Port netstack, hostinet, and netlink sockets to VFS2.

All three follow the same pattern:
1. Refactor VFS1 sockets into socketOpsCommon, so that most of the methods can
   be shared with VFS2.
2. Create a FileDescriptionImpl with the corresponding socket operations,
   rewriting the few that cannot be shared with VFS1.
3. Set up a VFS2 socket provider that creates a socket by setting up a dentry
   in the global Kernel.socketMount and connecting it with a new
   FileDescription.

This mostly completes the work for porting sockets to VFS2, and many syscall
tests can be enabled as a result.
There are several networking-related syscall tests that are still not passing:
1. net gofer tests
2. socketpair gofer tests
2. sendfile tests (splice is not implemented in VFS2 yet)

Updates #1478, #1484, #1485

PiperOrigin-RevId: 309457331

5 files changed, 262 insertions, 29 deletions

diff --git a/pkg/sentry/socket/netlink/BUILD b/pkg/sentry/socket/netlink/BUILD
index 1911cd9b8..09ca00a4a 100644
--- a/pkg/sentry/socket/netlink/BUILD
+++ b/pkg/sentry/socket/netlink/BUILD
@@ -7,7 +7,9 @@ go_library(
     srcs = [
         "message.go",
         "provider.go",
+        "provider_vfs2.go",
         "socket.go",
+        "socket_vfs2.go",
     ],
     visibility = ["//pkg/sentry:internal"],
     deps = [
@@ -18,6 +20,8 @@ go_library(
         "//pkg/sentry/device",
         "//pkg/sentry/fs",
         "//pkg/sentry/fs/fsutil",
+        "//pkg/sentry/fsimpl/kernfs",
+        "//pkg/sentry/fsimpl/sockfs",
         "//pkg/sentry/kernel",
         "//pkg/sentry/kernel/auth",
         "//pkg/sentry/kernel/time",
@@ -25,6 +29,7 @@ go_library(
         "//pkg/sentry/socket/netlink/port",
         "//pkg/sentry/socket/unix",
         "//pkg/sentry/socket/unix/transport",
+        "//pkg/sentry/vfs",
         "//pkg/sync",
         "//pkg/syserr",
         "//pkg/syserror",
diff --git a/pkg/sentry/socket/netlink/provider.go b/pkg/sentry/socket/netlink/provider.go
index b0dc70e5c..0d45e5053 100644
--- a/pkg/sentry/socket/netlink/provider.go
+++ b/pkg/sentry/socket/netlink/provider.go
@@ -67,6 +67,8 @@ func RegisterProvider(protocol int, provider Provider) {
 	protocols[protocol] = provider
 }
 
+// LINT.IfChange
+
 // socketProvider implements socket.Provider.
 type socketProvider struct {
 }
@@ -105,7 +107,10 @@ func (*socketProvider) Pair(*kernel.Task, linux.SockType, int) (*fs.File, *fs.Fi
 	return nil, nil, syserr.ErrNotSupported
 }
 
