Add FUSE_INIT

This change allows the sentry to send FUSE_INIT request and process
the reply. It adds the corresponding structs, employs the fuse
device to send and read the message, and stores the results of negotiation
in corresponding places (inside connection struct).

It adds a CallAsync() function to the FUSE connection interface:

- like Call(), but it's for requests that do not expect immediate response (init, release, interrupt etc.)
- will block if the connection hasn't initialized, which is the same for Call()

6 files changed, 406 insertions, 52 deletions

diff --git a/pkg/sentry/fsimpl/fuse/BUILD b/pkg/sentry/fsimpl/fuse/BUILD
index 67649e811..999111deb 100644
--- a/pkg/sentry/fsimpl/fuse/BUILD
+++ b/pkg/sentry/fsimpl/fuse/BUILD
@@ -21,6 +21,7 @@ go_library(
         "connection.go",
         "dev.go",
         "fusefs.go",
+        "init.go",
         "register.go",
         "request_list.go",
     ],
@@ -44,14 +45,13 @@ go_library(
 )
 
 go_test(
-    name = "dev_test",
+    name = "fuse_test",
     size = "small",
     srcs = ["dev_test.go"],
     library = ":fuse",
     deps = [
         "//pkg/abi/linux",
         "//pkg/sentry/fsimpl/testutil",
-        "//pkg/sentry/fsimpl/tmpfs",
         "//pkg/sentry/kernel",
         "//pkg/sentry/kernel/auth",
         "//pkg/sentry/vfs",
diff --git a/pkg/sentry/fsimpl/fuse/connection.go b/pkg/sentry/fsimpl/fuse/connection.go
index f330da0bd..6df2728ab 100644
--- a/pkg/sentry/fsimpl/fuse/connection.go
+++ b/pkg/sentry/fsimpl/fuse/connection.go
@@ -17,6 +17,8 @@ package fuse
 import (
 	"errors"
 	"fmt"
+	"sync"
+	"sync/atomic"
 	"syscall"
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
@@ -25,18 +27,29 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/kernel"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
 	"gvisor.dev/gvisor/pkg/waiter"
 	"gvisor.dev/gvisor/tools/go_marshal/marshal"
 )
 
-// MaxActiveRequestsDefault is the default setting controlling the upper bound
+// maxActiveRequestsDefault is the default setting controlling the upper bound
 // on the number of active requests at any given time.
-const MaxActiveRequestsDefault = 10000
+const maxActiveRequestsDefault = 10000
 
-var (
-	// Ordinary requests have even IDs, while interrupts IDs are odd.
-	InitReqBit uint64 = 1
-	ReqIDStep  uint64 = 2
+// Ordinary requests have even IDs, while interrupts IDs are odd.
+// Used to increment the unique ID for each FUSE request.
+var reqIDStep uint64 = 2
+
+const (
+	// fuseDefaultMaxBackground is the default value for MaxBackground.
+	fuseDefaultMaxBackground = 12
+
+	// fuseDefaultCongestionThreshold is the default value for CongestionThreshold,
+	// and is 75% of the default maximum of MaxGround.
+	fuseDefaultCongestionThreshold = (fuseDefaultMaxBackground * 3 / 4)
+
+	// fuseDefaultMaxPagesPerReq is the default value for MaxPagesPerReq.
+	fuseDefaultMaxPagesPerReq = 32
 )
 
 // Request represents a FUSE operation request that hasn't been sent to the
@@ -61,17 +74,125 @@ type Response struct {
 	data   []byte
 }
 
-// Connection is the struct by which the sentry communicates with the FUSE server daemon.
-type Connection struct {
+// connection is the struct by which the sentry communicates with the FUSE server daemon.
+type connection struct {
 	fd *DeviceFD
 
