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

5 files changed, 26 insertions, 18 deletions

diff --git a/pkg/sentry/fsimpl/memfs/benchmark_test.go b/pkg/sentry/fsimpl/memfs/benchmark_test.go
index 23a846c08..ea6417ce7 100644
--- a/pkg/sentry/fsimpl/memfs/benchmark_test.go
+++ b/pkg/sentry/fsimpl/memfs/benchmark_test.go
@@ -176,7 +176,7 @@ func BenchmarkVFS2MemfsStat(b *testing.B) {
 			// Create VFS.
 			vfsObj := vfs.New()
 			vfsObj.MustRegisterFilesystemType("memfs", memfs.FilesystemType{})
-			mntns, err := vfsObj.NewMountNamespace(ctx, creds, "", "memfs", &vfs.NewFilesystemOptions{})
+			mntns, err := vfsObj.NewMountNamespace(ctx, creds, "", "memfs", &vfs.GetFilesystemOptions{})
 			if err != nil {
 				b.Fatalf("failed to create tmpfs root mount: %v", err)
 			}
@@ -365,7 +365,7 @@ func BenchmarkVFS2MemfsMountStat(b *testing.B) {
 			// Create VFS.
 			vfsObj := vfs.New()
 			vfsObj.MustRegisterFilesystemType("memfs", memfs.FilesystemType{})
-			mntns, err := vfsObj.NewMountNamespace(ctx, creds, "", "memfs", &vfs.NewFilesystemOptions{})
+			mntns, err := vfsObj.NewMountNamespace(ctx, creds, "", "memfs", &vfs.GetFilesystemOptions{})
 			if err != nil {
 				b.Fatalf("failed to create tmpfs root mount: %v", err)
 			}
@@ -394,7 +394,7 @@ func BenchmarkVFS2MemfsMountStat(b *testing.B) {
 			}
 			defer mountPoint.DecRef()
 			// Create and mount the submount.
-			if err := vfsObj.NewMount(ctx, creds, "", &pop, "memfs", &vfs.NewFilesystemOptions{}); err != nil {
+			if err := vfsObj.NewMount(ctx, creds, "", &pop, "memfs", &vfs.GetFilesystemOptions{}); err != nil {
 				b.Fatalf("failed to mount tmpfs submount: %v", err)
 			}
 			filePathBuilder.WriteString(mountPointName)
diff --git a/pkg/sentry/fsimpl/memfs/filesystem.go b/pkg/sentry/fsimpl/memfs/filesystem.go
index f006c40cd..08a9cb8ef 100644
--- a/pkg/sentry/fsimpl/memfs/filesystem.go
+++ b/pkg/sentry/fsimpl/memfs/filesystem.go
@@ -159,7 +159,7 @@ func (fs *filesystem) GetDentryAt(ctx context.Context, rp *vfs.ResolvingPath, op
 			return nil, err
 		}
 	}
-	inode.incRef() // vfsd.IncRef(&fs.vfsfs)
+	inode.incRef()
 	return vfsd, nil
 }
 
@@ -379,6 +379,7 @@ func (i *inode) open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentr
 			return nil, err
 		}
 	}
+	mnt := rp.Mount()
 	switch impl := i.impl.(type) {
 	case *regularFile:
 		var fd regularFileFD
@@ -386,12 +387,14 @@ func (i *inode) open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentr
 		fd.readable = vfs.MayReadFileWithOpenFlags(flags)
 		fd.writable = vfs.MayWriteFileWithOpenFlags(flags)
 		if fd.writable {
-			if err := rp.Mount().CheckBeginWrite(); err != nil {
+			if err := mnt.CheckBeginWrite(); err != nil {
 				return nil, err
 			}
-			// Mount.EndWrite() is called by regularFileFD.Release().
+			// mnt.EndWrite() is called by regularFileFD.Release().
 		}
-		fd.vfsfd.Init(&fd, rp.Mount(), vfsd)
+		mnt.IncRef()
+		vfsd.IncRef()
+		fd.vfsfd.Init(&fd, mnt, vfsd)
 		if flags&linux.O_TRUNC != 0 {
 			impl.mu.Lock()
 			impl.data = impl.data[:0]
@@ -405,7 +408,9 @@ func (i *inode) open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentr
 			return nil, syserror.EISDIR
 		}
 		var fd directoryFD
-		fd.vfsfd.Init(&fd, rp.Mount(), vfsd)
+		mnt.IncRef()
+		vfsd.IncRef()
+		fd.vfsfd.Init(&fd, mnt, vfsd)
 		fd.flags = flags
 		return &fd.vfsfd, nil
 	case *symlink:
diff --git a/pkg/sentry/fsimpl/memfs/memfs.go b/pkg/sentry/fsimpl/memfs/memfs.go
index 64c851c1a..4cb2a4e0f 100644
--- a/pkg/sentry/fsimpl/memfs/memfs.go
+++ b/pkg/sentry/fsimpl/memfs/memfs.go
@@ -52,10 +52,10 @@ type filesystem struct {
 	nextInoMinusOne uint64 // accessed using atomic memory operations
 }
 