+// LINT.ThenChange(./provider_vfs2.go)
+
 // init registers the socket provider.
 func init() {
 	socket.RegisterProvider(linux.AF_NETLINK, &socketProvider{})
+	socket.RegisterProviderVFS2(linux.AF_NETLINK, &socketProviderVFS2{})
 }
diff --git a/pkg/sentry/socket/netlink/provider_vfs2.go b/pkg/sentry/socket/netlink/provider_vfs2.go
new file mode 100644
index 000000000..dcd92b5cd
--- /dev/null
+++ b/pkg/sentry/socket/netlink/provider_vfs2.go
@@ -0,0 +1,71 @@
+// Copyright 2018 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 netlink
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/kernfs"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/sockfs"
+	"gvisor.dev/gvisor/pkg/sentry/kernel"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserr"
+)
+
+// socketProviderVFS2 implements socket.Provider.
+type socketProviderVFS2 struct {
+}
+
+// Socket implements socket.Provider.Socket.
+func (*socketProviderVFS2) Socket(t *kernel.Task, stype linux.SockType, protocol int) (*vfs.FileDescription, *syserr.Error) {
+	// Netlink sockets must be specified as datagram or raw, but they
+	// behave the same regardless of type.
+	if stype != linux.SOCK_DGRAM && stype != linux.SOCK_RAW {
+		return nil, syserr.ErrSocketNotSupported
+	}
+
+	provider, ok := protocols[protocol]
+	if !ok {
+		return nil, syserr.ErrProtocolNotSupported
+	}
+
+	p, err := provider(t)
+	if err != nil {
+		return nil, err
+	}
+
+	s, err := NewVFS2(t, stype, p)
+	if err != nil {
+		return nil, err
+	}
+
+	vfsfd := &s.vfsfd
+	mnt := t.Kernel().SocketMount()
+	fs := mnt.Filesystem().Impl().(*kernfs.Filesystem)
+	d := sockfs.NewDentry(t.Credentials(), fs.NextIno())
+	if err := vfsfd.Init(s, linux.O_RDWR, mnt, d, &vfs.FileDescriptionOptions{
+		DenyPRead:         true,
+		DenyPWrite:        true,
+		UseDentryMetadata: true,
+	}); err != nil {
+		return nil, syserr.FromError(err)
+	}
+	return vfsfd, nil
+}
+
+// Pair implements socket.Provider.Pair by returning an error.
+func (*socketProviderVFS2) Pair(*kernel.Task, linux.SockType, int) (*vfs.FileDescription, *vfs.FileDescription, *syserr.Error) {
+	// Netlink sockets never supports creating socket pairs.
+	return nil, nil, syserr.ErrNotSupported
+}
diff --git a/pkg/sentry/socket/netlink/socket.go b/pkg/sentry/socket/netlink/socket.go
index 2ca02567d..81f34c5a2 100644
--- a/pkg/sentry/socket/netlink/socket.go
+++ b/pkg/sentry/socket/netlink/socket.go
@@ -58,6 +58,8 @@ var errNoFilter = syserr.New("no filter attached", linux.ENOENT)
 // netlinkSocketDevice is the netlink socket virtual device.
 var netlinkSocketDevice = device.NewAnonDevice()
 
+// LINT.IfChange
+
 // Socket is the base socket type for netlink sockets.
 //
 // This implementation only supports userspace sending and receiving messages
@@ -74,6 +76,14 @@ type Socket struct {
 	fsutil.FileNoSplice             `state:"nosave"`
 	fsutil.FileNoopFlush            `state:"nosave"`
 	fsutil.FileUseInodeUnstableAttr `state:"nosave"`
+
+	socketOpsCommon
+}
+
+// socketOpsCommon contains the socket operations common to VFS1 and VFS2.
+//
+// +stateify savable
+type socketOpsCommon struct {
 	socket.SendReceiveTimeout
 
 	// ports provides netlink port allocation.
@@ -140,17 +150,19 @@ func NewSocket(t *kernel.Task, skType linux.SockType, protocol Protocol) (*Socke
 	}
 
 	return &Socket{
-		ports:          t.Kernel().NetlinkPorts(),
-		protocol:       protocol,
-		skType:         skType,
-		ep:             ep,
-		connection:     connection,
-		sendBufferSize: defaultSendBufferSize,
+		socketOpsCommon: socketOpsCommon{
+			ports:          t.Kernel().NetlinkPorts(),
+			protocol:       protocol,
+			skType:         skType,
+			ep:             ep,
+			connection:     connection,
+			sendBufferSize: defaultSendBufferSize,
+		},
 	}, nil
 }
 
 // Release implements fs.FileOperations.Release.
-func (s *Socket) Release() {
+func (s *socketOpsCommon) Release() {
 	s.connection.Release()
 	s.ep.Close()
 
@@ -160,7 +172,7 @@ func (s *Socket) Release() {
 }
 
 // Readiness implements waiter.Waitable.Readiness.
-func (s *Socket) Readiness(mask waiter.EventMask) waiter.EventMask {
+func (s *socketOpsCommon) Readiness(mask waiter.EventMask) waiter.EventMask {
 	// ep holds messages to be read and thus handles EventIn readiness.
 	ready := s.ep.Readiness(mask)
 
