runc create/run: warn on rootless + shared pidns + no cgroup

Shared pid namespace means `runc kill` (or `runc delete -f`) have to
kill all container processes, not just init. To do so, it needs a cgroup
to read the PIDs from.

If there is no cgroup, processes will be leaked, and so such
configuration is bad and should not be allowed. To keep backward
compatibility, though, let's merely warn about this for now.

Alas, the only way to know if cgroup access is available is by returning
an error from Manager.Apply. Amend fs cgroup managers to do so (systemd
doesn't need it, since v1 can't work with rootless, and cgroup v2 does
not have a special rootless case).

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/cgroups.go

@@ -11,6 +11,11 @@ var (
 	// is not configured to set device rules.
 	ErrDevicesUnsupported = errors.New("cgroup manager is not configured to set device rules")
 
+	// ErrRootless is returned by [Manager.Apply] when there is an error
+	// creating cgroup directory, and cgroup.Rootless is set. In general,
+	// this error is to be ignored.
+	ErrRootless = errors.New("cgroup manager can not access cgroup (rootless container)")
+
 	// DevicesSetV1 and DevicesSetV2 are functions to set devices for
 	// cgroup v1 and v2, respectively. Unless
 	// [github.com/opencontainers/runc/libcontainer/cgroups/devices]

libcontainer/cgroups/fs/fs.go

@@ -105,7 +105,7 @@ func isIgnorableError(rootless bool, err error) bool {
 	return false
 }
 
-func (m *Manager) Apply(pid int) (err error) {
+func (m *Manager) Apply(pid int) (retErr error) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 
@@ -129,6 +129,7 @@ func (m *Manager) Apply(pid int) (err error) {
 			// later by Set, which fails with a friendly error (see
 			// if path == "" in Set).
 			if isIgnorableError(c.Rootless, err) && c.Path == "" {
+				retErr = cgroups.ErrRootless
 				delete(m.paths, name)
 				continue
 			}
@@ -136,7 +137,7 @@ func (m *Manager) Apply(pid int) (err error) {
 		}
 
 	}
-	return nil
+	return retErr
 }
 
 func (m *Manager) Destroy() error {

libcontainer/cgroups/fs2/fs2.go

@@ -71,7 +71,7 @@ func (m *Manager) Apply(pid int) error {
 		if m.config.Rootless {
 			if m.config.Path == "" {
 				if blNeed, nErr := needAnyControllers(m.config.Resources); nErr == nil && !blNeed {
-					return nil
+					return cgroups.ErrRootless
 				}
 				return fmt.Errorf("rootless needs no limits + no cgrouppath when no permission is granted for cgroups: %w", err)
 			}

libcontainer/process_linux.go

@@ -580,7 +580,18 @@ func (p *initProcess) start() (retErr error) {
 	// cgroup. We don't need to worry about not doing this and not being root
 	// because we'd be using the rootless cgroup manager in that case.
 	if err := p.manager.Apply(p.pid()); err != nil {
-		return fmt.Errorf("unable to apply cgroup configuration: %w", err)
+		if errors.Is(err, cgroups.ErrRootless) {
+			// ErrRootless is to be ignored except when
+			// the container doesn't have private pidns.
+			if !p.config.Config.Namespaces.IsPrivate(configs.NEWPID) {
+				// TODO: make this an error in runc 1.3.
+				logrus.Warn("Creating a rootless container with no cgroup and no private pid namespace. " +
+					"Such configuration is strongly discouraged (as it is impossible to properly kill all container's processes) " +
+					"and will result in an error in a future runc version.")
+			}
+		} else {
+			return fmt.Errorf("unable to apply cgroup configuration: %w", err)
+		}
 	}
 	if p.intelRdtManager != nil {
 		if err := p.intelRdtManager.Apply(p.pid()); err != nil {

tests/integration/create.bats

@@ -81,3 +81,31 @@ function teardown() {
 
 	testcontainer test_busybox running
 }
+
+# https://github.com/opencontainers/runc/issues/4394#issuecomment-2334926257
+@test "runc create [shared pidns + rootless]" {
+	# Remove pidns so it's shared with the host.
+	update_config '	  .linux.namespaces -= [{"type": "pid"}]'
+	if [ $EUID -ne 0 ]; then
+		if rootless_cgroup; then
+			# Rootless containers have empty cgroup path by default.
+			set_cgroups_path
+		fi
+		# Can't mount real /proc when rootless + no pidns,
+		# so change it to a bind-mounted one from the host.
+		update_config '	  .mounts |= map((select(.type == "proc")
+                                | .type = "none"
+                                | .source = "/proc"
+                                | .options = ["rbind", "nosuid", "nodev", "noexec"]
+                          ) // .)'
+	fi
+
+	exp="Such configuration is strongly discouraged"
+	runc create --console-socket "$CONSOLE_SOCKET" test
+	[ "$status" -eq 0 ]
+	if [ $EUID -ne 0 ] && ! rootless_cgroup; then
+		[[ "$output" = *"$exp"* ]]
+	else
+		[[ "$output" != *"$exp"* ]]
+	fi
+}