-	// MaxWrite is the daemon's maximum size of a write buffer.
-	// This is negotiated during FUSE_INIT.
-	MaxWrite uint32
+	// The following FUSE_INIT flags are currently unsupported by this implementation:
+	// - FUSE_ATOMIC_O_TRUNC: requires open(..., O_TRUNC)
+	// - FUSE_EXPORT_SUPPORT
+	// - FUSE_HANDLE_KILLPRIV
+	// - FUSE_POSIX_LOCKS: requires POSIX locks
+	// - FUSE_FLOCK_LOCKS: requires POSIX locks
+	// - FUSE_AUTO_INVAL_DATA: requires page caching eviction
+	// - FUSE_EXPLICIT_INVAL_DATA: requires page caching eviction
+	// - FUSE_DO_READDIRPLUS/FUSE_READDIRPLUS_AUTO: requires FUSE_READDIRPLUS implementation
+	// - FUSE_ASYNC_DIO
+	// - FUSE_POSIX_ACL: affects defaultPermissions, posixACL, xattr handler
+
+	// initialized after receiving FUSE_INIT reply.
+	// Until it's set, suspend sending FUSE requests.
+	// Use SetInitialized() and IsInitialized() for atomic access.
+	initialized int32
+
+	// initializedChan is used to block requests before initialization.
+	initializedChan chan struct{}
+
+	// blocked when there are too many outstading backgrounds requests (NumBackground == MaxBackground).
+	// TODO(gvisor.dev/issue/3185): update the numBackground accordingly; use a channel to block.
+	blocked bool
+
+	// connected (connection established) when a new FUSE file system is created.
+	// Set to false when:
+	//   umount,
+	//   connection abort,
+	//   device release.
+	connected bool
+
+	// aborted via sysfs.
+	// TODO(gvisor.dev/issue/3185): abort all queued requests.
+	aborted bool
+
+	// connInitError if FUSE_INIT encountered error (major version mismatch).
+	// Only set in INIT.
+	connInitError bool
+
+	// connInitSuccess if FUSE_INIT is successful.
+	// Only set in INIT.
+	// Used for destory.
+	connInitSuccess bool
+
+	// TODO(gvisor.dev/issue/3185): All the queue logic are working in progress.
+
+	// NumberBackground is the number of requests in the background.
+	numBackground uint16
+
+	// congestionThreshold for NumBackground.
+	// Negotiated in FUSE_INIT.
+	congestionThreshold uint16
+
+	// maxBackground is the maximum number of NumBackground.
+	// Block connection when it is reached.
+	// Negotiated in FUSE_INIT.
+	maxBackground uint16
+
+	// numActiveBackground is the number of requests in background and has being marked as active.
+	numActiveBackground uint16
+
+	// numWating is the number of requests waiting for completion.
+	numWaiting uint32
+
+	// TODO(gvisor.dev/issue/3185): BgQueue
+	// some queue for background queued requests.
+
+	// bgLock protects:
+	// MaxBackground, CongestionThreshold, NumBackground,
+	// NumActiveBackground, BgQueue, Blocked.
+	bgLock sync.Mutex
+
+	// maxRead is the maximum size of a read buffer in in bytes.
+	maxRead uint32
+
+	// maxWrite is the maximum size of a write buffer in bytes.
+	// Negotiated in FUSE_INIT.
+	maxWrite uint32
+
+	// maxPages is the maximum number of pages for a single request to use.
+	// Negotiated in FUSE_INIT.
+	maxPages uint16
+
+	// minor version of the FUSE protocol.
+	// Negotiated and only set in INIT.
+	minor uint32
+
+	// asyncRead if read pages asynchronously.
+	// Negotiated and only set in INIT.
+	asyncRead bool
+
+	// abortErr is true if kernel need to return an unique read error after abort.
+	// Negotiated and only set in INIT.
+	abortErr bool
+
+	// writebackCache is true for write-back cache policy,
+	// false for write-through policy.
+	// Negotiated and only set in INIT.
+	writebackCache bool
+
+	// cacheSymlinks if filesystem needs to cache READLINK responses in page cache.
+	// Negotiated and only set in INIT.
+	cacheSymlinks bool
+
+	// bigWrites if doing multi-page cached writes.
+	// Negotiated and only set in INIT.
+	bigWrites bool
+
+	// dontMask if filestestem does not apply umask to creation modes.
+	// Negotiated in INIT.
+	dontMask bool
 }
 