@@ -174,18 +186,18 @@ func (s *Socket) Readiness(mask waiter.EventMask) waiter.EventMask {
 }
 
 // EventRegister implements waiter.Waitable.EventRegister.
-func (s *Socket) EventRegister(e *waiter.Entry, mask waiter.EventMask) {
+func (s *socketOpsCommon) EventRegister(e *waiter.Entry, mask waiter.EventMask) {
 	s.ep.EventRegister(e, mask)
 	// Writable readiness never changes, so no registration is needed.
 }
 
 // EventUnregister implements waiter.Waitable.EventUnregister.
-func (s *Socket) EventUnregister(e *waiter.Entry) {
+func (s *socketOpsCommon) EventUnregister(e *waiter.Entry) {
 	s.ep.EventUnregister(e)
 }
 
 // Passcred implements transport.Credentialer.Passcred.
-func (s *Socket) Passcred() bool {
+func (s *socketOpsCommon) Passcred() bool {
 	s.mu.Lock()
 	passcred := s.passcred
 	s.mu.Unlock()
@@ -193,7 +205,7 @@ func (s *Socket) Passcred() bool {
 }
 
 // ConnectedPasscred implements transport.Credentialer.ConnectedPasscred.
-func (s *Socket) ConnectedPasscred() bool {
+func (s *socketOpsCommon) ConnectedPasscred() bool {
 	// This socket is connected to the kernel, which doesn't need creds.
 	//
 	// This is arbitrary, as ConnectedPasscred on this type has no callers.
@@ -227,7 +239,7 @@ func ExtractSockAddr(b []byte) (*linux.SockAddrNetlink, *syserr.Error) {
 // port of 0 defaults to the ThreadGroup ID.
 //
 // Preconditions: mu is held.
-func (s *Socket) bindPort(t *kernel.Task, port int32) *syserr.Error {
+func (s *socketOpsCommon) bindPort(t *kernel.Task, port int32) *syserr.Error {
 	if s.bound {
 		// Re-binding is only allowed if the port doesn't change.
 		if port != s.portID {
@@ -251,7 +263,7 @@ func (s *Socket) bindPort(t *kernel.Task, port int32) *syserr.Error {
 }
 
 // Bind implements socket.Socket.Bind.
-func (s *Socket) Bind(t *kernel.Task, sockaddr []byte) *syserr.Error {
+func (s *socketOpsCommon) Bind(t *kernel.Task, sockaddr []byte) *syserr.Error {
 	a, err := ExtractSockAddr(sockaddr)
 	if err != nil {
 		return err
@@ -269,7 +281,7 @@ func (s *Socket) Bind(t *kernel.Task, sockaddr []byte) *syserr.Error {
 }
 
 // Connect implements socket.Socket.Connect.
-func (s *Socket) Connect(t *kernel.Task, sockaddr []byte, blocking bool) *syserr.Error {
+func (s *socketOpsCommon) Connect(t *kernel.Task, sockaddr []byte, blocking bool) *syserr.Error {
 	a, err := ExtractSockAddr(sockaddr)
 	if err != nil {
 		return err
@@ -300,25 +312,25 @@ func (s *Socket) Connect(t *kernel.Task, sockaddr []byte, blocking bool) *syserr
 }
 
 // Accept implements socket.Socket.Accept.
-func (s *Socket) Accept(t *kernel.Task, peerRequested bool, flags int, blocking bool) (int32, linux.SockAddr, uint32, *syserr.Error) {
+func (s *socketOpsCommon) Accept(t *kernel.Task, peerRequested bool, flags int, blocking bool) (int32, linux.SockAddr, uint32, *syserr.Error) {
 	// Netlink sockets never support accept.
 	return 0, nil, 0, syserr.ErrNotSupported
 }
 
 // Listen implements socket.Socket.Listen.
-func (s *Socket) Listen(t *kernel.Task, backlog int) *syserr.Error {
+func (s *socketOpsCommon) Listen(t *kernel.Task, backlog int) *syserr.Error {
 	// Netlink sockets never support listen.
 	return syserr.ErrNotSupported
 }
 
