libcontainer: fix a bug when setting shared rootfs propagation mode

So far when the input mount flags contain `MS_SHARED`, the flag has
not been applied to the container rootfs. That's because we call
`rootfsParentMountPrivate()` after applying the original mount flags.
As a result, the original flags are overwritten. Though it's also true
that we actually need to mount the container rootfs with `MS_PRIVATE`,
to avoid failure from `pivot_root()` in the Linux kernel.

Thus if the mount flags contain `MS_SHARED`, we need a special case
handling. First do `pivotRoot()` (or `msMoveRoot`, `chroot`) with the
rootfs with a mount flag `MS_PRIVATE`. Then after `pivotRoot()`, again
mount the rootfs with `MS_SHARED`.

With this fix, `validation/linux_rootfs_propagation.t` of runtime-tools
works well with the shared mode finally.

Fixes https://github.com/opencontainers/runc/issues/1755

Signed-off-by: Dongsu Park <dongsu@kinvolk.io>

libcontainer/rootfs_linux.go

@@ -110,6 +110,15 @@ func prepareRootfs(pipe io.ReadWriter, iConfig *initConfig) (err error) {
 		return newSystemErrorWithCause(err, "jailing process inside rootfs")
 	}
 
+	// mount with MS_SHARED flag does not work well with pivotRoot, because
+	// of the checks in the Linux kernel. So we need to first pivotRoot with
+	// a private rootfs, and after that make it shared.
+	if config.RootPropagation&unix.MS_SHARED != 0 {
+		if err := rootfsParentMountShared(config.Rootfs); err != nil {
+			return err
+		}
+	}
+
 	if setupDev {
 		if err := reOpenDevNull(); err != nil {
 			return newSystemErrorWithCause(err, "reopening /dev/null inside container")
@@ -608,6 +617,31 @@ func rootfsParentMountPrivate(rootfs string) error {
 	return nil
 }
 
+// Make parent mount shared if it was not shared
+func rootfsParentMountShared(rootfs string) error {
+	sharedMount := false
+
+	parentMount, optionalOpts, err := getParentMount(rootfs)
+	if err != nil {
+		return err
+	}
+
+	optsSplit := strings.Split(optionalOpts, " ")
+	for _, opt := range optsSplit {
+		if strings.HasPrefix(opt, "shared:") {
+			sharedMount = true
+			break
+		}
+	}
+
+	// Make parent mount SHARED if it was not shared.
+	if !sharedMount {
+		return unix.Mount("", parentMount, "", unix.MS_SHARED, "")
+	}
+
+	return nil
+}
+
 func prepareRoot(config *configs.Config) error {
 	flag := unix.MS_SLAVE | unix.MS_REC
 	if config.RootPropagation != 0 {