-// NewFUSEConnection creates a FUSE connection to fd
-func NewFUSEConnection(_ context.Context, fd *vfs.FileDescription, maxInFlightRequests uint64) (*Connection, error) {
+// newFUSEConnection creates a FUSE connection to fd.
+func newFUSEConnection(_ context.Context, fd *vfs.FileDescription, maxInFlightRequests uint64) (*connection, error) {
 	// 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 := fd.Impl().(*DeviceFD)
@@ -84,16 +205,41 @@ func NewFUSEConnection(_ context.Context, fd *vfs.FileDescription, maxInFlightRe
 	fuseFD.fullQueueCh = make(chan struct{}, maxInFlightRequests)
 	fuseFD.writeCursor = 0
 
-	return &Connection{
-		fd: fuseFD,
+	return &connection{
+		fd:                  fuseFD,
+		maxBackground:       fuseDefaultMaxBackground,
+		congestionThreshold: fuseDefaultCongestionThreshold,
+		maxPages:            fuseDefaultMaxPagesPerReq,
+		initializedChan:     make(chan struct{}),
+		connected:           true,
 	}, nil
 }
 
+// SetInitialized atomically sets the connection as initialized.
+func (conn *connection) SetInitialized() {
+	// Unblock the requests sent before INIT.
+	close(conn.initializedChan)
+
+	// Close the channel first to avoid the non-atomic situation
+	// where conn.initialized is true but there are
+	// tasks being blocked on the channel.
+	// And it prevents the newer tasks from gaining
+	// unnecessary higher chance to be issued before the blocked one.
+
+	atomic.StoreInt32(&(conn.initialized), int32(1))
+}
+
+// IsInitialized atomically check if the connection is initialized.
+// pairs with SetInitialized().
+func (conn *connection) Initialized() bool {
+	return atomic.LoadInt32(&(conn.initialized)) != 0
+}
+
 // NewRequest creates a new request that can be sent to the FUSE server.
-func (conn *Connection) NewRequest(creds *auth.Credentials, pid uint32, ino uint64, opcode linux.FUSEOpcode, payload marshal.Marshallable) (*Request, error) {
+func (conn *connection) NewRequest(creds *auth.Credentials, pid uint32, ino uint64, opcode linux.FUSEOpcode, payload marshal.Marshallable) (*Request, error) {
 	conn.fd.mu.Lock()
 	defer conn.fd.mu.Unlock()
-	conn.fd.nextOpID += linux.FUSEOpID(ReqIDStep)
+	conn.fd.nextOpID += linux.FUSEOpID(reqIDStep)
 
 	hdrLen := (*linux.FUSEHeaderIn)(nil).SizeBytes()
 	hdr := linux.FUSEHeaderIn{
@@ -118,13 +264,49 @@ func (conn *Connection) NewRequest(creds *auth.Credentials, pid uint32, ino uint
 }
 
