tmpfs and ramfs Dirs should drop references on children in Release().

This is the source of many warnings like:
AtomicRefCount 0x7f5ff84e3500 owned by "fs.Inode" garbage collected with ref count of 1 (want 0)

PiperOrigin-RevId: 261197093

pkg/sentry/fs/ramfs/dir.go

@@ -53,7 +53,6 @@ type Dir struct {
 	fsutil.InodeGenericChecker `state:"nosave"`
 	fsutil.InodeIsDirAllocate  `state:"nosave"`
 	fsutil.InodeIsDirTruncate  `state:"nosave"`
-	fsutil.InodeNoopRelease    `state:"nosave"`
 	fsutil.InodeNoopWriteOut   `state:"nosave"`
 	fsutil.InodeNotMappable    `state:"nosave"`
 	fsutil.InodeNotSocket      `state:"nosave"`
@@ -84,7 +83,8 @@ type Dir struct {
 
 var _ fs.InodeOperations = (*Dir)(nil)
 
-// NewDir returns a new Dir with the given contents and attributes.
+// NewDir returns a new Dir with the given contents and attributes. A reference
+// on each fs.Inode in the `contents` map will be donated to this Dir.
 func NewDir(ctx context.Context, contents map[string]*fs.Inode, owner fs.FileOwner, perms fs.FilePermissions) *Dir {
 	d := &Dir{
 		InodeSimpleAttributes: fsutil.NewInodeSimpleAttributes(ctx, owner, perms, linux.RAMFS_MAGIC),
@@ -138,7 +138,7 @@ func (d *Dir) addChildLocked(ctx context.Context, name string, inode *fs.Inode)
 	d.NotifyModificationAndStatusChange(ctx)
 }
 
-// AddChild adds a child to this dir.
+// AddChild adds a child to this dir, inheriting its reference.
 func (d *Dir) AddChild(ctx context.Context, name string, inode *fs.Inode) {
 	d.mu.Lock()
 	defer d.mu.Unlock()
@@ -172,7 +172,9 @@ func (d *Dir) Children() ([]string, map[string]fs.DentAttr) {
 	return namesCopy, entriesCopy
 }
 
-// removeChildLocked attempts to remove an entry from this directory.
+// removeChildLocked attempts to remove an entry from this directory. It
+// returns the removed fs.Inode along with its reference, which callers are
+// responsible for decrementing.
 func (d *Dir) removeChildLocked(ctx context.Context, name string) (*fs.Inode, error) {
 	inode, ok := d.children[name]
 	if !ok {
@@ -253,7 +255,8 @@ func (d *Dir) RemoveDirectory(ctx context.Context, _ *fs.Inode, name string) err
 	return nil
 }
 
-// Lookup loads an inode at p into a Dirent.
+// Lookup loads an inode at p into a Dirent. It returns the fs.Dirent along
+// with a reference.
 func (d *Dir) Lookup(ctx context.Context, _ *fs.Inode, p string) (*fs.Dirent, error) {
 	if len(p) > linux.NAME_MAX {
 		return nil, syserror.ENAMETOOLONG
@@ -408,6 +411,16 @@ func (*Dir) Rename(ctx context.Context, inode *fs.Inode, oldParent *fs.Inode, ol
 	return Rename(ctx, oldParent.InodeOperations, oldName, newParent.InodeOperations, newName, replacement)
 }
 
+// Release implements fs.InodeOperation.Release.
+func (d *Dir) Release(_ context.Context) {
+	// Drop references on all children.
+	d.mu.Lock()
+	for _, i := range d.children {
+		i.DecRef()
+	}
+	d.mu.Unlock()
+}
+
 // dirFileOperations implements fs.FileOperations for a ramfs directory.
 //
 // +stateify savable

pkg/sentry/fs/tmpfs/tmpfs.go

@@ -56,7 +56,6 @@ func rename(ctx context.Context, oldParent *fs.Inode, oldName string, newParent
 type Dir struct {
 	fsutil.InodeGenericChecker `state:"nosave"`
 	fsutil.InodeIsDirTruncate  `state:"nosave"`
-	fsutil.InodeNoopRelease    `state:"nosave"`
 	fsutil.InodeNoopWriteOut   `state:"nosave"`
 	fsutil.InodeNotMappable    `state:"nosave"`
 	fsutil.InodeNotSocket      `state:"nosave"`
@@ -252,6 +251,11 @@ func (d *Dir) Allocate(ctx context.Context, node *fs.Inode, offset, length int64
 	return d.ramfsDir.Allocate(ctx, node, offset, length)
 }
 
+// Release implements fs.InodeOperations.Release.
+func (d *Dir) Release(ctx context.Context) {
+	d.ramfsDir.Release(ctx)
+}
+
 // Symlink is a symlink.
 //
 // +stateify savable