-// NewFilesystem implements vfs.FilesystemType.NewFilesystem.
-func (fstype FilesystemType) NewFilesystem(ctx context.Context, creds *auth.Credentials, source string, opts vfs.NewFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
+// GetFilesystem implements vfs.FilesystemType.GetFilesystem.
+func (fstype FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials, source string, opts vfs.GetFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
 	var fs filesystem
-	fs.vfsfs.Init(&fs)
+	fs.vfsfs.Init(vfsObj, &fs)
 	root := fs.newDentry(fs.newDirectory(creds, 01777))
 	return &fs.vfsfs, &root.vfsd, nil
 }
@@ -99,17 +99,17 @@ func (fs *filesystem) newDentry(inode *inode) *dentry {
 }
 
 // IncRef implements vfs.DentryImpl.IncRef.
-func (d *dentry) IncRef(vfsfs *vfs.Filesystem) {
+func (d *dentry) IncRef() {
 	d.inode.incRef()
 }
 
 // TryIncRef implements vfs.DentryImpl.TryIncRef.
-func (d *dentry) TryIncRef(vfsfs *vfs.Filesystem) bool {
+func (d *dentry) TryIncRef() bool {
 	return d.inode.tryIncRef()
 }
 
 // DecRef implements vfs.DentryImpl.DecRef.
-func (d *dentry) DecRef(vfsfs *vfs.Filesystem) {
+func (d *dentry) DecRef() {
 	d.inode.decRef()
 }
 
@@ -266,11 +266,11 @@ type fileDescription struct {
 }
 
 func (fd *fileDescription) filesystem() *filesystem {
-	return fd.vfsfd.VirtualDentry().Mount().Filesystem().Impl().(*filesystem)
+	return fd.vfsfd.Mount().Filesystem().Impl().(*filesystem)
 }
 
 func (fd *fileDescription) inode() *inode {
-	return fd.vfsfd.VirtualDentry().Dentry().Impl().(*dentry).inode
+	return fd.vfsfd.Dentry().Impl().(*dentry).inode
 }
 
 // StatusFlags implements vfs.FileDescriptionImpl.StatusFlags.
diff --git a/pkg/sentry/fsimpl/memfs/named_pipe.go b/pkg/sentry/fsimpl/memfs/named_pipe.go
index 732ed7c58..91cb4b1fc 100644
--- a/pkg/sentry/fsimpl/memfs/named_pipe.go
+++ b/pkg/sentry/fsimpl/memfs/named_pipe.go
@@ -54,6 +54,9 @@ func newNamedPipeFD(ctx context.Context, np *namedPipe, rp *vfs.ResolvingPath, v
 	if err != nil {
 		return nil, err
 	}
-	fd.vfsfd.Init(&fd, rp.Mount(), vfsd)
+	mnt := rp.Mount()
+	mnt.IncRef()
+	vfsd.IncRef()
+	fd.vfsfd.Init(&fd, mnt, vfsd)
 	return &fd.vfsfd, nil
 }
diff --git a/pkg/sentry/fsimpl/memfs/pipe_test.go b/pkg/sentry/fsimpl/memfs/pipe_test.go
index 0674b81a3..a3a870571 100644
--- a/pkg/sentry/fsimpl/memfs/pipe_test.go
+++ b/pkg/sentry/fsimpl/memfs/pipe_test.go
@@ -152,7 +152,7 @@ func setup(t *testing.T) (context.Context, *auth.Credentials, *vfs.VirtualFilesy
 	// Create VFS.
 	vfsObj := vfs.New()
 	vfsObj.MustRegisterFilesystemType("memfs", FilesystemType{})
-	mntns, err := vfsObj.NewMountNamespace(ctx, creds, "", "memfs", &vfs.NewFilesystemOptions{})
+	mntns, err := vfsObj.NewMountNamespace(ctx, creds, "", "memfs", &vfs.GetFilesystemOptions{})
 	if err != nil {
 		t.Fatalf("failed to create tmpfs root mount: %v", err)
 	}