 // Call makes a request to the server and blocks the invoking task until a
-// server responds with a response.
-// NOTE: If no task is provided then the Call will simply enqueue the request
-// and return a nil response. No blocking will happen in this case. Instead,
-// this is used to signify that the processing of this request will happen by
-// the kernel.Task that writes the response. See FUSE_INIT for such an
-// invocation.
-func (conn *Connection) Call(t *kernel.Task, r *Request) (*Response, error) {
+// server responds with a response. Task should never be nil.
+// Requests will not be sent before the connection is initialized.
+// For async tasks, use CallAsync().
+func (conn *connection) Call(t *kernel.Task, r *Request) (*Response, error) {
+	// Block requests sent before connection is initalized.
+	if !conn.Initialized() {
+		if err := t.Block(conn.initializedChan); err != nil {
+			return nil, err
+		}
+	}
+
+	return conn.call(t, r)
+}
+
+// CallAsync makes an async (aka background) request.
+// Those requests either do not expect a response (e.g. release) or
+// the response should be handled by others (e.g. init).
+// Return immediately unless the connection is blocked (before initialization).
+// Async call example: init, release, forget, aio, interrupt.
+// When the Request is FUSE_INIT, it will not be blocked before initialization.
+func (conn *connection) CallAsync(t *kernel.Task, r *Request) error {
+	// Block requests sent before connection is initalized.
+	if !conn.Initialized() && r.hdr.Opcode != linux.FUSE_INIT {
+		if err := t.Block(conn.initializedChan); err != nil {
+			return err
+		}
+	}
+
+	// This should be the only place that invokes call() with a nil task.
+	_, err := conn.call(nil, r)
+	return err
+}
+
+// call makes a call without blocking checks.
+func (conn *connection) call(t *kernel.Task, r *Request) (*Response, error) {
+	if !conn.connected {
+		return nil, syserror.ENOTCONN
+	}
+
+	if conn.connInitError {
+		return nil, syserror.ECONNREFUSED
+	}
+
 	fut, err := conn.callFuture(t, r)
 	if err != nil {
 		return nil, err
@@ -160,7 +342,7 @@ func (r *Response) UnmarshalPayload(m marshal.Marshallable) error {
 
 // callFuture makes a request to the server and returns a future response.
 // Call resolve() when the response needs to be fulfilled.
-func (conn *Connection) callFuture(t *kernel.Task, r *Request) (*futureResponse, error) {
+func (conn *connection) callFuture(t *kernel.Task, r *Request) (*futureResponse, error) {
 	conn.fd.mu.Lock()
 	defer conn.fd.mu.Unlock()
 
@@ -195,7 +377,7 @@ func (conn *Connection) callFuture(t *kernel.Task, r *Request) (*futureResponse,
 }
 
 // callFutureLocked makes a request to the server and returns a future response.
-func (conn *Connection) callFutureLocked(t *kernel.Task, r *Request) (*futureResponse, error) {
+func (conn *connection) callFutureLocked(t *kernel.Task, r *Request) (*futureResponse, error) {
 	conn.fd.queue.PushBack(r)
 	conn.fd.numActiveRequests += 1
 	fut := newFutureResponse(r.hdr.Opcode)
diff --git a/pkg/sentry/fsimpl/fuse/dev.go b/pkg/sentry/fsimpl/fuse/dev.go
index f3443ac71..2225076bc 100644
--- a/pkg/sentry/fsimpl/fuse/dev.go
+++ b/pkg/sentry/fsimpl/fuse/dev.go
@@ -21,6 +21,7 @@ import (
 	"gvisor.dev/gvisor/pkg/context"
 	"gvisor.dev/gvisor/pkg/log"
 	"gvisor.dev/gvisor/pkg/sentry/kernel"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 	"gvisor.dev/gvisor/pkg/sync"
 	"gvisor.dev/gvisor/pkg/syserror"
@@ -98,7 +99,9 @@ type DeviceFD struct {
 }
 
 // Release implements vfs.FileDescriptionImpl.Release.
-func (fd *DeviceFD) Release() {}
+func (fd *DeviceFD) Release() {
+	fd.fs.conn.connected = false
+}
 
