try to install busybox into a tmpfs /bin (with fallback)

This code path requires gokrazy/tools at this commit or newer:
37e2f95c5c

And gokrazy/serial-breakglass at this commit or newer:
bf9bc19235

Afterwards, with an ~/.ssh/config entry like this:

    Host scan2drive
        ProxyCommand breakglass -proxy %h

…using Emacs TRAMP should just work:

    emacs /ssh:scan2drive:/perm/keep/index.md

busybox.go

@@ -0,0 +1,53 @@
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"os/exec"
+	"strings"
+	"syscall"
+)
+
+const wellKnownBusybox = "/usr/local/bin/busybox"
+
+// mountBin bind-mounts /bin to a tmpfs.
+func mountBin() error {
+	b, err := os.ReadFile("/proc/self/mountinfo")
+	if err != nil {
+		return err
+	}
+	for _, line := range strings.Split(strings.TrimSpace(string(b)), "\n") {
+		parts := strings.Fields(line)
+		if len(parts) < 5 {
+			continue
+		}
+		mountpoint := parts[4]
+		log.Printf("Found mountpoint %q", parts[4])
+		if mountpoint == "/bin" {
+			log.Printf("/bin file system already mounted, nothing to do")
+			return nil
+		}
+	}
+
+	if err := syscall.Mount("tmpfs", "/bin", "tmpfs", 0, ""); err != nil {
+		return fmt.Errorf("mounting tmpfs on /bin: %v", err)
+	}
+
+	return nil
+}
+
+func installBusybox() error {
+	// /bin is read-only by default, so mount a tmpfs over it
+	if err := mountBin(); err != nil {
+		return err
+	}
+
+	install := exec.Command(wellKnownBusybox, "--install", "-s", "/bin")
+	install.Stdout = os.Stdout
+	install.Stderr = os.Stderr
+	if err := install.Run(); err != nil {
+		return fmt.Errorf("%v: %v", install.Args, err)
+	}
+	return nil
+}

ssh.go

@@ -215,6 +215,16 @@ type exitStatus struct {
 }
 
 func findShell() string {
+	if _, err := os.Stat(wellKnownBusybox); err == nil {
+		// Install busybox to /bin to provide the typical userspace utilities
+		// in standard locations (makes Emacs TRAMP work, for example).
+		if err := installBusybox(); err != nil {
+			log.Printf("installing busybox failed: %v", err)
+			// fallthrough
+		} else {
+			return "/bin/sh" // available after installation
+		}
+	}
 	if path, err := exec.LookPath("sh"); err == nil {
 		return path
 	}