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, 42 insertions, 0 deletions

diff --git a/pkg/sentry/fsimpl/fuse/register.go b/pkg/sentry/fsimpl/fuse/register.go
new file mode 100644
index 000000000..b5b581152
--- /dev/null
+++ b/pkg/sentry/fsimpl/fuse/register.go
@@ -0,0 +1,42 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package fuse
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/devtmpfs"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+)
+
+// Register registers the FUSE device with vfsObj.
+func Register(vfsObj *vfs.VirtualFilesystem) error {
+	if err := vfsObj.RegisterDevice(vfs.CharDevice, linux.MISC_MAJOR, fuseDevMinor, fuseDevice{}, &vfs.RegisterDeviceOptions{
+		GroupName: "misc",
+	}); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// CreateDevtmpfsFile creates a device special file in devtmpfs.
+func CreateDevtmpfsFile(ctx context.Context, dev *devtmpfs.Accessor) error {
+	if err := dev.CreateDeviceFile(ctx, "fuse", vfs.CharDevice, linux.MISC_MAJOR, fuseDevMinor, 0666 /* mode */); err != nil {
+		return err
+	}
+
+	return nil
+}