 // Shutdown implements socket.Socket.Shutdown.
-func (s *Socket) Shutdown(t *kernel.Task, how int) *syserr.Error {
+func (s *socketOpsCommon) Shutdown(t *kernel.Task, how int) *syserr.Error {
 	// Netlink sockets never support shutdown.
 	return syserr.ErrNotSupported
 }
 
 // GetSockOpt implements socket.Socket.GetSockOpt.
-func (s *Socket) GetSockOpt(t *kernel.Task, level int, name int, outPtr usermem.Addr, outLen int) (interface{}, *syserr.Error) {
+func (s *socketOpsCommon) GetSockOpt(t *kernel.Task, level int, name int, outPtr usermem.Addr, outLen int) (interface{}, *syserr.Error) {
 	switch level {
 	case linux.SOL_SOCKET:
 		switch name {
@@ -369,7 +381,7 @@ func (s *Socket) GetSockOpt(t *kernel.Task, level int, name int, outPtr usermem.
 }
 
 // SetSockOpt implements socket.Socket.SetSockOpt.
-func (s *Socket) SetSockOpt(t *kernel.Task, level int, name int, opt []byte) *syserr.Error {
+func (s *socketOpsCommon) SetSockOpt(t *kernel.Task, level int, name int, opt []byte) *syserr.Error {
 	switch level {
 	case linux.SOL_SOCKET:
 		switch name {
@@ -466,7 +478,7 @@ func (s *Socket) SetSockOpt(t *kernel.Task, level int, name int, opt []byte) *sy
 }
 
 // GetSockName implements socket.Socket.GetSockName.
-func (s *Socket) GetSockName(t *kernel.Task) (linux.SockAddr, uint32, *syserr.Error) {
+func (s *socketOpsCommon) GetSockName(t *kernel.Task) (linux.SockAddr, uint32, *syserr.Error) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 
@@ -478,7 +490,7 @@ func (s *Socket) GetSockName(t *kernel.Task) (linux.SockAddr, uint32, *syserr.Er
 }
 
 // GetPeerName implements socket.Socket.GetPeerName.
-func (s *Socket) GetPeerName(t *kernel.Task) (linux.SockAddr, uint32, *syserr.Error) {
+func (s *socketOpsCommon) GetPeerName(t *kernel.Task) (linux.SockAddr, uint32, *syserr.Error) {
 	sa := &linux.SockAddrNetlink{
 		Family: linux.AF_NETLINK,
 		// TODO(b/68878065): Support non-kernel peers. For now the peer
@@ -489,7 +501,7 @@ func (s *Socket) GetPeerName(t *kernel.Task) (linux.SockAddr, uint32, *syserr.Er
 }
 
 // RecvMsg implements socket.Socket.RecvMsg.
-func (s *Socket) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags int, haveDeadline bool, deadline ktime.Time, senderRequested bool, controlDataLen uint64) (int, int, linux.SockAddr, uint32, socket.ControlMessages, *syserr.Error) {
+func (s *socketOpsCommon) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags int, haveDeadline bool, deadline ktime.Time, senderRequested bool, controlDataLen uint64) (int, int, linux.SockAddr, uint32, socket.ControlMessages, *syserr.Error) {
 	from := &linux.SockAddrNetlink{
 		Family: linux.AF_NETLINK,
 		PortID: 0,
@@ -590,7 +602,7 @@ func (kernelSCM) Credentials(*kernel.Task) (kernel.ThreadID, auth.UID, auth.GID)
 var kernelCreds = &kernelSCM{}
 
 // sendResponse sends the response messages in ms back to userspace.
-func (s *Socket) sendResponse(ctx context.Context, ms *MessageSet) *syserr.Error {
+func (s *socketOpsCommon) sendResponse(ctx context.Context, ms *MessageSet) *syserr.Error {
 	// Linux combines multiple netlink messages into a single datagram.
 	bufs := make([][]byte, 0, len(ms.Messages))
 	for _, m := range ms.Messages {
@@ -666,7 +678,7 @@ func dumpAckMesage(hdr linux.NetlinkMessageHeader, ms *MessageSet) {
 
