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, 30 insertions, 3 deletions

diff --git a/pkg/sentry/vfs/file_description.go b/pkg/sentry/vfs/file_description.go
index 3a9665800..34007eb57 100644
--- a/pkg/sentry/vfs/file_description.go
+++ b/pkg/sentry/vfs/file_description.go
@@ -47,15 +47,14 @@ type FileDescription struct {
 	impl FileDescriptionImpl
 }
 
-// Init must be called before first use of fd. It takes references on mnt and
-// d.
+// Init must be called before first use of fd. It takes ownership of references
+// on mnt and d held by the caller.
 func (fd *FileDescription) Init(impl FileDescriptionImpl, mnt *Mount, d *Dentry) {
 	fd.refs = 1
 	fd.vd = VirtualDentry{
 		mount:  mnt,
 		dentry: d,
 	}
-	fd.vd.IncRef()
 	fd.impl = impl
 }
 
@@ -64,6 +63,18 @@ func (fd *FileDescription) Impl() FileDescriptionImpl {
 	return fd.impl
 }
 
+// Mount returns the mount on which fd was opened. It does not take a reference
+// on the returned Mount.
+func (fd *FileDescription) Mount() *Mount {
+	return fd.vd.mount
+}
+
+// Dentry returns the dentry at which fd was opened. It does not take a
+// reference on the returned Dentry.
+func (fd *FileDescription) Dentry() *Dentry {
+	return fd.vd.dentry
+}
+
 // VirtualDentry returns the location at which fd was opened. It does not take
 // a reference on the returned VirtualDentry.
 func (fd *FileDescription) VirtualDentry() VirtualDentry {
@@ -75,6 +86,22 @@ func (fd *FileDescription) IncRef() {
 	atomic.AddInt64(&fd.refs, 1)
 }
 
+// TryIncRef increments fd's reference count and returns true. If fd's
+// reference count is already zero, TryIncRef does nothing and returns false.
+//
+// TryIncRef does not require that a reference is held on fd.
+func (fd *FileDescription) TryIncRef() bool {
+	for {
+		refs := atomic.LoadInt64(&fd.refs)
+		if refs <= 0 {
+			return false
+		}
+		if atomic.CompareAndSwapInt64(&fd.refs, refs, refs+1) {
+			return true
+		}
+	}
+}
+
 // DecRef decrements fd's reference count.
 func (fd *FileDescription) DecRef() {
 	if refs := atomic.AddInt64(&fd.refs, -1); refs == 0 {