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, 16 insertions, 4 deletions

diff --git a/pkg/sentry/vfs/filesystem.go b/pkg/sentry/vfs/filesystem.go
index 7a074b718..76ff8cf51 100644
--- a/pkg/sentry/vfs/filesystem.go
+++ b/pkg/sentry/vfs/filesystem.go
@@ -33,29 +33,41 @@ type Filesystem struct {
 	// operations.
 	refs int64
 
+	// vfs is the VirtualFilesystem that uses this Filesystem. vfs is
+	// immutable.
+	vfs *VirtualFilesystem
+
 	// impl is the FilesystemImpl associated with this Filesystem. impl is
 	// immutable. This should be the last field in Dentry.
 	impl FilesystemImpl
 }
 
 // Init must be called before first use of fs.
-func (fs *Filesystem) Init(impl FilesystemImpl) {
+func (fs *Filesystem) Init(vfsObj *VirtualFilesystem, impl FilesystemImpl) {
 	fs.refs = 1
+	fs.vfs = vfsObj
 	fs.impl = impl
 }
 
+// VirtualFilesystem returns the containing VirtualFilesystem.
+func (fs *Filesystem) VirtualFilesystem() *VirtualFilesystem {
+	return fs.vfs
+}
+
 // Impl returns the FilesystemImpl associated with fs.
 func (fs *Filesystem) Impl() FilesystemImpl {
 	return fs.impl
 }
 
-func (fs *Filesystem) incRef() {
+// IncRef increments fs' reference count.
+func (fs *Filesystem) IncRef() {
 	if atomic.AddInt64(&fs.refs, 1) <= 1 {
-		panic("Filesystem.incRef() called without holding a reference")
+		panic("Filesystem.IncRef() called without holding a reference")
 	}
 }
 
-func (fs *Filesystem) decRef() {
+// DecRef decrements fs' reference count.
+func (fs *Filesystem) DecRef() {
 	if refs := atomic.AddInt64(&fs.refs, -1); refs == 0 {
 		fs.impl.Release()
 	} else if refs < 0 {