 // processMessages handles each message in buf, passing it to the protocol
 // handler for final handling.
-func (s *Socket) processMessages(ctx context.Context, buf []byte) *syserr.Error {
+func (s *socketOpsCommon) processMessages(ctx context.Context, buf []byte) *syserr.Error {
 	for len(buf) > 0 {
 		msg, rest, ok := ParseMessage(buf)
 		if !ok {
@@ -698,7 +710,7 @@ func (s *Socket) processMessages(ctx context.Context, buf []byte) *syserr.Error
 }
 
 // sendMsg is the core of message send, used for SendMsg and Write.
-func (s *Socket) sendMsg(ctx context.Context, src usermem.IOSequence, to []byte, flags int, controlMessages socket.ControlMessages) (int, *syserr.Error) {
+func (s *socketOpsCommon) sendMsg(ctx context.Context, src usermem.IOSequence, to []byte, flags int, controlMessages socket.ControlMessages) (int, *syserr.Error) {
 	dstPort := int32(0)
 
 	if len(to) != 0 {
@@ -745,7 +757,7 @@ func (s *Socket) sendMsg(ctx context.Context, src usermem.IOSequence, to []byte,
 }
 
 // SendMsg implements socket.Socket.SendMsg.
-func (s *Socket) SendMsg(t *kernel.Task, src usermem.IOSequence, to []byte, flags int, haveDeadline bool, deadline ktime.Time, controlMessages socket.ControlMessages) (int, *syserr.Error) {
+func (s *socketOpsCommon) SendMsg(t *kernel.Task, src usermem.IOSequence, to []byte, flags int, haveDeadline bool, deadline ktime.Time, controlMessages socket.ControlMessages) (int, *syserr.Error) {
 	return s.sendMsg(t, src, to, flags, controlMessages)
 }
 
@@ -756,11 +768,13 @@ func (s *Socket) Write(ctx context.Context, _ *fs.File, src usermem.IOSequence,
 }
 
 // State implements socket.Socket.State.
-func (s *Socket) State() uint32 {
+func (s *socketOpsCommon) State() uint32 {
 	return s.ep.State()
 }
 