 // PRead implements vfs.FileDescriptionImpl.PRead.
 func (fd *DeviceFD) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts vfs.ReadOptions) (int64, error) {
@@ -124,7 +127,7 @@ func (fd *DeviceFD) Read(ctx context.Context, dst usermem.IOSequence, opts vfs.R
 	minBuffSize := linux.FUSE_MIN_READ_BUFFER
 	inHdrLen := uint32((*linux.FUSEHeaderIn)(nil).SizeBytes())
 	writeHdrLen := uint32((*linux.FUSEWriteIn)(nil).SizeBytes())
-	negotiatedMinBuffSize := inHdrLen + writeHdrLen + fd.fs.conn.MaxWrite
+	negotiatedMinBuffSize := inHdrLen + writeHdrLen + fd.fs.conn.maxWrite
 	if minBuffSize < negotiatedMinBuffSize {
 		minBuffSize = negotiatedMinBuffSize
 	}
@@ -385,9 +388,9 @@ func (fd *DeviceFD) sendError(ctx context.Context, errno int32, req *Request) er
 // FUSE_INIT.
 func (fd *DeviceFD) noReceiverAction(ctx context.Context, r *Response) error {
 	if r.opcode == linux.FUSE_INIT {
-		// TODO: process init response here.
-		// Maybe get the creds from the context?
-		// creds := auth.CredentialsFromContext(ctx)
+		creds := auth.CredentialsFromContext(ctx)
+		rootUserNs := kernel.KernelFromContext(ctx).RootUserNamespace()
+		return fd.fs.conn.InitRecv(r, creds.HasCapabilityIn(linux.CAP_SYS_ADMIN, rootUserNs))
 	}
 
 	return nil
diff --git a/pkg/sentry/fsimpl/fuse/dev_test.go b/pkg/sentry/fsimpl/fuse/dev_test.go
index fcd77832a..122415275 100644
--- a/pkg/sentry/fsimpl/fuse/dev_test.go
+++ b/pkg/sentry/fsimpl/fuse/dev_test.go
@@ -22,7 +22,6 @@ import (
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/sentry/fsimpl/testutil"
-	"gvisor.dev/gvisor/pkg/sentry/fsimpl/tmpfs"
 	"gvisor.dev/gvisor/pkg/sentry/kernel"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
@@ -60,25 +59,25 @@ func TestFUSECommunication(t *testing.T) {
 			Name:              "SingleClientSingleServer",
 			NumClients:        1,
 			NumServers:        1,
-			MaxActiveRequests: MaxActiveRequestsDefault,
+			MaxActiveRequests: maxActiveRequestsDefault,
 		},
 		{
 			Name:              "SingleClientMultipleServers",
 			NumClients:        1,
 			NumServers:        10,
-			MaxActiveRequests: MaxActiveRequestsDefault,
+			MaxActiveRequests: maxActiveRequestsDefault,
 		},
 		{
 			Name:              "MultipleClientsSingleServer",
 			NumClients:        10,
 			NumServers:        1,
-			MaxActiveRequests: MaxActiveRequestsDefault,
+			MaxActiveRequests: maxActiveRequestsDefault,
 		},
 		{
 			Name:              "MultipleClientsMultipleServers",
 			NumClients:        10,
 			NumServers:        10,
-			MaxActiveRequests: MaxActiveRequestsDefault,
+			MaxActiveRequests: maxActiveRequestsDefault,
 		},
 		{
 			Name:              "RequestCapacityFull",
@@ -145,7 +144,7 @@ func TestFUSECommunication(t *testing.T) {
 // CallTest makes a request to the server and blocks the invoking
 // goroutine until a server responds with a response. Doesn't block
 // a kernel.Task. Analogous to Connection.Call but used for testing.
-func CallTest(conn *Connection, t *kernel.Task, r *Request, i uint32) (*Response, error) {
+func CallTest(conn *connection, t *kernel.Task, r *Request, i uint32) (*Response, error) {
 	conn.fd.mu.Lock()
 
