[vfs] kernfs: Fix inode memory leak issue.

This change aims to fix the memory leak issue reported in #3933.

Background:
VFS2 kernfs kept accumulating invalid dentries if those dentries were not
walked on. After substantial consideration of the problem by our team, we
decided to have an LRU cache solution. This change is the first part to that
solution, where we don't cache anything. The LRU cache can be added on top of
this.

What has changed:
- Introduced the concept of an inode tree in kernfs.OrderedChildren.
  This is helpful is cases where the lifecycle of an inode is different from
  that of a dentry.
    - OrderedChildren now deals with initialized inodes instead of initialized
      dentries. It now implements Lookup() where it constructs a new dentry
      using the inode.
    - OrderedChildren holds a ref on all its children inodes. With this change,
      now an inode can "outlive" a dentry pointing to it. See comments in
      kernfs.OrderedChildren.
- The kernfs dentry tree is solely maintained by kernfs only. Inode
  implementations can not modify the dentry tree.
- Dentries that reach ref count 0 are removed from the dentry tree.
- revalidateChildLocked now defer-DecRefs the newly created dentry from
  Inode.Lookup(), limiting its life to the current filesystem operation. If
  refs are picked on the dentry during the FS op (via an FD or something),
  then it will stick around and will be removed when the FD is closed. So there
  is essentially _no caching_ for Look()ed up dentries.
- kernfs.DecRef does not have the precondition that fs.mu must be locked.

Fixes #3933

PiperOrigin-RevId: 336768576

1 files changed, 28 insertions, 35 deletions

diff --git a/pkg/sentry/fsimpl/fuse/fusefs.go b/pkg/sentry/fsimpl/fuse/fusefs.go
index 65786e42a..1a388f54f 100644
--- a/pkg/sentry/fsimpl/fuse/fusefs.go
+++ b/pkg/sentry/fsimpl/fuse/fusefs.go
@@ -249,14 +249,12 @@ func (fs *filesystem) Release(ctx context.Context) {
 // +stateify savable
 type inode struct {
 	inodeRefs
+	kernfs.InodeAlwaysValid
 	kernfs.InodeAttrs
 	kernfs.InodeDirectoryNoNewChildren
-	kernfs.InodeNoDynamicLookup
 	kernfs.InodeNotSymlink
 	kernfs.OrderedChildren
 
-	dentry kernfs.Dentry
-
 	// the owning filesystem. fs is immutable.
 	fs *filesystem
 
@@ -284,26 +282,24 @@ type inode struct {
 }
 
 func (fs *filesystem) newRootInode(creds *auth.Credentials, mode linux.FileMode) *kernfs.Dentry {
-	i := &inode{fs: fs}
+	i := &inode{fs: fs, nodeID: 1}
 	i.InodeAttrs.Init(creds, linux.UNNAMED_MAJOR, fs.devMinor, 1, linux.ModeDirectory|0755)
 	i.OrderedChildren.Init(kernfs.OrderedChildrenOptions{})
 	i.EnableLeakCheck()
-	i.dentry.Init(i)
-	i.nodeID = 1
 
-	return &i.dentry
+	var d kernfs.Dentry
+	d.Init(&fs.Filesystem, i)
+	return &d
 }
 
-func (fs *filesystem) newInode(nodeID uint64, attr linux.FUSEAttr) *kernfs.Dentry {
+func (fs *filesystem) newInode(nodeID uint64, attr linux.FUSEAttr) kernfs.Inode {
 	i := &inode{fs: fs, nodeID: nodeID}
 	creds := auth.Credentials{EffectiveKGID: auth.KGID(attr.UID), EffectiveKUID: auth.KUID(attr.UID)}
 	i.InodeAttrs.Init(&creds, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), linux.FileMode(attr.Mode))
 	atomic.StoreUint64(&i.size, attr.Size)
 	i.OrderedChildren.Init(kernfs.OrderedChildrenOptions{})
 	i.EnableLeakCheck()
-	i.dentry.Init(i)
-
-	return &i.dentry
+	return i
 }
 
 // Open implements kernfs.Inode.Open.
@@ -410,23 +406,27 @@ func (i *inode) Open(ctx context.Context, rp *vfs.ResolvingPath, d *kernfs.Dentr
 }
 
 // Lookup implements kernfs.Inode.Lookup.
-func (i *inode) Lookup(ctx context.Context, name string) (*kernfs.Dentry, error) {
+func (i *inode) Lookup(ctx context.Context, name string) (kernfs.Inode, error) {
 	in := linux.FUSELookupIn{Name: name}
 	return i.newEntry(ctx, name, 0, linux.FUSE_LOOKUP, &in)
 }
 
+// Keep implements kernfs.Inode.Keep.
+func (i *inode) Keep() bool {
+	// Return true so that kernfs keeps the new dentry pointing to this
+	// inode in the dentry tree. This is needed because inodes created via
+	// Lookup are not temporary. They might refer to existing files on server
+	// that can be Unlink'd/Rmdir'd.
+	return true
+}
+
 // IterDirents implements kernfs.Inode.IterDirents.
 func (*inode) IterDirents(ctx context.Context, callback vfs.IterDirentsCallback, offset, relOffset int64) (int64, error) {
 	return offset, nil
 }
 
