Implement cgroupfs.

A skeleton implementation of cgroupfs. It supports trivial cpu and
memory controllers with no support for hierarchies.

PiperOrigin-RevId: 366561126

2 files changed, 14 insertions, 6 deletions

diff --git a/pkg/sentry/vfs/file_description_impl_util.go b/pkg/sentry/vfs/file_description_impl_util.go
index 1556b41a3..b87d9690a 100644
--- a/pkg/sentry/vfs/file_description_impl_util.go
+++ b/pkg/sentry/vfs/file_description_impl_util.go
@@ -252,6 +252,9 @@ type WritableDynamicBytesSource interface {
 // are backed by a bytes.Buffer that is regenerated when necessary, consistent
 // with Linux's fs/seq_file.c:single_open().
 //
+// If data additionally implements WritableDynamicBytesSource, writes are
+// dispatched to the implementer. The source data is not automatically modified.
+//
 // DynamicBytesFileDescriptionImpl.SetDataSource() must be called before first
 // use.
 //
diff --git a/pkg/sentry/vfs/mount.go b/pkg/sentry/vfs/mount.go
index 922f9e697..7cdab6945 100644
--- a/pkg/sentry/vfs/mount.go
+++ b/pkg/sentry/vfs/mount.go
@@ -970,17 +970,22 @@ func superBlockOpts(mountPath string, mnt *Mount) string {
 		opts += "," + mopts
 	}
 
-	// NOTE(b/147673608): If the mount is a cgroup, we also need to include
-	// the cgroup name in the options. For now we just read that from the
-	// path.
+	// NOTE(b/147673608): If the mount is a ramdisk-based fake cgroupfs, we also
+	// need to include the cgroup name in the options. For now we just read that
+	// from the path. Note that this is only possible when "cgroup" isn't
+	// registered as a valid filesystem type.
 	//
-	// TODO(gvisor.dev/issue/190): Once gVisor has full cgroup support, we
-	// should get this value from the cgroup itself, and not rely on the
-	// path.
+	// TODO(gvisor.dev/issue/190): Once we removed fake cgroupfs support, we
+	// should remove this.
+	if cgroupfs := mnt.vfs.getFilesystemType("cgroup"); cgroupfs != nil && cgroupfs.opts.AllowUserMount {
+		// Real cgroupfs available.
+		return opts
+	}
 	if mnt.fs.FilesystemType().Name() == "cgroup" {
 		splitPath := strings.Split(mountPath, "/")
 		cgroupType := splitPath[len(splitPath)-1]
 		opts += "," + cgroupType
 	}
+
 	return opts
 }