		//
-		// We use the backlog to allocate a channel of that size, hence enforce
-		// a hard limit for the backlog.
 		backlog = maxListenBacklog
 	}
 
+	// 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
+	//
+	// In case of unix domain sockets, the following check
+	// https://github.com/torvalds/linux/blob/7d6beb71da3/net/unix/af_unix.c#L1293
+	// will allow 1 connect through since it checks for a receive queue len >
+	// backlog and not >=.
+	backlog++
+
 	return 0, nil, s.Listen(t, int(backlog)).ToError()
 }
 
diff --git a/pkg/sentry/syscalls/linux/vfs2/socket.go b/pkg/sentry/syscalls/linux/vfs2/socket.go
index 936614eab..6edde0ed1 100644
--- a/pkg/sentry/syscalls/linux/vfs2/socket.go
+++ b/pkg/sentry/syscalls/linux/vfs2/socket.go
@@ -387,12 +387,19 @@ func Listen(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscal
 		// Linux treats incoming backlog as uint with a limit defined by
 		// sysctl_somaxconn.
 		// https://github.com/torvalds/linux/blob/7acac4b3196/net/socket.c#L1666
-		//
-		// We use the backlog to allocate a channel of that size, hence enforce
-		// a hard limit for the backlog.
 		backlog = maxListenBacklog
 	}
 
+	// 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
+	//
+	// In case of unix domain sockets, the following check
+	// https://github.com/torvalds/linux/blob/7d6beb71da3/net/unix/af_unix.c#L1293
+	// will allow 1 connect through since it checks for a receive queue len >
+	// backlog and not >=.
+	backlog++
+
 	return 0, nil, s.Listen(t, int(backlog)).ToError()
 }
 
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)
 	}
 
diff --git a/test/syscalls/linux/socket_unix_unbound_abstract.cc b/test/syscalls/linux/socket_unix_unbound_abstract.cc
index 8b1762000..dd3d25450 100644
--- a/test/syscalls/linux/socket_unix_unbound_abstract.cc
+++ b/test/syscalls/linux/socket_unix_unbound_abstract.cc
@@ -72,6 +72,52 @@ TEST_P(UnboundAbstractUnixSocketPairTest, BindNothing) {
               SyscallSucceeds());
 }
 
+TEST_P(UnboundAbstractUnixSocketPairTest, ListenZeroBacklog) {
+  SKIP_IF((GetParam().type & SOCK_DGRAM) != 0);
+  auto sockets = ASSERT_NO_ERRNO_AND_VALUE(NewSocketPair());
+  struct sockaddr_un addr = {};
+  addr.sun_family = AF_UNIX;
+  constexpr char kPath[] = "\x00/foo_bar";
+  memcpy(addr.sun_path, kPath, sizeof(kPath));
+  ASSERT_THAT(bind(sockets->first_fd(),
+                   reinterpret_cast<struct sockaddr*>(&addr), sizeof(addr)),
+              SyscallSucceeds());
+  ASSERT_THAT(listen(sockets->first_fd(), 0 /* backlog */), SyscallSucceeds());
+  ASSERT_THAT(connect(sockets->second_fd(),
+                      reinterpret_cast<struct sockaddr*>(&addr), sizeof(addr)),
+              SyscallSucceeds());
+  auto sockets2 = ASSERT_NO_ERRNO_AND_VALUE(NewSocketPair());
+  {
+    // Set the FD to O_NONBLOCK.
+    int opts;
+    int orig_opts;
+    ASSERT_THAT(opts = fcntl(sockets2->first_fd(), F_GETFL), SyscallSucceeds());
+    orig_opts = opts;
+    opts |= O_NONBLOCK;
+    ASSERT_THAT(fcntl(sockets2->first_fd(), F_SETFL, opts), SyscallSucceeds());
+
+    ASSERT_THAT(
+        connect(sockets2->first_fd(), reinterpret_cast<struct sockaddr*>(&addr),
+                sizeof(addr)),
+        SyscallFailsWithErrno(EAGAIN));
+  }
+  {
+    // Set the FD to O_NONBLOCK.
+    int opts;
+    int orig_opts;
+    ASSERT_THAT(opts = fcntl(sockets2->second_fd(), F_GETFL),
+                SyscallSucceeds());
+    orig_opts = opts;
+    opts |= O_NONBLOCK;
+    ASSERT_THAT(fcntl(sockets2->second_fd(), F_SETFL, opts), SyscallSucceeds());
+
+    ASSERT_THAT(
+        connect(sockets2->second_fd(),
+                reinterpret_cast<struct sockaddr*>(&addr), sizeof(addr)),
+        SyscallFailsWithErrno(EAGAIN));
+  }
+}
+
 TEST_P(UnboundAbstractUnixSocketPairTest, GetSockNameFullLength) {
   auto sockets = ASSERT_NO_ERRNO_AND_VALUE(NewSocketPair());