[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, 20 insertions, 20 deletions

diff --git a/pkg/sentry/fsimpl/proc/task_net.go b/pkg/sentry/fsimpl/proc/task_net.go
index e7f748655..3425e8698 100644
--- a/pkg/sentry/fsimpl/proc/task_net.go
+++ b/pkg/sentry/fsimpl/proc/task_net.go
@@ -37,12 +37,12 @@ import (
 	"gvisor.dev/gvisor/pkg/usermem"
 )
 
-func (fs *filesystem) newTaskNetDir(task *kernel.Task) *kernfs.Dentry {
+func (fs *filesystem) newTaskNetDir(task *kernel.Task) kernfs.Inode {
 	k := task.Kernel()
 	pidns := task.PIDNamespace()
 	root := auth.NewRootCredentials(pidns.UserNamespace())
 
-	var contents map[string]*kernfs.Dentry
+	var contents map[string]kernfs.Inode
 	if stack := task.NetworkNamespace().Stack(); stack != nil {
 		const (
 			arp       = "IP address       HW type     Flags       HW address            Mask     Device\n"
@@ -56,34 +56,34 @@ func (fs *filesystem) newTaskNetDir(task *kernel.Task) *kernfs.Dentry {
 
 		// TODO(gvisor.dev/issue/1833): Make sure file contents reflect the task
 		// network namespace.
-		contents = map[string]*kernfs.Dentry{
-			"dev":  fs.newDentry(root, fs.NextIno(), 0444, &netDevData{stack: stack}),
-			"snmp": fs.newDentry(root, fs.NextIno(), 0444, &netSnmpData{stack: stack}),
+		contents = map[string]kernfs.Inode{
+			"dev":  fs.newInode(root, 0444, &netDevData{stack: stack}),
+			"snmp": fs.newInode(root, 0444, &netSnmpData{stack: stack}),
 
 			// The following files are simple stubs until they are implemented in
 			// netstack, if the file contains a header the stub is just the header
 			// otherwise it is an empty file.
-			"arp":       fs.newDentry(root, fs.NextIno(), 0444, newStaticFile(arp)),
-			"netlink":   fs.newDentry(root, fs.NextIno(), 0444, newStaticFile(netlink)),
-			"netstat":   fs.newDentry(root, fs.NextIno(), 0444, &netStatData{}),
-			"packet":    fs.newDentry(root, fs.NextIno(), 0444, newStaticFile(packet)),
-			"protocols": fs.newDentry(root, fs.NextIno(), 0444, newStaticFile(protocols)),
+			"arp":       fs.newInode(root, 0444, newStaticFile(arp)),
+			"netlink":   fs.newInode(root, 0444, newStaticFile(netlink)),
+			"netstat":   fs.newInode(root, 0444, &netStatData{}),
+			"packet":    fs.newInode(root, 0444, newStaticFile(packet)),
+			"protocols": fs.newInode(root, 0444, newStaticFile(protocols)),
 
 			// Linux sets psched values to: nsec per usec, psched tick in ns, 1000000,
 			// high res timer ticks per sec (ClockGetres returns 1ns resolution).
-			"psched": fs.newDentry(root, fs.NextIno(), 0444, newStaticFile(psched)),
-			"ptype":  fs.newDentry(root, fs.NextIno(), 0444, newStaticFile(ptype)),
-			"route":  fs.newDentry(root, fs.NextIno(), 0444, &netRouteData{stack: stack}),
-			"tcp":    fs.newDentry(root, fs.NextIno(), 0444, &netTCPData{kernel: k}),
-			"udp":    fs.newDentry(root, fs.NextIno(), 0444, &netUDPData{kernel: k}),
-			"unix":   fs.newDentry(root, fs.NextIno(), 0444, &netUnixData{kernel: k}),
+			"psched": fs.newInode(root, 0444, newStaticFile(psched)),
+			"ptype":  fs.newInode(root, 0444, newStaticFile(ptype)),
+			"route":  fs.newInode(root, 0444, &netRouteData{stack: stack}),
+			"tcp":    fs.newInode(root, 0444, &netTCPData{kernel: k}),
+			"udp":    fs.newInode(root, 0444, &netUDPData{kernel: k}),
+			"unix":   fs.newInode(root, 0444, &netUnixData{kernel: k}),
 		}
 
 		if stack.SupportsIPv6() {
-			contents["if_inet6"] = fs.newDentry(root, fs.NextIno(), 0444, &ifinet6{stack: stack})
-			contents["ipv6_route"] = fs.newDentry(root, fs.NextIno(), 0444, newStaticFile(""))
-			contents["tcp6"] = fs.newDentry(root, fs.NextIno(), 0444, &netTCP6Data{kernel: k})
-			contents["udp6"] = fs.newDentry(root, fs.NextIno(), 0444, newStaticFile(upd6))
+			contents["if_inet6"] = fs.newInode(root, 0444, &ifinet6{stack: stack})
+			contents["ipv6_route"] = fs.newInode(root, 0444, newStaticFile(""))
+			contents["tcp6"] = fs.newInode(root, 0444, &netTCP6Data{kernel: k})
+			contents["udp6"] = fs.newInode(root, 0444, newStaticFile(upd6))
 		}
 	}