Fix tmpfs mode opts when dir already exists

When a directory already exists (or after a container is restarted) the
perms of the directory being mounted to were being used even when a
different permission is set on the tmpfs mount options.

This prepends the original directory perms to the mount options.
If the perms were already set in the mount opts then those perms will
win.
This eliminates the need to perform a chmod after mount entirely.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

libcontainer/rootfs_linux.go

@@ -443,29 +443,28 @@ func mountToRootfs(c *mountConfig, m mountEntry) error {
 		}
 		return label.SetFileLabel(dest, mountLabel)
 	case "tmpfs":
-		stat, err := os.Stat(dest)
-		if err != nil {
-			if err := os.MkdirAll(dest, 0o755); err != nil {
+		var mode os.FileMode = 0o755
+		if stat, err := os.Stat(dest); err != nil {
+			if err := os.MkdirAll(dest, mode); err != nil {
 				return err
 			}
+		} else {
+			mode = stat.Mode()
+		}
+
+		dt := fmt.Sprintf("mode=%04o", mode)
+		if m.Data != "" {
+			dt = dt + "," + m.Data
 		}
+		m.Data = dt
 
 		if m.Extensions&configs.EXT_COPYUP == configs.EXT_COPYUP {
 			err = doTmpfsCopyUp(m, rootfs, mountLabel)
 		} else {
 			err = mountPropagate(m, rootfs, mountLabel)
 		}
 
-		if err != nil {
-			return err
-		}
-
-		if stat != nil {
-			if err = os.Chmod(dest, stat.Mode()); err != nil {
-				return err
-			}
-		}
-		return nil
+		return err
 	case "bind":
 		if err := prepareBindMount(m, rootfs); err != nil {
 			return err

tests/integration/run.bats

@@ -76,3 +76,37 @@ function teardown() {
 	[ "$status" -eq 0 ]
 	[[ "${lines[0]}" == *'mydomainname'* ]]
 }
+
+@test "runc run with tmpfs perms" {
+	# shellcheck disable=SC2016
+	update_config '.process.args = ["sh", "-c", "stat -c %a /tmp/test"]'
+	update_config '.mounts += [{"destination": "/tmp/test", "type": "tmpfs", "source": "tmpfs", "options": ["mode=0444"]}]'
+
+	# Directory is to be created by runc.
+	runc run test_tmpfs
+	[ "$status" -eq 0 ]
+	[ "$output" = "444" ]
+
+	# Run a 2nd time with the pre-existing directory.
+	# Ref: https://github.com/opencontainers/runc/issues/3911
+	runc run test_tmpfs
+	[ "$status" -eq 0 ]
+	[ "$output" = "444" ]
+
+	# Existing directory, custom perms, no mode on the mount,
+	# so it should use the directory's perms.
+	update_config '.mounts[-1].options = []'
+	chmod 0710 rootfs/tmp/test
+	# shellcheck disable=SC2016
+	runc run test_tmpfs
+	[ "$status" -eq 0 ]
+	[ "$output" = "710" ]
+
+	# Add back the mode on the mount, and it should use that instead.
+	# Just for fun, use different perms than was used earlier.
+	# shellcheck disable=SC2016
+	update_config '.mounts[-1].options = ["mode=0410"]'
+	runc run test_tmpfs
+	[ "$status" -eq 0 ]
+	[ "$output" = "410" ]
+}