 // Type implements socket.Socket.Type.
-func (s *Socket) Type() (family int, skType linux.SockType, protocol int) {
+func (s *socketOpsCommon) Type() (family int, skType linux.SockType, protocol int) {
 	return linux.AF_NETLINK, s.skType, s.protocol.Protocol()
 }
+
+// LINT.ThenChange(./socket_vfs2.go)
diff --git a/pkg/sentry/socket/netlink/socket_vfs2.go b/pkg/sentry/socket/netlink/socket_vfs2.go
new file mode 100644
index 000000000..b854bf990
--- /dev/null
+++ b/pkg/sentry/socket/netlink/socket_vfs2.go
@@ -0,0 +1,138 @@
+// Copyright 2018 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 netlink
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/sentry/arch"
+	"gvisor.dev/gvisor/pkg/sentry/kernel"
+	"gvisor.dev/gvisor/pkg/sentry/socket"
+	"gvisor.dev/gvisor/pkg/sentry/socket/unix"
+	"gvisor.dev/gvisor/pkg/sentry/socket/unix/transport"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserr"
+	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/tcpip"
+	"gvisor.dev/gvisor/pkg/usermem"
+	"gvisor.dev/gvisor/pkg/waiter"
+)
+
+// SocketVFS2 is the base VFS2 socket type for netlink sockets.
+//
+// This implementation only supports userspace sending and receiving messages
+// to/from the kernel.
+//
+// SocketVFS2 implements socket.SocketVFS2 and transport.Credentialer.
+type SocketVFS2 struct {
+	vfsfd vfs.FileDescription
+	vfs.FileDescriptionDefaultImpl
+	vfs.DentryMetadataFileDescriptionImpl
+
+	socketOpsCommon
+}
+
+var _ socket.SocketVFS2 = (*SocketVFS2)(nil)
+var _ transport.Credentialer = (*SocketVFS2)(nil)
+
+// NewVFS2 creates a new SocketVFS2.
+func NewVFS2(t *kernel.Task, skType linux.SockType, protocol Protocol) (*SocketVFS2, *syserr.Error) {
+	// Datagram endpoint used to buffer kernel -> user messages.
+	ep := transport.NewConnectionless(t)
+
+	// Bind the endpoint for good measure so we can connect to it. The
+	// bound address will never be exposed.
+	if err := ep.Bind(tcpip.FullAddress{Addr: "dummy"}, nil); err != nil {
+		ep.Close()
+		return nil, err
+	}
+
+	// Create a connection from which the kernel can write messages.
+	connection, err := ep.(transport.BoundEndpoint).UnidirectionalConnect(t)
+	if err != nil {
+		ep.Close()
+		return nil, err
+	}
+
+	return &SocketVFS2{
+		socketOpsCommon: socketOpsCommon{
+			ports:          t.Kernel().NetlinkPorts(),
+			protocol:       protocol,
+			skType:         skType,
+			ep:             ep,
+			connection:     connection,
+			sendBufferSize: defaultSendBufferSize,
+		},
+	}, nil
+}
+
+// Readiness implements waiter.Waitable.Readiness.
+func (s *SocketVFS2) Readiness(mask waiter.EventMask) waiter.EventMask {
+	return s.socketOpsCommon.Readiness(mask)
+}
+
+// EventRegister implements waiter.Waitable.EventRegister.
+func (s *SocketVFS2) EventRegister(e *waiter.Entry, mask waiter.EventMask) {
+	s.socketOpsCommon.EventRegister(e, mask)
+}
+
+// EventUnregister implements waiter.Waitable.EventUnregister.
+func (s *SocketVFS2) EventUnregister(e *waiter.Entry) {
+	s.socketOpsCommon.EventUnregister(e)
+}
+
+// Ioctl implements vfs.FileDescriptionImpl.
+func (*SocketVFS2) Ioctl(context.Context, usermem.IO, arch.SyscallArguments) (uintptr, error) {
+	// TODO(b/68878065): no ioctls supported.
+	return 0, syserror.ENOTTY
+}
+
+// PRead implements vfs.FileDescriptionImpl.
+func (s *SocketVFS2) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts vfs.ReadOptions) (int64, error) {
+	return 0, syserror.ESPIPE
+}
+
+// Read implements vfs.FileDescriptionImpl.
+func (s *SocketVFS2) Read(ctx context.Context, dst usermem.IOSequence, opts vfs.ReadOptions) (int64, error) {
+	// All flags other than RWF_NOWAIT should be ignored.
+	// TODO(gvisor.dev/issue/2601): Support RWF_NOWAIT.
+	if opts.Flags != 0 {
+		return 0, syserror.EOPNOTSUPP
+	}
+
+	if dst.NumBytes() == 0 {
+		return 0, nil
+	}
+	return dst.CopyOutFrom(ctx, &unix.EndpointReader{
+		Endpoint: s.ep,
+	})
+}
+
+// PWrite implements vfs.FileDescriptionImpl.
+func (s *SocketVFS2) PWrite(ctx context.Context, src usermem.IOSequence, offset int64, opts vfs.WriteOptions) (int64, error) {
+	return 0, syserror.ESPIPE
+}
+
+// Write implements vfs.FileDescriptionImpl.
+func (s *SocketVFS2) Write(ctx context.Context, src usermem.IOSequence, opts vfs.WriteOptions) (int64, error) {
+	// All flags other than RWF_NOWAIT should be ignored.
+	// TODO(gvisor.dev/issue/2601): Support RWF_NOWAIT.
+	if opts.Flags != 0 {
+		return 0, syserror.EOPNOTSUPP
+	}
+
+	n, err := s.sendMsg(ctx, src, nil, 0, socket.ControlMessages{})
+	return int64(n), err.ToError()
+}