ssh/tailssh: fix /usr/bin/login args on macOS

Fixes #4931

Signed-off-by: Adam Eijdenberg <adam@continusec.com>
(cherry picked from commit 7f807fef6cfde123ae987cc659a7e89dc74b84c1)

ssh/tailssh/incubator.go

@@ -206,7 +206,7 @@ func beIncubator(args []string) error {
 		// If we are trying to launch a login shell, just exec into login
 		// instead. We can only do this if a TTY was requested, otherwise login
 		// exits immediately, which breaks things likes mosh and VSCode.
-		return unix.Exec(ia.loginCmdPath, []string{ia.loginCmdPath, "-f", ia.localUser, "-h", ia.remoteIP, "-p"}, os.Environ())
+		return unix.Exec(ia.loginCmdPath, ia.loginArgs(), os.Environ())
 	}
 
 	// Inform the system that we are about to log someone in.

ssh/tailssh/incubator_darwin.go

@@ -0,0 +1,9 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tailssh
+
+func (ia *incubatorArgs) loginArgs() []string {
+	return []string{ia.loginCmdPath, "-fp", "-h", ia.remoteIP, ia.localUser}
+}

ssh/tailssh/incubator_linux.go

@@ -173,3 +173,7 @@ func maybeStartLoginSessionLinux(logf logger.Logf, ia incubatorArgs) (func() err
 	}
 	return nil, nil
 }
+
+func (ia *incubatorArgs) loginArgs() []string {
+	return []string{ia.loginCmdPath, "-f", ia.localUser, "-h", ia.remoteIP, "-p"}
+}