[vfs2] Add FilesystemType.Release to avoid reference leaks.

Singleton filesystem like devpts and devtmpfs have a single filesystem shared
among all mounts, so they acquire a "self-reference" when initialized that
must be released when the entire virtual filesystem is released at sandbox
exit.

PiperOrigin-RevId: 336828852

pkg/sentry/fsimpl/devpts/BUILD

@@ -35,6 +35,7 @@ go_library(
         "//pkg/refs",
         "//pkg/safemem",
         "//pkg/sentry/arch",
+        "//pkg/sentry/fs",
         "//pkg/sentry/fs/lock",
         "//pkg/sentry/fsimpl/kernfs",
         "//pkg/sentry/kernel",

pkg/sentry/fsimpl/devpts/devpts.go

@@ -37,27 +37,51 @@ const Name = "devpts"
 // FilesystemType implements vfs.FilesystemType.
 //
 // +stateify savable
-type FilesystemType struct{}
+type FilesystemType struct {
+	initOnce sync.Once `state:"nosave"` // FIXME(gvisor.dev/issue/1663): not yet supported.
+	initErr  error
+
+	// fs backs all mounts of this FilesystemType. root is fs' root. fs and root
+	// are immutable.
+	fs   *vfs.Filesystem
+	root *vfs.Dentry
+}
 
 // Name implements vfs.FilesystemType.Name.
-func (FilesystemType) Name() string {
+func (*FilesystemType) Name() string {
 	return Name
 }
 
-var _ vfs.FilesystemType = (*FilesystemType)(nil)
-
 // 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) {
+func (fstype *FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials, source string, opts vfs.GetFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
 	// No data allowed.
 	if opts.Data != "" {
 		return nil, nil, syserror.EINVAL
 	}
 
-	fs, root, err := fstype.newFilesystem(vfsObj, creds)
-	if err != nil {
-		return nil, nil, err
+	fstype.initOnce.Do(func() {
+		fs, root, err := fstype.newFilesystem(vfsObj, creds)
+		if err != nil {
+			fstype.initErr = err
+			return
+		}
+		fstype.fs = fs.VFSFilesystem()
+		fstype.root = root.VFSDentry()
+	})
+	if fstype.initErr != nil {
+		return nil, nil, fstype.initErr
+	}
+	fstype.fs.IncRef()
+	fstype.root.IncRef()
+	return fstype.fs, fstype.root, nil
+}
+
+// Release implements vfs.FilesystemType.Release.
+func (fstype *FilesystemType) Release(ctx context.Context) {
+	if fstype.fs != nil {
+		fstype.root.DecRef(ctx)
+		fstype.fs.DecRef(ctx)
 	}
-	return fs.Filesystem.VFSFilesystem(), root.VFSDentry(), nil
 }
 
 // +stateify savable
@@ -69,7 +93,7 @@ type filesystem struct {
 
 // newFilesystem creates a new devpts filesystem with root directory and ptmx
 // master inode. It returns the filesystem and root Dentry.
-func (fstype FilesystemType) newFilesystem(vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials) (*filesystem, *kernfs.Dentry, error) {
+func (fstype *FilesystemType) newFilesystem(vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials) (*filesystem, *kernfs.Dentry, error) {
 	devMinor, err := vfsObj.GetAnonBlockDevMinor()
 	if err != nil {
 		return nil, nil, err

pkg/sentry/fsimpl/devtmpfs/devtmpfs.go

@@ -71,6 +71,15 @@ func (fst *FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virtua
 	return fst.fs, fst.root, nil
 }
 
+// Release implements vfs.FilesystemType.Release.
+func (fst *FilesystemType) Release(ctx context.Context) {
+	if fst.fs != nil {
+		// Release the original reference obtained when creating the filesystem.
+		fst.root.DecRef(ctx)
+		fst.fs.DecRef(ctx)
+	}
+}
+
 // Accessor allows devices to create device special files in devtmpfs.
 type Accessor struct {
 	vfsObj *vfs.VirtualFilesystem

pkg/sentry/fsimpl/ext/ext.go

@@ -38,9 +38,6 @@ const Name = "ext"
 // +stateify savable
 type FilesystemType struct{}
 