 	// Wait until we're certain that a new request can be processed.
@@ -214,7 +213,7 @@ func ReadTest(serverTask *kernel.Task, fd *vfs.FileDescription, inIOseq usermem.
 // fuseClientRun emulates all the actions of a normal FUSE request. It creates
 // a header, a payload, calls the server, waits for the response, and processes
 // the response.
-func fuseClientRun(t *testing.T, s *testutil.System, k *kernel.Kernel, conn *Connection, creds *auth.Credentials, pid uint32, inode uint64, clientDone chan struct{}) {
+func fuseClientRun(t *testing.T, s *testutil.System, k *kernel.Kernel, conn *connection, creds *auth.Credentials, pid uint32, inode uint64, clientDone chan struct{}) {
 	defer func() { clientDone <- struct{}{} }()
 
 	tc := k.NewThreadGroup(nil, k.RootPIDNamespace(), kernel.NewSignalHandlers(), linux.SIGCHLD, k.GlobalInit().Limits())
@@ -343,7 +342,7 @@ func setup(t *testing.T) *testutil.System {
 		AllowUserMount: true,
 	})
 
-	mntns, err := k.VFS().NewMountNamespace(ctx, creds, "", tmpfs.Name, &vfs.GetFilesystemOptions{})
+	mntns, err := k.VFS().NewMountNamespace(ctx, creds, "", "tmpfs", &vfs.GetFilesystemOptions{})
 	if err != nil {
 		t.Fatalf("NewMountNamespace(): %v", err)
 	}
@@ -353,7 +352,7 @@ func setup(t *testing.T) *testutil.System {
 
 // newTestConnection creates a fuse connection that the sentry can communicate with
 // and the FD for the server to communicate with.
-func newTestConnection(system *testutil.System, k *kernel.Kernel, maxActiveRequests uint64) (*Connection, *vfs.FileDescription, error) {
+func newTestConnection(system *testutil.System, k *kernel.Kernel, maxActiveRequests uint64) (*connection, *vfs.FileDescription, error) {
 	vfsObj := &vfs.VirtualFilesystem{}
 	fuseDev := &DeviceFD{}
 
@@ -426,4 +425,4 @@ func (t *testPayload) CopyIn(task marshal.Task, addr usermem.Addr) (int, error)
 // WriteTo implements io.WriterTo.WriteTo.
 func (t *testPayload) WriteTo(w io.Writer) (int64, error) {
 	panic("not implemented")
-}
+}
\ No newline at end of file
diff --git a/pkg/sentry/fsimpl/fuse/fusefs.go b/pkg/sentry/fsimpl/fuse/fusefs.go
index 911b6f7cb..200a93bbf 100644
--- a/pkg/sentry/fsimpl/fuse/fusefs.go
+++ b/pkg/sentry/fsimpl/fuse/fusefs.go
@@ -65,7 +65,7 @@ type filesystem struct {
 
 	// conn is used for communication between the FUSE server
 	// daemon and the sentry fusefs.
-	conn *Connection
+	conn *connection
 
 	// opts is the options the fusefs is initialized with.
 	opts *filesystemOptions
@@ -140,7 +140,7 @@ func (fsType FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 	fsopts.rootMode = rootMode
 
 	// Set the maxInFlightRequests option.
-	fsopts.maxActiveRequests = MaxActiveRequestsDefault
+	fsopts.maxActiveRequests = maxActiveRequestsDefault
 
 	// Check for unparsed options.
 	if len(mopts) != 0 {
@@ -157,8 +157,12 @@ func (fsType FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 
 	fs.VFSFilesystem().Init(vfsObj, &fsType, fs)
 
-	// TODO: dispatch a FUSE_INIT request to the FUSE daemon server before
-	//  returning. Mount will not block on this dispatched request.
+	// Send a FUSE_INIT request to the FUSE daemon server before returning.
+	// This call is not blocking.
+	if err := fs.conn.InitSend(creds, uint32(kernelTask.ThreadID())); err != nil {
+		log.Warningf("%s.InitSend: failed with error: %v", fsType.Name(), err)
+		return nil, nil, err
+	}
 
