Add FUSE character device

This change adds a FUSE character device backed by devtmpfs. This
device will be used to establish a connection between the FUSE
server daemon and fusefs. The FileDescriptionImpl methods will
be implemented as we flesh out fusefs some more. The tests assert
that the device can be opened and used.

3 files changed, 152 insertions, 0 deletions

diff --git a/pkg/sentry/devices/miscdev/BUILD b/pkg/sentry/devices/miscdev/BUILD
new file mode 100644
index 000000000..aaa76c5d2
--- /dev/null
+++ b/pkg/sentry/devices/miscdev/BUILD
@@ -0,0 +1,20 @@
+load("//tools:defs.bzl", "go_library")
+
+licenses(["notice"])
+
+go_library(
+    name = "miscdev",
+    srcs = [
+        "fuse.go",
+        "miscdev.go",
+    ],
+    visibility = ["//pkg/sentry:internal"],
+    deps = [
+        "//pkg/abi/linux",
+        "//pkg/context",
+        "//pkg/sentry/fsimpl/devtmpfs",
+        "//pkg/sentry/vfs",
+        "//pkg/syserror",
+        "//pkg/usermem",
+    ],
+)
diff --git a/pkg/sentry/devices/miscdev/fuse.go b/pkg/sentry/devices/miscdev/fuse.go
new file mode 100644
index 000000000..d0a963191
--- /dev/null
+++ b/pkg/sentry/devices/miscdev/fuse.go
@@ -0,0 +1,78 @@
+// 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 miscdev
+
+import (
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/usermem"
+)
+
+const fuseDevMinor = 229
+
+// fuseDevice implements vfs.Device for /dev/fuse.
+type fuseDevice struct{}
+
+// Open implements vfs.Device.Open.
+func (fuseDevice) Open(ctx context.Context, mnt *vfs.Mount, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
+	var fd FUSEDeviceFile
+	if err := fd.vfsfd.Init(&fd, opts.Flags, mnt, vfsd, &vfs.FileDescriptionOptions{
+		UseDentryMetadata: true,
+	}); err != nil {
+		return nil, err
+	}
+	return &fd.vfsfd, nil
+}
+
+// FUSEDeviceFile implements vfs.FileDescriptionImpl for /dev/fuse.
+type FUSEDeviceFile struct {
+	vfsfd vfs.FileDescription
+	vfs.FileDescriptionDefaultImpl
+	vfs.DentryMetadataFileDescriptionImpl
+	vfs.NoLockFD
+
+	// TODO(gvisor.dev/issue/2987): Add all the data structures needed to enqueue
+	// and deque requests, control synchronization and establish communication
+	// between the FUSE kernel module and the /dev/fuse character device.
+}
+
+// Release implements vfs.FileDescriptionImpl.Release.
+func (fd *FUSEDeviceFile) Release() {}
+
+// PRead implements vfs.FileDescriptionImpl.PRead.
+func (fd *FUSEDeviceFile) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts vfs.ReadOptions) (int64, error) {
+	return 0, syserror.ENOSYS
+}
+
+// Read implements vfs.FileDescriptionImpl.Read.
+func (fd *FUSEDeviceFile) Read(ctx context.Context, dst usermem.IOSequence, opts vfs.ReadOptions) (int64, error) {
+	return 0, syserror.ENOSYS
+}
+
+// PWrite implements vfs.FileDescriptionImpl.PWrite.
+func (fd *FUSEDeviceFile) PWrite(ctx context.Context, src usermem.IOSequence, offset int64, opts vfs.WriteOptions) (int64, error) {
+	return 0, syserror.ENOSYS
+}
+
+// Write implements vfs.FileDescriptionImpl.Write.
+func (fd *FUSEDeviceFile) Write(ctx context.Context, src usermem.IOSequence, opts vfs.WriteOptions) (int64, error) {
+	return 0, syserror.ENOSYS
+}
+
+// Seek implements vfs.FileDescriptionImpl.Seek.
+func (fd *FUSEDeviceFile) Seek(ctx context.Context, offset int64, whence int32) (int64, error) {
+	return 0, syserror.ENOSYS
+}
diff --git a/pkg/sentry/devices/miscdev/miscdev.go b/pkg/sentry/devices/miscdev/miscdev.go
new file mode 100644
index 000000000..500d92ed9
--- /dev/null
+++ b/pkg/sentry/devices/miscdev/miscdev.go
@@ -0,0 +1,54 @@
+// 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 miscdev implements "misc" character devices, as implemented in Linux
+// by drivers/char/misc.c and fs/fuse/dev.c.
+package miscdev
+
+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"
+)
+
+// miscDevMajor is the major device number for devices defined in this package.
+const miscDevMajor = linux.MISC_MAJOR
+
+// Register registers all devices implemented by this package in vfsObj.
+func Register(vfsObj *vfs.VirtualFilesystem) error {
+	for minor, dev := range map[uint32]vfs.Device{
+		fuseDevMinor: fuseDevice{},
+	} {
+		if err := vfsObj.RegisterDevice(vfs.CharDevice, miscDevMajor, minor, dev, &vfs.RegisterDeviceOptions{
+			GroupName: "misc",
+		}); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// CreateDevtmpfsFiles creates device special files in dev representing all
+// devices implemented by this package.
+func CreateDevtmpfsFiles(ctx context.Context, dev *devtmpfs.Accessor) error {
+	for minor, name := range map[uint32]string{
+		fuseDevMinor: "fuse",
+	} {
+		if err := dev.CreateDeviceFile(ctx, name, vfs.CharDevice, miscDevMajor, minor, 0666 /* mode */); err != nil {
+			return err
+		}
+	}
+	return nil
+}