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, 143 insertions, 0 deletions

diff --git a/pkg/abi/linux/fuse.go b/pkg/abi/linux/fuse.go
new file mode 100644
index 000000000..d3ebbccc4
--- /dev/null
+++ b/pkg/abi/linux/fuse.go
@@ -0,0 +1,143 @@
+// 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 linux
+
+// +marshal
+type FUSEOpcode uint32
+
+// +marshal
+type FUSEOpID uint64
+
+// Opcodes for FUSE operations. Analogous to the opcodes in include/linux/fuse.h.
+const (
+	FUSE_LOOKUP   FUSEOpcode = 1
+	FUSE_FORGET              = 2 /* no reply */
+	FUSE_GETATTR             = 3
+	FUSE_SETATTR             = 4
+	FUSE_READLINK            = 5
+	FUSE_SYMLINK             = 6
+	_
+	FUSE_MKNOD   = 8
+	FUSE_MKDIR   = 9
+	FUSE_UNLINK  = 10
+	FUSE_RMDIR   = 11
+	FUSE_RENAME  = 12
+	FUSE_LINK    = 13
+	FUSE_OPEN    = 14
+	FUSE_READ    = 15
+	FUSE_WRITE   = 16
+	FUSE_STATFS  = 17
+	FUSE_RELEASE = 18
+	_
+	FUSE_FSYNC        = 20
+	FUSE_SETXATTR     = 21
+	FUSE_GETXATTR     = 22
+	FUSE_LISTXATTR    = 23
+	FUSE_REMOVEXATTR  = 24
+	FUSE_FLUSH        = 25
+	FUSE_INIT         = 26
+	FUSE_OPENDIR      = 27
+	FUSE_READDIR      = 28
+	FUSE_RELEASEDIR   = 29
+	FUSE_FSYNCDIR     = 30
+	FUSE_GETLK        = 31
+	FUSE_SETLK        = 32
+	FUSE_SETLKW       = 33
+	FUSE_ACCESS       = 34
+	FUSE_CREATE       = 35
+	FUSE_INTERRUPT    = 36
+	FUSE_BMAP         = 37
+	FUSE_DESTROY      = 38
+	FUSE_IOCTL        = 39
+	FUSE_POLL         = 40
+	FUSE_NOTIFY_REPLY = 41
+	FUSE_BATCH_FORGET = 42
+)
+
+const (
+	// FUSE_MIN_READ_BUFFER is the minimum size the read can be for any FUSE filesystem.
+	// This is the minimum size Linux supports. See linux.fuse.h.
+	FUSE_MIN_READ_BUFFER uint32 = 8192
+)
+
+// FUSEHeaderIn is the header read by the daemon with each request.
+//
+// +marshal
+type FUSEHeaderIn struct {
+	// Len specifies the total length of the data, including this header.
+	Len uint32
+
+	// Opcode specifies the kind of operation of the request.
+	Opcode FUSEOpcode
+
+	// Unique specifies the unique identifier for this request.
+	Unique FUSEOpID
+
+	// NodeID is the ID of the filesystem object being operated on.
+	NodeID uint64
+
+	// UID is the UID of the requesting process.
+	UID uint32
+
+	// GID is the GID of the requesting process.
+	GID uint32
+
+	// PID is the PID of the requesting process.
+	PID uint32
+
+	_ uint32
+}
+
+// FUSEHeaderOut is the header written by the daemon when it processes
+// a request and wants to send a reply (almost all operations require a
+// reply; if they do not, this will be explicitly documented).
+//
+// +marshal
+type FUSEHeaderOut struct {
+	// Len specifies the total length of the data, including this header.
+	Len uint32
+
+	// Error specifies the error that occurred (0 if none).
+	Error int32
+
+	// Unique specifies the unique identifier of the corresponding request.
+	Unique FUSEOpID
+}
+
+// FUSEWriteIn is the header written by a daemon when it makes a
+// write request to the FUSE filesystem.
+//
+// +marshal
+type FUSEWriteIn struct {
+	// Fh specifies the file handle that is being written to.
+	Fh uint64
+
+	// Offset is the offset of the write.
+	Offset uint64
+
+	// Size is the size of data being written.
+	Size uint32
+
+	// WriteFlags is the flags used during the write.
+	WriteFlags uint32
+
+	// LockOwner is the ID of the lock owner.
+	LockOwner uint64
+
+	// Flags is the flags for the request.
+	Flags uint32
+
+	_ uint32
+}