 	// root is the fusefs root directory.
 	root := fs.newInode(creds, fsopts.rootMode)
@@ -173,7 +177,7 @@ func NewFUSEFilesystem(ctx context.Context, devMinor uint32, opts *filesystemOpt
 		opts:     opts,
 	}
 
-	conn, err := NewFUSEConnection(ctx, device, opts.maxActiveRequests)
+	conn, err := newFUSEConnection(ctx, device, opts.maxActiveRequests)
 	if err != nil {
 		log.Warningf("fuse.NewFUSEFilesystem: NewFUSEConnection failed with error: %v", err)
 		return nil, syserror.EINVAL
@@ -192,8 +196,8 @@ func (fs *filesystem) Release() {
 	fs.Filesystem.Release()
 }
 
-// Inode implements kernfs.Inode.
-type Inode struct {
+// inode implements kernfs.Inode.
+type inode struct {
 	kernfs.InodeAttrs
 	kernfs.InodeNoDynamicLookup
 	kernfs.InodeNotSymlink
@@ -206,7 +210,7 @@ type Inode struct {
 }
 
 func (fs *filesystem) newInode(creds *auth.Credentials, mode linux.FileMode) *kernfs.Dentry {
-	i := &Inode{}
+	i := &inode{}
 	i.InodeAttrs.Init(creds, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), linux.ModeDirectory|0755)
 	i.OrderedChildren.Init(kernfs.OrderedChildrenOptions{})
 	i.dentry.Init(i)
@@ -215,7 +219,7 @@ func (fs *filesystem) newInode(creds *auth.Credentials, mode linux.FileMode) *ke
 }
 
