Merge pull request #2445 from alexlarsson/fix-shared-root-from-dm

Fix lxc start for shared root filesystems

container.go

@@ -894,7 +894,13 @@ func (container *Container) Start() (err error) {
 		return err
 	}
 
+	var lxcStart string = "lxc-start"
+	if container.hostConfig.Privileged && container.runtime.capabilities.AppArmor {
+		lxcStart = path.Join(container.runtime.config.Root, "lxc-start-unconfined")
+	}
+
 	params := []string{
+		lxcStart,
 		"-n", container.ID,
 		"-f", container.lxcConfigPath(),
 		"--",
@@ -987,11 +993,24 @@ func (container *Container) Start() (err error) {
 	params = append(params, "--", container.Path)
 	params = append(params, container.Args...)
 
-	var lxcStart string = "lxc-start"
-	if container.hostConfig.Privileged && container.runtime.capabilities.AppArmor {
-		lxcStart = path.Join(container.runtime.config.Root, "lxc-start-unconfined")
+	if RootIsShared() {
+		// lxc-start really needs / to be non-shared, or all kinds of stuff break
+		// when lxc-start unmount things and those unmounts propagate to the main
+		// mount namespace.
+		// What we really want is to clone into a new namespace and then
+		// mount / MS_REC|MS_SLAVE, but since we can't really clone or fork
+		// without exec in go we have to do this horrible shell hack...
+		shellString :=
+			"mount --make-rslave /; exec " +
+				utils.ShellQuoteArguments(params)
+
+		params = []string{
+			"unshare", "-m", "--", "/bin/sh", "-c", shellString,
+		}
 	}
-	container.cmd = exec.Command(lxcStart, params...)
+
+	container.cmd = exec.Command(params[0], params[1:]...)
+
 	// Setup logging of stdout and stderr to disk
 	if err := container.runtime.LogToDisk(container.stdout, container.logPath("json"), "stdout"); err != nil {
 		return err

utils.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"github.com/dotcloud/docker/namesgenerator"
 	"github.com/dotcloud/docker/utils"
+	"io/ioutil"
 	"strconv"
 	"strings"
 )
@@ -310,6 +311,20 @@ func parseLink(rawLink string) (map[string]string, error) {
 	return utils.PartParser("name:alias", rawLink)
 }
 
+func RootIsShared() bool {
+	if data, err := ioutil.ReadFile("/proc/self/mountinfo"); err == nil {
+		for _, line := range strings.Split(string(data), "\n") {
+			cols := strings.Split(line, " ")
+			if len(cols) >= 6 && cols[4] == "/" {
+				return strings.HasPrefix(cols[6], "shared")
+			}
+		}
+	}
+
+	// No idea, probably safe to assume so
+	return true
+}
+
 type checker struct {
 	runtime *Runtime
 }

utils/utils.go

@@ -1179,6 +1179,41 @@ func (e *StatusError) Error() string {
 	return fmt.Sprintf("Status: %d", e.Status)
 }
 
+func quote(word string, buf *bytes.Buffer) {
+	// Bail out early for "simple" strings
+	if word != "" && !strings.ContainsAny(word, "\\'\"`${[|&;<>()~*?! \t\n") {
+		buf.WriteString(word)
+		return
+	}
+
+	buf.WriteString("'")
+
+	for i := 0; i < len(word); i++ {
+		b := word[i]
+		if b == '\'' {
+			// Replace literal ' with a close ', a \', and a open '
+			buf.WriteString("'\\''")
+		} else {
+			buf.WriteByte(b)
+		}
+	}
+
+	buf.WriteString("'")
+}
+
+// Take a list of strings and escape them so they will be handled right
+// when passed as arguments to an program via a shell
+func ShellQuoteArguments(args []string) string {
+	var buf bytes.Buffer
+	for i, arg := range args {
+		if i != 0 {
+			buf.WriteByte(' ')
+		}
+		quote(arg, &buf)
+	}
+	return buf.String()
+}
+
 func IsClosedError(err error) bool {
 	/* This comparison is ugly, but unfortunately, net.go doesn't export errClosing.
 	 * See: