Create the cgroupfs mount point in sysfs.

Create the /sys/fs/cgroup directory when cgroups are available. This
creates the empty directory to serve as the mountpoint, actually
mounting cgroups is left to the launcher/userspace. This is consistent
with Linux behaviour.

Without this mountpoint, getdents(2) on /sys/fs indicates an empty
directory even if the launcher mounts cgroupfs at /sys/fs/cgroup. The
launcher can't create the mountpoint directory since sysfs doesn't
support mkdir.

PiperOrigin-RevId: 398596698

1 files changed, 14 insertions, 0 deletions

diff --git a/pkg/sentry/fsimpl/sys/sys_test.go b/pkg/sentry/fsimpl/sys/sys_test.go
index 0a0d914cc..0c46a3a13 100644
--- a/pkg/sentry/fsimpl/sys/sys_test.go
+++ b/pkg/sentry/fsimpl/sys/sys_test.go
@@ -87,3 +87,17 @@ func TestSysRootContainsExpectedEntries(t *testing.T) {
 		"power":    linux.DT_DIR,
 	})
 }
+
+func TestCgroupMountpointExists(t *testing.T) {
+	// Note: The mountpoint is only created if cgroups are available. This is
+	// the VFS2 implementation of sysfs and the test runs with VFS2 enabled, so
+	// we expect to see the mount point unconditionally.
+	s := newTestSystem(t)
+	defer s.Destroy()
+	pop := s.PathOpAtRoot("/fs")
+	s.AssertAllDirentTypes(s.ListDirents(pop), map[string]testutil.DirentType{
+		"cgroup": linux.DT_DIR,
+	})
+	pop = s.PathOpAtRoot("/fs/cgroup")
+	s.AssertAllDirentTypes(s.ListDirents(pop), map[string]testutil.DirentType{ /*empty*/ })
+}