Merge pull request #5537 from kolyshkin/kir/host-mntns-prop

libct: don't change host mounts propagation

CHANGELOG.md

@@ -28,6 +28,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   mount namespace, and without one, container mounts (such as `/proc`) were
   mounted on top of the host ones. (#5535)
 
+### Fixed ###
+- Running a container which does not have its own mount namespace no longer
+  changes the propagation of host's `/` and of the parent mount of the
+  container rootfs (they were made private or slave). (#5537)
+
 ## [1.6.0-rc.1] - 2026-10-06
 
 > Lo bueno, si breve, dos veces bueno.

libcontainer/rootfs_linux.go

@@ -169,7 +169,11 @@ func setupAndMountToRootfs(pipe *syncSocket, config *configs.Config, mountConfig
 // finalizeRootfs after this function to finish setting up the rootfs.
 func prepareRootfs(pipe *syncSocket, iConfig *initConfig) (err error) {
 	config := iConfig.Config
-	if err := prepareRoot(config); err != nil {
+	prepare := prepareRoot
+	if !config.Namespaces.Contains(configs.NEWNS) {
+		prepare = prepareRootInHostMountNS
+	}
+	if err := prepare(config); err != nil {
 		return fmt.Errorf("error preparing rootfs: %w", err)
 	}
 
@@ -1109,6 +1113,18 @@ func prepareRoot(config *configs.Config) error {
 	return mount(config.Rootfs, config.Rootfs, "bind", unix.MS_BIND|unix.MS_REC, "")
 }
 
+// prepareRootInHostMountNS is like prepareRoot, but for a container that
+// runs in the host mount namespace. The propagation of existing mounts
+// must not be changed, as this would affect the host.
+func prepareRootInHostMountNS(config *configs.Config) error {
+	if err := mount(config.Rootfs, config.Rootfs, "bind", unix.MS_BIND|unix.MS_REC, ""); err != nil {
+		return err
+	}
+	// Make the new rootfs mount a slave, so that container mounts
+	// do not propagate to the host (and other mount namespaces).
+	return mount("", config.Rootfs, "", unix.MS_SLAVE|unix.MS_REC, "")
+}
+
 func setReadonly() error {
 	flags := uintptr(unix.MS_BIND | unix.MS_REMOUNT | unix.MS_RDONLY)
 

tests/integration/host-mntns.bats

@@ -17,6 +17,25 @@ function teardown() {
 	teardown_bundle
 }
 
+# This test goes first, since without the fix, any test which runs
+# a container in the host mount namespace changes the host mounts
+# propagation, so the bug would go unnoticed.
+@test "runc run [host mount ns] must not change host mounts propagation" {
+	update_config '	  .process.args = ["true"]
+			| .linux.namespaces -= [{"type": "mount"}]
+			| .linux.maskedPaths = []
+			| .linux.readonlyPaths = []
+			| .root.readonly = false'
+
+	# Check / and the mount the container bundle resides on.
+	before=$(findmnt -n -o TARGET,PROPAGATION / && findmnt -n -o TARGET,PROPAGATION -T "$ROOT/bundle")
+	run -0 runc run test_host_mntns
+	after=$(findmnt -n -o TARGET,PROPAGATION / && findmnt -n -o TARGET,PROPAGATION -T "$ROOT/bundle")
+	echo "before: $before"
+	echo "after: $after"
+	[ "$before" = "$after" ]
+}
+
 @test "runc run [host mount ns + hooks]" {
 	update_config '	  .process.args = ["/bin/echo", "Hello World"]
 			| .hooks |= . + {"createRuntime": [{"path": "/bin/sh", "args": ["/bin/sh", "-c", "touch createRuntimeHook.$$"]}]}