 // Open implements kernfs.Inode.Open.
-func (i *Inode) Open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
+func (i *inode) Open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
 	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &i.OrderedChildren, &i.locks, &opts)
 	if err != nil {
 		return nil, err
diff --git a/pkg/sentry/fsimpl/fuse/init.go b/pkg/sentry/fsimpl/fuse/init.go
new file mode 100644
index 000000000..779c2bd3f
--- /dev/null
+++ b/pkg/sentry/fsimpl/fuse/init.go
@@ -0,0 +1,166 @@
+// 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/sentry/kernel/auth"
+)
+
+// consts used by FUSE_INIT negotiation.
+const (
+	// fuseMaxMaxPages is the maximum value for MaxPages received in InitOut.
+	// Follow the same behavior as unix fuse implementation.
+	fuseMaxMaxPages = 256
+
+	// Maximum value for the time granularity for file time stamps, 1s.
+	// Follow the same behavior as unix fuse implementation.
+	fuseMaxTimeGranNs = 1000000000
+
+	// Minimum value for MaxWrite.
+	// Follow the same behavior as unix fuse implementation.
+	fuseMinMaxWrite = 4096
+
+	// Temporary default value for max readahead, 128kb.
+	fuseDefaultMaxReadahead = 131072
+
+	// The FUSE_INIT_IN flags sent to the daemon.
+	// TODO(gvisor.dev/issue/3199): complete the flags.
+	fuseDefaultInitFlags = linux.FUSE_MAX_PAGES
+)
+
+// Adjustable maximums for Connection's cogestion control parameters.
+// Used as the upperbound of the config values.
+// Currently we do not support adjustment to them.
+var (
+	MaxUserBackgroundRequest   uint16 = fuseDefaultMaxBackground
+	MaxUserCongestionThreshold uint16 = fuseDefaultCongestionThreshold
+)
+
+// InitSend sends a FUSE_INIT request.
+func (conn *connection) InitSend(creds *auth.Credentials, pid uint32) error {
+	in := linux.FUSEInitIn{
+		Major: linux.FUSE_KERNEL_VERSION,
+		Minor: linux.FUSE_KERNEL_MINOR_VERSION,
+		// TODO(gvisor.dev/issue/3196): find appropriate way to calculate this
+		MaxReadahead: fuseDefaultMaxReadahead,
+		Flags:        fuseDefaultInitFlags,
+	}
+
+	req, err := conn.NewRequest(creds, pid, 0, linux.FUSE_INIT, &in)
+	if err != nil {
+		return err
+	}
+
+	// Since there is no task to block on and FUSE_INIT is the request
+	// to unblock other requests, use nil.
+	return conn.CallAsync(nil, req)
+}
+
+// InitRecv receives a FUSE_INIT reply and process it.
+func (conn *connection) InitRecv(res *Response, hasSysAdminCap bool) error {
+	if err := res.Error(); err != nil {
+		return err
+	}
+
+	var out linux.FUSEInitOut
+	if err := res.UnmarshalPayload(&out); err != nil {
+		return err
+	}
+
+	return conn.initProcessReply(&out, hasSysAdminCap)
+}
+
+// Process the FUSE_INIT reply from the FUSE server.
+func (conn *connection) initProcessReply(out *linux.FUSEInitOut, hasSysAdminCap bool) error {
+	// No support for old major fuse versions.
+	if out.Major != linux.FUSE_KERNEL_VERSION {
+		conn.connInitError = true
+
+		// Set the connection as initialized and unblock the blocked requests
+		// (i.e. return error for them).
+		conn.SetInitialized()
+
+		return nil
+	}
+
+	// Start processing the reply.
+	conn.connInitSuccess = true
+	conn.minor = out.Minor
+
+	// No support for limits before minor version 13.
+	if out.Minor >= 13 {
+		conn.bgLock.Lock()
+
+		if out.MaxBackground > 0 {
+			conn.maxBackground = out.MaxBackground
+
+			if !hasSysAdminCap &&
+				conn.maxBackground > MaxUserBackgroundRequest {
+				conn.maxBackground = MaxUserBackgroundRequest
+			}
+		}
+
+		if out.CongestionThreshold > 0 {
+			conn.congestionThreshold = out.CongestionThreshold
+
+			if !hasSysAdminCap &&
+				conn.congestionThreshold > MaxUserCongestionThreshold {
+				conn.congestionThreshold = MaxUserCongestionThreshold
+			}
+		}
+
+		conn.bgLock.Unlock()
+	}
+
+	// No support for the following flags before minor version 6.
+	if out.Minor >= 6 {
+		conn.asyncRead = out.Flags&linux.FUSE_ASYNC_READ != 0
+		conn.bigWrites = out.Flags&linux.FUSE_BIG_WRITES != 0
+		conn.dontMask = out.Flags&linux.FUSE_DONT_MASK != 0
+		conn.writebackCache = out.Flags&linux.FUSE_WRITEBACK_CACHE != 0
+		conn.cacheSymlinks = out.Flags&linux.FUSE_CACHE_SYMLINKS != 0
+		conn.abortErr = out.Flags&linux.FUSE_ABORT_ERROR != 0
+
+		// TODO(gvisor.dev/issue/3195): figure out how to use TimeGran (0 < TimeGran <= fuseMaxTimeGranNs).
+
+		if out.Flags&linux.FUSE_MAX_PAGES != 0 {
+			maxPages := out.MaxPages
+			if maxPages < 1 {
+				maxPages = 1
+			}
+			if maxPages > fuseMaxMaxPages {
+				maxPages = fuseMaxMaxPages
+			}
+			conn.maxPages = maxPages
+		}
+	}
+
+	// No support for negotiating MaxWrite before minor version 5.
+	if out.Minor >= 5 {
+		conn.maxWrite = out.MaxWrite
+	} else {
+		conn.maxWrite = fuseMinMaxWrite
+	}
+	if conn.maxWrite < fuseMinMaxWrite {
+		conn.maxWrite = fuseMinMaxWrite
+	}
+
+	// Set connection as initialized and unblock the requests
+	// issued before init.
+	conn.SetInitialized()
+
+	return nil
+}