ssh/tailssh: fix regression after LDAP support

58ab66ec51 added LDAP support
for #4945 by shelling out to getdent.

It was supposed to fall back to the old method when getdent wasn't
found, but some variable name confusion (uid vs username) meant the
old path wasn't calling the right lookup function (user.LookupId
instead of user.Lookup).

Which meant that changed probably also broke FreeBSD and macOS SSH
support in addition to the reported OpenWRT regression.

The gokrazy support didn't look right either.

Fixes #8180

Change-Id: I273bbe96fe98b2517fbf0335fd476b483c051554
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ssh/tailssh/tailssh_test.go

@@ -851,7 +851,7 @@ func TestSSH(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	um, err := userLookup(u.Uid)
+	um, err := userLookup(u.Username)
 	if err != nil {
 		t.Fatal(err)
 	}

ssh/tailssh/user.go

@@ -48,19 +48,26 @@ func (u *userMeta) GroupIds() ([]string, error) {
 	return u.User.GroupIds()
 }
 
-// userLookup is like os/user.LookupId but it returns a *userMeta wrapper
+// userLookup is like os/user.Lookup but it returns a *userMeta wrapper
 // around a *user.User with extra fields.
-func userLookup(uid string) (*userMeta, error) {
+func userLookup(username string) (*userMeta, error) {
 	if runtime.GOOS != "linux" {
-		return userLookupStd(uid)
+		return userLookupStd(username)
 	}
 
 	// No getent on Gokrazy. So hard-code the login shell.
 	if distro.Get() == distro.Gokrazy {
-		um, err := userLookupStd(uid)
-		if err == nil {
-			um.loginShellCached = "/tmp/serial-busybox/ash"
+		um, err := userLookupStd(username)
+		if err != nil {
+			um.User = user.User{
+				Uid:      "0",
+				Gid:      "0",
+				Username: "root",
+				Name:     "Gokrazy",
+				HomeDir:  "/",
+			}
 		}
+		um.loginShellCached = "/tmp/serial-busybox/ash"
 		return um, err
 	}
 
@@ -70,7 +77,7 @@ func userLookup(uid string) (*userMeta, error) {
 	// os/user without cgo won't get (because of no libc hooks).
 	// But if "getent" fails, userLookupGetent falls back to the standard
 	// library anyway.
-	return userLookupGetent(uid)
+	return userLookupGetent(username)
 }
 
 func validUsername(uid string) bool {
@@ -85,18 +92,18 @@ func validUsername(uid string) bool {
 	return true
 }
 
-func userLookupGetent(uid string) (*userMeta, error) {
+func userLookupGetent(username string) (*userMeta, error) {
 	// Do some basic validation before passing this string to "getent", even though
 	// getent should do its own validation.
-	if !validUsername(uid) {
+	if !validUsername(username) {
 		return nil, errors.New("invalid username")
 	}
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 	defer cancel()
-	out, err := exec.CommandContext(ctx, "getent", "passwd", uid).Output()
+	out, err := exec.CommandContext(ctx, "getent", "passwd", username).Output()
 	if err != nil {
-		log.Printf("error calling getent for user %q: %v", uid, err)
-		return userLookupStd(uid)
+		log.Printf("error calling getent for user %q: %v", username, err)
+		return userLookupStd(username)
 	}
 	// output is "alice:x:1001:1001:Alice Smith,,,:/home/alice:/bin/bash"
 	f := strings.SplitN(strings.TrimSpace(string(out)), ":", 10)
@@ -116,8 +123,8 @@ func userLookupGetent(uid string) (*userMeta, error) {
 	return um, nil
 }
 
-func userLookupStd(uid string) (*userMeta, error) {
-	u, err := user.LookupId(uid)
+func userLookupStd(username string) (*userMeta, error) {
+	u, err := user.Lookup(username)
 	if err != nil {
 		return nil, err
 	}