Fix check shell before executing

ssh.go

@@ -14,6 +14,7 @@ import (
 	"strings"
 	"sync"
 	"syscall"
+	"time"
 	"unsafe"
 
 	"github.com/gokrazy/gokrazy"
@@ -220,6 +221,14 @@ type exitStatus struct {
 	Status uint32
 }
 
+func shellWorks(shell string) bool {
+	ctx, cancel := context.WithTimeout(context.Background(), 200*time.Millisecond)
+	defer cancel()
+	cmd := exec.CommandContext(ctx, shell, "-c", "exit 58")
+	cmd.Run()
+	return cmd.ProcessState != nil && cmd.ProcessState.ExitCode() == 58
+}
+
 func findShell() string {
 	if _, err := os.Stat(wellKnownBusybox); err == nil {
 		// Install busybox to /bin to provide the typical userspace utilities
@@ -229,13 +238,13 @@ func findShell() string {
 			// fallthrough, we don't return /bin/sh as we read /etc/passwd
 		}
 	}
-	if _, err := exec.LookPath(shell); path.IsAbs(shell) && err == nil {
+	if _, err := exec.LookPath(shell); path.IsAbs(shell) && shellWorks(shell) && err == nil {
 		return shell
 	}
-	if path, err := exec.LookPath("bash"); err == nil {
+	if path, err := exec.LookPath("bash"); shellWorks(path) && err == nil {
 		return path
 	}
-	if path, err := exec.LookPath("sh"); err == nil {
+	if path, err := exec.LookPath("sh"); shellWorks(path) && err == nil {
 		return path
 	}
 	const wellKnownSerialShell = "/tmp/serial-busybox/ash"