[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, 28 deletions

diff --git a/pkg/sentry/fsimpl/proc/tasks.go b/pkg/sentry/fsimpl/proc/tasks.go
index d8f5dd509..3259c3732 100644
--- a/pkg/sentry/fsimpl/proc/tasks.go
+++ b/pkg/sentry/fsimpl/proc/tasks.go
@@ -38,10 +38,11 @@ const (
 // +stateify savable
 type tasksInode struct {
 	implStatFS
-	kernfs.AlwaysValid
+	kernfs.InodeAlwaysValid
 	kernfs.InodeAttrs
 	kernfs.InodeDirectoryNoNewChildren
 	kernfs.InodeNotSymlink
+	kernfs.InodeTemporary // This holds no meaning as this inode can't be Looked up and is always valid.
 	kernfs.OrderedChildren
 	tasksInodeRefs
 
@@ -52,8 +53,6 @@ type tasksInode struct {
 
 	// '/proc/self' and '/proc/thread-self' have custom directory offsets in
 	// Linux. So handle them outside of OrderedChildren.
-	selfSymlink       *kernfs.Dentry
-	threadSelfSymlink *kernfs.Dentry
 
 	// cgroupControllers is a map of controller name to directory in the
 	// cgroup hierarchy. These controllers are immutable and will be listed
@@ -63,52 +62,53 @@ type tasksInode struct {
 
 var _ kernfs.Inode = (*tasksInode)(nil)
 
-func (fs *filesystem) newTasksInode(k *kernel.Kernel, pidns *kernel.PIDNamespace, cgroupControllers map[string]string) (*tasksInode, *kernfs.Dentry) {
+func (fs *filesystem) newTasksInode(k *kernel.Kernel, pidns *kernel.PIDNamespace, cgroupControllers map[string]string) *tasksInode {
 	root := auth.NewRootCredentials(pidns.UserNamespace())
-	contents := map[string]*kernfs.Dentry{
-		"cpuinfo":     fs.newDentry(root, fs.NextIno(), 0444, newStaticFileSetStat(cpuInfoData(k))),
-		"filesystems": fs.newDentry(root, fs.NextIno(), 0444, &filesystemsData{}),
-		"loadavg":     fs.newDentry(root, fs.NextIno(), 0444, &loadavgData{}),
+	contents := map[string]kernfs.Inode{
+		"cpuinfo":     fs.newInode(root, 0444, newStaticFileSetStat(cpuInfoData(k))),
+		"filesystems": fs.newInode(root, 0444, &filesystemsData{}),
+		"loadavg":     fs.newInode(root, 0444, &loadavgData{}),
 		"sys":         fs.newSysDir(root, k),
-		"meminfo":     fs.newDentry(root, fs.NextIno(), 0444, &meminfoData{}),
+		"meminfo":     fs.newInode(root, 0444, &meminfoData{}),
 		"mounts":      kernfs.NewStaticSymlink(root, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), "self/mounts"),
 		"net":         kernfs.NewStaticSymlink(root, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), "self/net"),
-		"stat":        fs.newDentry(root, fs.NextIno(), 0444, &statData{}),
-		"uptime":      fs.newDentry(root, fs.NextIno(), 0444, &uptimeData{}),
-		"version":     fs.newDentry(root, fs.NextIno(), 0444, &versionData{}),
+		"stat":        fs.newInode(root, 0444, &statData{}),
+		"uptime":      fs.newInode(root, 0444, &uptimeData{}),
+		"version":     fs.newInode(root, 0444, &versionData{}),
 	}
 
 	inode := &tasksInode{
 		pidns:             pidns,
 		fs:                fs,
-		selfSymlink:       fs.newSelfSymlink(root, fs.NextIno(), pidns),
-		threadSelfSymlink: fs.newThreadSelfSymlink(root, fs.NextIno(), pidns),
 		cgroupControllers: cgroupControllers,
 	}
 	inode.InodeAttrs.Init(root, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), linux.ModeDirectory|0555)
 	inode.EnableLeakCheck()
 
-	dentry := &kernfs.Dentry{}
-	dentry.Init(inode)
-
 	inode.OrderedChildren.Init(kernfs.OrderedChildrenOptions{})
-	links := inode.OrderedChildren.Populate(dentry, contents)
+	links := inode.OrderedChildren.Populate(contents)
 	inode.IncLinks(links)
 
-	return inode, dentry
+	return inode
 }
 
-// Lookup implements kernfs.inodeDynamicLookup.Lookup.
-func (i *tasksInode) Lookup(ctx context.Context, name string) (*kernfs.Dentry, error) {
-	// Try to lookup a corresponding task.
+// Lookup implements kernfs.inodeDirectory.Lookup.
+func (i *tasksInode) Lookup(ctx context.Context, name string) (kernfs.Inode, error) {
+	// Check if a static entry was looked up.
+	if d, err := i.OrderedChildren.Lookup(ctx, name); err == nil {
+		return d, nil
+	}
+
+	// Not a static entry. Try to lookup a corresponding task.
 	tid, err := strconv.ParseUint(name, 10, 64)
 	if err != nil {
+		root := auth.NewRootCredentials(i.pidns.UserNamespace())
 		// If it failed to parse, check if it's one of the special handled files.
 		switch name {
 		case selfName:
-			return i.selfSymlink, nil
+			return i.newSelfSymlink(root), nil
 		case threadSelfName:
-			return i.threadSelfSymlink, nil
+			return i.newThreadSelfSymlink(root), nil
 		}
 		return nil, syserror.ENOENT
 	}
@@ -118,10 +118,10 @@ func (i *tasksInode) Lookup(ctx context.Context, name string) (*kernfs.Dentry, e
 		return nil, syserror.ENOENT
 	}
 
-	return i.fs.newTaskInode(task, i.pidns, true, i.cgroupControllers), nil
+	return i.fs.newTaskInode(task, i.pidns, true, i.cgroupControllers)
 }
 
-// IterDirents implements kernfs.inodeDynamicLookup.IterDirents.
+// IterDirents implements kernfs.inodeDirectory.IterDirents.
 func (i *tasksInode) IterDirents(ctx context.Context, cb vfs.IterDirentsCallback, offset, _ int64) (int64, error) {
 	// fs/proc/internal.h: #define FIRST_PROCESS_ENTRY 256
 	const FIRST_PROCESS_ENTRY = 256
@@ -229,8 +229,8 @@ func (i *tasksInode) Stat(ctx context.Context, vsfs *vfs.Filesystem, opts vfs.St
 }
 
 // DecRef implements kernfs.Inode.DecRef.
-func (i *tasksInode) DecRef(context.Context) {
-	i.tasksInodeRefs.DecRef(i.Destroy)
+func (i *tasksInode) DecRef(ctx context.Context) {
+	i.tasksInodeRefs.DecRef(func() { i.Destroy(ctx) })
 }
 
 // staticFileSetStat implements a special static file that allows inode