-// Compiles only if FilesystemType implements vfs.FilesystemType.
-var _ vfs.FilesystemType = (*FilesystemType)(nil)
-
 // getDeviceFd returns an io.ReaderAt to the underlying device.
 // Currently there are two ways of mounting an ext(2/3/4) fs:
 //   1. Specify a mount with our internal special MountType in the OCI spec.
@@ -101,6 +98,9 @@ func (FilesystemType) Name() string {
 	return Name
 }
 
+// Release implements vfs.FilesystemType.Release.
+func (FilesystemType) Release(ctx context.Context) {}
+
 // 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) {
 	// TODO(b/134676337): Ensure that the user is mounting readonly. If not,

pkg/sentry/fsimpl/fuse/fusefs.go

@@ -98,6 +98,9 @@ func (FilesystemType) Name() string {
 	return Name
 }
 
+// Release implements vfs.FilesystemType.Release.
+func (FilesystemType) Release(ctx context.Context) {}
+
 // 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) {
 	devMinor, err := vfsObj.GetAnonBlockDevMinor()

pkg/sentry/fsimpl/gofer/gofer.go

@@ -272,6 +272,9 @@ func (FilesystemType) Name() string {
 	return Name
 }
 
+// Release implements vfs.FilesystemType.Release.
+func (FilesystemType) Release(ctx context.Context) {}
+
 // 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) {
 	mfp := pgalloc.MemoryFileProviderFromContext(ctx)

pkg/sentry/fsimpl/host/host.go

@@ -151,6 +151,9 @@ func (filesystemType) Name() string {
 	return "none"
 }
 
+// Release implements vfs.FilesystemType.Release.
+func (filesystemType) Release(ctx context.Context) {}
+
 // NewFilesystem sets up and returns a new hostfs filesystem.
 //
 // Note that there should only ever be one instance of host.filesystem,

pkg/sentry/fsimpl/kernfs/kernfs_test.go

@@ -204,6 +204,8 @@ func (fsType) Name() string {
 	return "kernfs"
 }
 
+func (fsType) Release(ctx context.Context) {}
+
 func (fst fsType) GetFilesystem(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials, source string, opt vfs.GetFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
 	fs := &filesystem{}
 	fs.VFSFilesystem().Init(vfsObj, &fst, fs)

pkg/sentry/fsimpl/overlay/overlay.go

@@ -60,6 +60,9 @@ func (FilesystemType) Name() string {
 	return Name
 }
 
+// Release implements FilesystemType.Release.
+func (FilesystemType) Release(ctx context.Context) {}
+
 // FilesystemOptions may be passed as vfs.GetFilesystemOptions.InternalData to
 // FilesystemType.GetFilesystem.
 //

pkg/sentry/fsimpl/pipefs/pipefs.go

@@ -39,6 +39,9 @@ func (filesystemType) Name() string {
 	return "pipefs"
 }
 
+// Release implements vfs.FilesystemType.Release.
+func (filesystemType) Release(ctx context.Context) {}
+
 // GetFilesystem implements vfs.FilesystemType.GetFilesystem.
 func (filesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials, source string, opts vfs.GetFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
 	panic("pipefs.filesystemType.GetFilesystem should never be called")

pkg/sentry/fsimpl/proc/filesystem.go

@@ -34,13 +34,14 @@ const Name = "proc"
 // +stateify savable
 type FilesystemType struct{}
 
-var _ vfs.FilesystemType = (*FilesystemType)(nil)
-
 // Name implements vfs.FilesystemType.Name.
 func (FilesystemType) Name() string {
 	return Name
 }
 
