Add device implementation for /dev/fuse

This PR adds the following:
  - [x] Marshall-able structs for fuse headers
  - [x] Data structures needed in /dev/fuse to communicate with the daemon server
  - [x] Implementation of the device interface
  - [x] Go unit tests

This change adds the `/dev/fuse` implementation. `Connection` controls the
communication between the server and the sentry.  The FUSE server uses
the `FileDescription` interface to interact with the Sentry. The Sentry
implmenetation of fusefs, uses `Connection` and the Connection interface
to interact with the Server. All communication messages are in the form
of `go_marshal` backed structs defined in the ABI package.

This change also adds some go unit tests that test (pretty basically)
the interfaces and should be used as an example of an end to end FUSE
operation.

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/gvisor/pull/3083 from ridwanmsharif:ridwanmsharif/fuse-device-impl 69aa2ce970004938fe9f918168dfe57636ab856e
PiperOrigin-RevId: 323428180

1 files changed, 38 insertions, 14 deletions

diff --git a/pkg/sentry/fsimpl/fuse/fusefs.go b/pkg/sentry/fsimpl/fuse/fusefs.go
index f7775fb9b..911b6f7cb 100644
--- a/pkg/sentry/fsimpl/fuse/fusefs.go
+++ b/pkg/sentry/fsimpl/fuse/fusefs.go
@@ -51,6 +51,11 @@ type filesystemOptions struct {
 
 	// rootMode specifies the the file mode of the filesystem's root.
 	rootMode linux.FileMode
+
+	// maxActiveRequests specifies the maximum number of active requests that can
+	// exist at any time. Any further requests will block when trying to
+	// Call the server.
+	maxActiveRequests uint64
 }
 
 // filesystem implements vfs.FilesystemImpl.
@@ -58,12 +63,12 @@ type filesystem struct {
 	kernfs.Filesystem
 	devMinor uint32
 
-	// fuseFD is the FD returned when opening /dev/fuse. It is used for communication
-	// between the FUSE server daemon and the sentry fusefs.
-	fuseFD *DeviceFD
+	// conn is used for communication between the FUSE server
+	// daemon and the sentry fusefs.
+	conn *Connection
 
 	// opts is the options the fusefs is initialized with.
-	opts filesystemOptions
+	opts *filesystemOptions
 }
 
 // Name implements vfs.FilesystemType.Name.
@@ -100,7 +105,7 @@ func (fsType FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 	fuseFd := kernelTask.GetFileVFS2(int32(deviceDescriptor))
 
 	// Parse and set all the other supported FUSE mount options.
-	// TODO: Expand the supported mount options.
+	// TODO(gVisor.dev/issue/3229): Expand the supported mount options.
 	if userIDStr, ok := mopts["user_id"]; ok {
 		delete(mopts, "user_id")
 		userID, err := strconv.ParseUint(userIDStr, 10, 32)
@@ -134,21 +139,20 @@ func (fsType FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 	}
 	fsopts.rootMode = rootMode
 
+	// Set the maxInFlightRequests option.
+	fsopts.maxActiveRequests = MaxActiveRequestsDefault
+
 	// Check for unparsed options.
 	if len(mopts) != 0 {
 		log.Warningf("%s.GetFilesystem: unknown options: %v", fsType.Name(), mopts)
 		return nil, nil, syserror.EINVAL
 	}
 
-	// Mark the device as ready so it can be used. /dev/fuse can only be used if the FD was used to
-	// mount a FUSE filesystem.
-	fuseFD := fuseFd.Impl().(*DeviceFD)
-	fuseFD.mounted = true
-
-	fs := &filesystem{
-		devMinor: devMinor,
-		fuseFD:   fuseFD,
-		opts:     fsopts,
+	// Create a new FUSE filesystem.
+	fs, err := NewFUSEFilesystem(ctx, devMinor, &fsopts, fuseFd)
+	if err != nil {
+		log.Warningf("%s.NewFUSEFilesystem: failed with error: %v", fsType.Name(), err)
+		return nil, nil, err
 	}
 
 	fs.VFSFilesystem().Init(vfsObj, &fsType, fs)
@@ -162,6 +166,26 @@ func (fsType FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 	return fs.VFSFilesystem(), root.VFSDentry(), nil
 }
 
+// NewFUSEFilesystem creates a new FUSE filesystem.
+func NewFUSEFilesystem(ctx context.Context, devMinor uint32, opts *filesystemOptions, device *vfs.FileDescription) (*filesystem, error) {
+	fs := &filesystem{
+		devMinor: devMinor,
+		opts:     opts,
+	}
+
+	conn, err := NewFUSEConnection(ctx, device, opts.maxActiveRequests)
+	if err != nil {
+		log.Warningf("fuse.NewFUSEFilesystem: NewFUSEConnection failed with error: %v", err)
+		return nil, syserror.EINVAL
+	}
+
+	fs.conn = conn
+	fuseFD := device.Impl().(*DeviceFD)
+	fuseFD.fs = fs
+
+	return fs, nil
+}
+
 // Release implements vfs.FilesystemImpl.Release.
 func (fs *filesystem) Release() {
 	fs.Filesystem.VFSFilesystem().VirtualFilesystem().PutAnonBlockDevMinor(fs.devMinor)