From 4a496073b0bec3f07330fd1a0c1697d845b53016 Mon Sep 17 00:00:00 2001
From: Alfonso Acosta <fons@weave.works>
Date: Wed, 30 Mar 2016 23:57:31 +0000
Subject: [PATCH] More robust obtention of host shell command

---
 probe/host/controls.go        |  2 +-
 probe/host/controls_darwin.go |  4 +++-
 probe/host/controls_linux.go  | 26 ++++++++++++++++++--------
 probe/host/reporter.go        | 18 ++++++++++--------
 4 files changed, 32 insertions(+), 18 deletions(-)

diff --git a/probe/host/controls.go b/probe/host/controls.go
index 0ecad9f59..3f666c66c 100644
--- a/probe/host/controls.go
+++ b/probe/host/controls.go
@@ -24,7 +24,7 @@ func (*Reporter) deregisterControls() {
 }
 
 func (r *Reporter) execHost(req xfer.Request) xfer.Response {
-	cmd := exec.Command(hostShellCmd[0], hostShellCmd[1:]...)
+	cmd := exec.Command(r.hostShellCmd[0], r.hostShellCmd[1:]...)
 	cmd.Env = []string{"TERM=xterm"}
 	ptyPipe, err := pty.Start(cmd)
 	if err != nil {
diff --git a/probe/host/controls_darwin.go b/probe/host/controls_darwin.go
index 418293025..cae9c363e 100644
--- a/probe/host/controls_darwin.go
+++ b/probe/host/controls_darwin.go
@@ -1,3 +1,5 @@
 package host
 
-var hostShellCmd = []string{"/bin/bash"}
+func getHostShellCmd() []string {
+	return []string{"/bin/bash"}
+}
diff --git a/probe/host/controls_linux.go b/probe/host/controls_linux.go
index 2a4ef4459..f4870c33d 100644
--- a/probe/host/controls_linux.go
+++ b/probe/host/controls_linux.go
@@ -3,14 +3,14 @@ package host
 import (
 	"bytes"
 	"os/exec"
+	"strings"
 	"syscall"
 
+	log "github.com/Sirupsen/logrus"
 	"github.com/willdonnelly/passwd"
 )
 
-var hostShellCmd []string
-
-func init() {
+func getHostShellCmd() []string {
 	if isProbeContainerized() {
 		// Escape the container namespaces and jump into the ones from
 		// the host's init process.
@@ -19,17 +19,16 @@ func init() {
 		// but it doesn't hurt.
 		readPasswdCmd := []string{"/usr/bin/nsenter", "-t1", "-m", "--no-fork", "cat", "/etc/passwd"}
 		uid, gid, shell := getRootUserDetails(readPasswdCmd)
-		hostShellCmd = []string{
+		return []string{
 			"/usr/bin/nsenter", "-t1", "-m", "-i", "-n", "-p", "--no-fork",
 			"--setuid", uid,
 			"--setgid", gid,
 			shell,
 		}
-		return
 	}
 
 	_, _, shell := getRootUserDetails([]string{"cat", "/etc/passwd"})
-	hostShellCmd = []string{shell}
+	return []string{shell}
 }
 
 func getRootUserDetails(readPasswdCmd []string) (uid, gid, shell string) {
@@ -41,16 +40,23 @@ func getRootUserDetails(readPasswdCmd []string) (uid, gid, shell string) {
 	cmdBuffer := &bytes.Buffer{}
 	cmd.Stdout = cmdBuffer
 	if err := cmd.Run(); err != nil {
+		log.Warnf(
+			"getRootUserDetails(): error running read passwd command %q: %s",
+			strings.Join(readPasswdCmd, " "),
+			err,
+		)
 		return
 	}
 
 	entries, err := passwd.ParseReader(cmdBuffer)
 	if err != nil {
+		log.Warnf("getRootUserDetails(): error parsing passwd: %s", err)
 		return
 	}
 
 	entry, ok := entries["root"]
 	if !ok {
+		log.Warnf("getRootUserDetails(): no root entry in passwd")
 		return
 	}
 
@@ -65,12 +71,16 @@ func isProbeContainerized() bool {
 	// wouldn't have a way to escape the container anyhow).
 	var statT syscall.Stat_t
 
-	if err := syscall.Stat("/proc/self/ns/mnt", &statT); err != nil {
+	path := "/proc/self/ns/mnt"
+	if err := syscall.Stat(path, &statT); err != nil {
+		log.Warnf("isProbeContainerized(): stat() error on %q: %s", path, err)
 		return false
 	}
 	selfMountNamespaceID := statT.Ino
 
-	if err := syscall.Stat("/proc/1/ns/mnt", &statT); err != nil {
+	path = "/proc/1/ns/mnt"
+	if err := syscall.Stat(path, &statT); err != nil {
+		log.Warnf("isProbeContainerized(): stat() error on %q: %s", path, err)
 		return false
 	}
 
diff --git a/probe/host/reporter.go b/probe/host/reporter.go
index 23b7c48e4..04019d376 100644
--- a/probe/host/reporter.go
+++ b/probe/host/reporter.go
@@ -35,20 +35,22 @@ const (
 
 // Reporter generates Reports containing the host topology.
 type Reporter struct {
-	hostID   string
-	hostName string
-	probeID  string
-	pipes    controls.PipeClient
+	hostID       string
+	hostName     string
+	probeID      string
+	pipes        controls.PipeClient
+	hostShellCmd []string
 }
 
 // NewReporter returns a Reporter which produces a report containing host
 // topology for this host.
 func NewReporter(hostID, hostName, probeID string, pipes controls.PipeClient) *Reporter {
 	r := &Reporter{
-		hostID:   hostID,
-		hostName: hostName,
-		probeID:  probeID,
-		pipes:    pipes,
+		hostID:       hostID,
+		hostName:     hostName,
+		probeID:      probeID,
+		pipes:        pipes,
+		hostShellCmd: getHostShellCmd(),
 	}
 	r.registerControls()
 	return r