From 6a8318bdb5edd94065a6e2509fa9b3629b73d075 Mon Sep 17 00:00:00 2001
From: Michael Stapelberg <michael@stapelberg.de>
Date: Mon, 25 May 2020 08:58:37 +0200
Subject: [PATCH] pick up shell from github.com/gokrazy/serial-busybox if
 present

This allows users to use breakglass without having to upload their own shell (as
a fallback).
---
 go.mod |  8 ++++++++
 go.sum | 20 ++++++++++++++++++++
 ssh.go | 15 +++++++++++++--
 3 files changed, 41 insertions(+), 2 deletions(-)
 create mode 100644 go.sum

diff --git a/go.mod b/go.mod
index 6ba38eb..7f230f0 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,11 @@
 module github.com/gokrazy/breakglass
 
 go 1.12
+
+require (
+	github.com/gokrazy/gokrazy v0.0.0-20190321081644-520b8ca41de7
+	github.com/gokrazy/internal v0.0.0-20190630091051-de21a662e434 // indirect
+	github.com/google/shlex v0.0.0-20181106134648-c34317bd91bf
+	github.com/kr/pty v1.1.8
+	golang.org/x/crypto v0.0.0-20200429183012-4b2356b1ed79
+)
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..8473f0c
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,20 @@
+github.com/creack/pty v1.1.7 h1:6pwm8kMQKCmgUg0ZHTm5+/YvRK0s3THD/28+T6/kk4A=
+github.com/creack/pty v1.1.7/go.mod h1:lj5s0c3V2DBrqTV7llrYr5NG6My20zk30Fl46Y7DoTY=
+github.com/gokrazy/gokrazy v0.0.0-20190321081644-520b8ca41de7 h1:lpARmqp1bhgN6Co5NJpTwIxQoiz3L1TLzuJqsueDlbY=
+github.com/gokrazy/gokrazy v0.0.0-20190321081644-520b8ca41de7/go.mod h1:YbpshsItGhDXnytFAvMTRvZvGkVSpZV/4mwxQsvqzHg=
+github.com/gokrazy/internal v0.0.0-20190630091051-de21a662e434 h1:3NgMIyCbCOWhjO/9/yIloXsQCuP6MLolya2SItd1NCM=
+github.com/gokrazy/internal v0.0.0-20190630091051-de21a662e434/go.mod h1:c7C8E8dlEJG/vdLtGN5NJPdbKNzZi/puMD0sKC346TI=
+github.com/google/shlex v0.0.0-20181106134648-c34317bd91bf h1:7+FW5aGwISbqUtkfmIpZJGRgNFg2ioYPvFaUxdqpDsg=
+github.com/google/shlex v0.0.0-20181106134648-c34317bd91bf/go.mod h1:RpwtwJQFrIEPstU94h88MWPXP2ektJZ8cZ0YntAmXiE=
+github.com/kr/pty v1.1.8 h1:AkaSdXYQOWeaO3neb8EM634ahkXXe3jYbVh/F9lq+GI=
+github.com/kr/pty v1.1.8/go.mod h1:O1sed60cT9XZ5uDucP5qwvh+TE3NnUj51EiZO/lmSfw=
+golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
+golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550 h1:ObdrDkeb4kJdCP557AjRjq69pTHfNouLtWZG7j9rPN8=
+golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
+golang.org/x/crypto v0.0.0-20200429183012-4b2356b1ed79 h1:IaQbIIB2X/Mp/DKctl6ROxz1KyMlKp4uyvL6+kQ7C88=
+golang.org/x/crypto v0.0.0-20200429183012-4b2356b1ed79/go.mod h1:LzIPMQfyMNhhGPhUkYOs5KpL4U8rLKemX1yGLhDgUto=
+golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
+golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/sys v0.0.0-20190412213103-97732733099d h1:+R4KGOnez64A81RvjARKc4UT5/tI9ujCIVX+P5KiHuI=
+golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
diff --git a/ssh.go b/ssh.go
index 31e67df..d11de8c 100644
--- a/ssh.go
+++ b/ssh.go
@@ -115,6 +115,17 @@ type execR struct {
 	Command string
 }
 
+func findShell() string {
+	if path, err := exec.LookPath("sh"); err == nil {
+		return path
+	}
+	const wellKnownSerialShell = "/tmp/serial-busybox/ash"
+	if _, err := os.Stat(wellKnownSerialShell); err == nil {
+		return wellKnownSerialShell
+	}
+	return ""
+}
+
 func (s *session) request(ctx context.Context, req *ssh.Request) error {
 	switch req.Type {
 	case "pty-req":
@@ -170,8 +181,8 @@ func (s *session) request(ctx context.Context, req *ssh.Request) error {
 		}
 
 		var cmd *exec.Cmd
-		if _, err := exec.LookPath("sh"); err == nil {
-			cmd = exec.CommandContext(ctx, "sh", "-c", r.Command)
+		if shell := findShell(); shell != "" {
+			cmd = exec.CommandContext(ctx, shell, "-c", r.Command)
 		} else {
 			cmd = exec.CommandContext(ctx, cmdline[0], cmdline[1:]...)
 		}