-// Valid implements kernfs.Inode.Valid.
-func (*inode) Valid(ctx context.Context) bool {
-	return true
-}
-
 // NewFile implements kernfs.Inode.NewFile.
-func (i *inode) NewFile(ctx context.Context, name string, opts vfs.OpenOptions) (*kernfs.Dentry, error) {
+func (i *inode) NewFile(ctx context.Context, name string, opts vfs.OpenOptions) (kernfs.Inode, error) {
 	kernelTask := kernel.TaskFromContext(ctx)
 	if kernelTask == nil {
 		log.Warningf("fusefs.Inode.NewFile: couldn't get kernel task from context", i.nodeID)
@@ -444,7 +444,7 @@ func (i *inode) NewFile(ctx context.Context, name string, opts vfs.OpenOptions)
 }
 
 // NewNode implements kernfs.Inode.NewNode.
-func (i *inode) NewNode(ctx context.Context, name string, opts vfs.MknodOptions) (*kernfs.Dentry, error) {
+func (i *inode) NewNode(ctx context.Context, name string, opts vfs.MknodOptions) (kernfs.Inode, error) {
 	in := linux.FUSEMknodIn{
 		MknodMeta: linux.FUSEMknodMeta{
 			Mode:  uint32(opts.Mode),
@@ -457,7 +457,7 @@ func (i *inode) NewNode(ctx context.Context, name string, opts vfs.MknodOptions)
 }
 
 // NewSymlink implements kernfs.Inode.NewSymlink.
-func (i *inode) NewSymlink(ctx context.Context, name, target string) (*kernfs.Dentry, error) {
+func (i *inode) NewSymlink(ctx context.Context, name, target string) (kernfs.Inode, error) {
 	in := linux.FUSESymLinkIn{
 		Name:   name,
 		Target: target,
@@ -466,7 +466,7 @@ func (i *inode) NewSymlink(ctx context.Context, name, target string) (*kernfs.De
 }
 
 // Unlink implements kernfs.Inode.Unlink.
-func (i *inode) Unlink(ctx context.Context, name string, child *kernfs.Dentry) error {
+func (i *inode) Unlink(ctx context.Context, name string, child kernfs.Inode) error {
 	kernelTask := kernel.TaskFromContext(ctx)
 	if kernelTask == nil {
 		log.Warningf("fusefs.Inode.newEntry: couldn't get kernel task from context", i.nodeID)
@@ -482,14 +482,11 @@ func (i *inode) Unlink(ctx context.Context, name string, child *kernfs.Dentry) e
 		return err
 	}
 	// only return error, discard res.
-	if err := res.Error(); err != nil {
-		return err
-	}
-	return i.dentry.RemoveChildLocked(name, child)
+	return res.Error()
 }
 
 // NewDir implements kernfs.Inode.NewDir.
-func (i *inode) NewDir(ctx context.Context, name string, opts vfs.MkdirOptions) (*kernfs.Dentry, error) {
+func (i *inode) NewDir(ctx context.Context, name string, opts vfs.MkdirOptions) (kernfs.Inode, error) {
 	in := linux.FUSEMkdirIn{
 		MkdirMeta: linux.FUSEMkdirMeta{
 			Mode:  uint32(opts.Mode),
@@ -501,7 +498,7 @@ func (i *inode) NewDir(ctx context.Context, name string, opts vfs.MkdirOptions)
 }
 
 // RmDir implements kernfs.Inode.RmDir.
-func (i *inode) RmDir(ctx context.Context, name string, child *kernfs.Dentry) error {
+func (i *inode) RmDir(ctx context.Context, name string, child kernfs.Inode) error {
 	fusefs := i.fs
 	task, creds := kernel.TaskFromContext(ctx), auth.CredentialsFromContext(ctx)
 
@@ -515,16 +512,12 @@ func (i *inode) RmDir(ctx context.Context, name string, child *kernfs.Dentry) er
 	if err != nil {
 		return err
 	}
-	if err := res.Error(); err != nil {
-		return err
-	}
-
-	return i.dentry.RemoveChildLocked(name, child)
+	return res.Error()
 }
 
 // newEntry calls FUSE server for entry creation and allocates corresponding entry according to response.
 // Shared by FUSE_MKNOD, FUSE_MKDIR, FUSE_SYMLINK, FUSE_LINK and FUSE_LOOKUP.
-func (i *inode) newEntry(ctx context.Context, name string, fileType linux.FileMode, opcode linux.FUSEOpcode, payload marshal.Marshallable) (*kernfs.Dentry, error) {
+func (i *inode) newEntry(ctx context.Context, name string, fileType linux.FileMode, opcode linux.FUSEOpcode, payload marshal.Marshallable) (kernfs.Inode, error) {
 	kernelTask := kernel.TaskFromContext(ctx)
 	if kernelTask == nil {
 		log.Warningf("fusefs.Inode.newEntry: couldn't get kernel task from context", i.nodeID)
@@ -734,8 +727,8 @@ func (i *inode) Stat(ctx context.Context, fs *vfs.Filesystem, opts vfs.StatOptio
 }
 
 // DecRef implements kernfs.Inode.DecRef.
-func (i *inode) DecRef(context.Context) {
-	i.inodeRefs.DecRef(i.Destroy)
+func (i *inode) DecRef(ctx context.Context) {
+	i.inodeRefs.DecRef(func() { i.Destroy(ctx) })
 }
 
 // StatFS implements kernfs.Inode.StatFS.