MountSource.Root() should return a refernce on the dirent.

PiperOrigin-RevId: 202038397
Change-Id: I074d525f2e2d9bcd43b247b62f86f9129c101b78

pkg/sentry/fs/mount.go

@@ -198,10 +198,12 @@ func (msrc *MountSource) Submounts() []*MountSource {
 	return ms
 }
 
-// Root returns the root dirent of this mount.
+// Root returns the root dirent of this mount. Callers must call DecRef on the
+// returned dirent.
 func (msrc *MountSource) Root() *Dirent {
 	msrc.mu.Lock()
 	defer msrc.mu.Unlock()
+	msrc.root.IncRef()
 	return msrc.root
 }
 

pkg/sentry/fs/mount_test.go

@@ -204,7 +204,9 @@ func mountPathsAre(root *Dirent, got []*MountSource, want ...string) error {
 	}
 	gotPaths := make(map[string]struct{}, len(got))
 	for _, g := range got {
-		n, _ := g.Root().FullName(root)
+		groot := g.Root()
+		n, _ := groot.FullName(root)
+		groot.DecRef()
 		gotPaths[n] = struct{}{}
 	}
 	for _, w := range want {

pkg/sentry/fs/proc/mounts.go

@@ -44,7 +44,9 @@ func forEachMountSource(t *kernel.Task, fn func(string, *fs.MountSource)) {
 		return ms[i].ID() < ms[j].ID()
 	})
 	for _, m := range ms {
-		mountPath, desc := m.Root().FullName(rootDir)
+		mroot := m.Root()
+		mountPath, desc := mroot.FullName(rootDir)
+		mroot.DecRef()
 		if !desc {
 			// MountSources that are not descendants of the chroot jail are ignored.
 			continue
@@ -88,7 +90,9 @@ func (mif *mountInfoFile) ReadSeqFileData(handle seqfile.SeqHandle) ([]seqfile.S
 
 		// (3) Major:Minor device ID. We don't have a superblock, so we
 		// just use the root inode device number.
-		sa := m.Root().Inode.StableAttr
+		mroot := m.Root()
+		sa := mroot.Inode.StableAttr
+		mroot.DecRef()
 		fmt.Fprintf(&buf, "%d:%d ", sa.DeviceFileMajor, sa.DeviceFileMinor)
 
 		// (4) Root: the pathname of the directory in the filesystem