+// Release implements vfs.FilesystemType.Release.
+func (FilesystemType) Release(ctx context.Context) {}
+
 // +stateify savable
 type filesystem struct {
 	kernfs.Filesystem

pkg/sentry/fsimpl/sockfs/sockfs.go

@@ -46,6 +46,9 @@ func (filesystemType) Name() string {
 	return "sockfs"
 }
 
+// Release implements vfs.FilesystemType.Release.
+func (filesystemType) Release(ctx context.Context) {}
+
 // +stateify savable
 type filesystem struct {
 	kernfs.Filesystem

pkg/sentry/fsimpl/sys/sys.go

@@ -52,6 +52,9 @@ func (FilesystemType) Name() string {
 	return Name
 }
 
+// Release implements vfs.FilesystemType.Release.
+func (FilesystemType) Release(ctx context.Context) {}
+
 // 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) {
 	devMinor, err := vfsObj.GetAnonBlockDevMinor()

pkg/sentry/fsimpl/tmpfs/tmpfs.go

@@ -81,6 +81,9 @@ func (FilesystemType) Name() string {
 	return Name
 }
 
+// Release implements vfs.FilesystemType.Release.
+func (FilesystemType) Release(ctx context.Context) {}
+
 // FilesystemOpts is used to pass configuration data to tmpfs.
 //
 // +stateify savable

pkg/sentry/fsimpl/verity/verity.go

@@ -156,6 +156,9 @@ func isEnabled(d *dentry) bool {
 	return !d.fs.allowRuntimeEnable || len(d.hash) != 0
 }
 
+// Release implements vfs.FilesystemType.Release.
+func (FilesystemType) Release(ctx context.Context) {}
+
 // alertIntegrityViolation alerts a violation of integrity, which usually means
 // unexpected modification to the file system is detected. In
 // noCrashOnVerificationFailure mode, it returns an error, otherwise it panic.

pkg/sentry/vfs/anonfs.go

@@ -61,11 +61,14 @@ func (anonFilesystemType) GetFilesystem(context.Context, *VirtualFilesystem, *au
 	panic("cannot instaniate an anon filesystem")
 }
 
-// Name implemenents FilesystemType.Name.
+// Name implements FilesystemType.Name.
 func (anonFilesystemType) Name() string {
 	return "none"
 }
 
+// Release implemenents FilesystemType.Release.
+func (anonFilesystemType) Release(ctx context.Context) {}
+
 // anonFilesystem is the implementation of FilesystemImpl that backs
 // VirtualDentries returned by VirtualFilesystem.NewAnonVirtualDentry().
 //

pkg/sentry/vfs/filesystem_type.go

@@ -33,6 +33,9 @@ type FilesystemType interface {
 
 	// Name returns the name of this FilesystemType.
 	Name() string
+
+	// Release releases all resources held by this FilesystemType.
+	Release(ctx context.Context)
 }
 
 // GetFilesystemOptions contains options to FilesystemType.GetFilesystem.

pkg/sentry/vfs/vfs.go

@@ -122,13 +122,6 @@ type VirtualFilesystem struct {
 	filesystems   map[*Filesystem]struct{}
 }
 
-// Release drops references on filesystem objects held by vfs.
-//
-// Precondition: This must be called after VFS.Init() has succeeded.
-func (vfs *VirtualFilesystem) Release(ctx context.Context) {
-	vfs.anonMount.DecRef(ctx)
-}
-
 // Init initializes a new VirtualFilesystem with no mounts or FilesystemTypes.
 func (vfs *VirtualFilesystem) Init(ctx context.Context) error {
 	if vfs.mountpoints != nil {
@@ -165,6 +158,16 @@ func (vfs *VirtualFilesystem) Init(ctx context.Context) error {
 	return nil
 }
 
+// Release drops references on filesystem objects held by vfs.
+//
+// Precondition: This must be called after VFS.Init() has succeeded.
+func (vfs *VirtualFilesystem) Release(ctx context.Context) {
+	vfs.anonMount.DecRef(ctx)
+	for _, fst := range vfs.fsTypes {
+		fst.fsType.Release(ctx)
+	}
+}
+
 // PathOperation specifies the path operated on by a VFS method.
 //
 // PathOperation is passed to VFS methods by pointer to reduce memory copying: