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

3 files changed, 22 insertions, 8 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()
 }