Minor VFS2 interface changes.

- Remove the Filesystem argument from DentryImpl.*Ref(); in general DentryImpls
  that need the Filesystem for reference counting will probably also need it
  for other interface methods that don't plumb Filesystem, so it's easier to
  just store a pointer to the filesystem in the DentryImpl.

- Add a pointer to the VirtualFilesystem to Filesystem, which is needed by the
  gofer client to disown dentries for cache eviction triggered by dentry
  reference count changes.

- Rename FilesystemType.NewFilesystem to GetFilesystem; in some cases (e.g.
  sysfs, cgroupfs) it's much cleaner for there to be only one Filesystem that
  is used by all mounts, and in at least one case (devtmpfs) it's visibly
  incorrect not to do so, so NewFilesystem doesn't always actually create and
  return a *new* Filesystem.

- Require callers of FileDescription.Init() to increment Mount/Dentry
  references. This is because the gofer client may, in the OpenAt() path, take
  a reference on a dentry with 0 references, which is safe due to
  synchronization that is outside the scope of this CL, and it would be safer
  to still have its implementation of DentryImpl.IncRef() check for an
  increment for 0 references in other cases.

- Add FileDescription.TryIncRef. This is used by the gofer client to take
  references on "special file descriptions" (FDs for files such as pipes,
  sockets, and devices), which use per-FD handles (fids) instead of
  dentry-shared handles, for sync() and syncfs().

PiperOrigin-RevId: 282473364

1 files changed, 14 insertions, 10 deletions

diff --git a/pkg/sentry/vfs/dentry.go b/pkg/sentry/vfs/dentry.go
index 09ed5a70e..40f4c1d09 100644
--- a/pkg/sentry/vfs/dentry.go
+++ b/pkg/sentry/vfs/dentry.go
@@ -118,7 +118,7 @@ func (d *Dentry) Impl() DentryImpl {
 type DentryImpl interface {
 	// IncRef increments the Dentry's reference count. A Dentry with a non-zero
 	// reference count must remain coherent with the state of the filesystem.
-	IncRef(fs *Filesystem)
+	IncRef()
 
 	// TryIncRef increments the Dentry's reference count and returns true. If
 	// the Dentry's reference count is zero, TryIncRef may do nothing and
@@ -126,10 +126,10 @@ type DentryImpl interface {
 	// guarantee that the Dentry is coherent with the state of the filesystem.)
 	//
 	// TryIncRef does not require that a reference is held on the Dentry.
-	TryIncRef(fs *Filesystem) bool
+	TryIncRef() bool
 
 	// DecRef decrements the Dentry's reference count.
-	DecRef(fs *Filesystem)
+	DecRef()
 }
 
 // IsDisowned returns true if d is disowned.
@@ -146,16 +146,20 @@ func (d *Dentry) isMounted() bool {
 	return atomic.LoadUint32(&d.mounts) != 0
 }
 
-func (d *Dentry) incRef(fs *Filesystem) {
-	d.impl.IncRef(fs)
+// IncRef increments d's reference count.
+func (d *Dentry) IncRef() {
+	d.impl.IncRef()
 }
 
-func (d *Dentry) tryIncRef(fs *Filesystem) bool {
-	return d.impl.TryIncRef(fs)
+// TryIncRef increments d's reference count and returns true. If d's reference
+// count is zero, TryIncRef may instead do nothing and return false.
+func (d *Dentry) TryIncRef() bool {
+	return d.impl.TryIncRef()
 }
 
-func (d *Dentry) decRef(fs *Filesystem) {
-	d.impl.DecRef(fs)
+// DecRef decrements d's reference count.
+func (d *Dentry) DecRef() {
+	d.impl.DecRef()
 }
 
 // These functions are exported so that filesystem implementations can use
@@ -420,6 +424,6 @@ func (vfs *VirtualFilesystem) forgetDisownedMountpoint(d *Dentry) {
 		vd.DecRef()
 	}
 	for _, mnt := range mountsToDecRef {
-		mnt.decRef()
+		mnt.DecRef()
 	}
 }