gvisor/pkg/sentry/syscalls/linux/sys_socket.go

// 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 linux

import (
	"fmt"
	"time"

	"golang.org/x/sys/unix"
	"gvisor.dev/gvisor/pkg/abi/linux"
	"gvisor.dev/gvisor/pkg/context"
	"gvisor.dev/gvisor/pkg/errors/linuxerr"
	"gvisor.dev/gvisor/pkg/hostarch"
	"gvisor.dev/gvisor/pkg/marshal"
	"gvisor.dev/gvisor/pkg/marshal/primitive"
	"gvisor.dev/gvisor/pkg/sentry/arch"
	"gvisor.dev/gvisor/pkg/sentry/fsimpl/host"
	"gvisor.dev/gvisor/pkg/sentry/kernel"
	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
	"gvisor.dev/gvisor/pkg/sentry/ktime"
	"gvisor.dev/gvisor/pkg/sentry/socket"
	"gvisor.dev/gvisor/pkg/sentry/socket/control"
	"gvisor.dev/gvisor/pkg/sentry/socket/unix/transport"
	"gvisor.dev/gvisor/pkg/sentry/vfs"
	"gvisor.dev/gvisor/pkg/syserr"
	"gvisor.dev/gvisor/pkg/usermem"
)

// maxAddrLen is the maximum socket address length we're willing to accept.
const maxAddrLen = 200

// maxOptLen is the maximum sockopt parameter length we're willing to accept.
const maxOptLen = 1024 * 8

// maxControlLen is the maximum length of the msghdr.msg_control buffer we're
// willing to accept. Note that this limit is smaller than Linux, which allows
// buffers upto INT_MAX.
const maxControlLen = 10 * 1024 * 1024

// maxListenBacklog is the maximum limit of listen backlog supported.
const maxListenBacklog = 1024

// nameLenOffset is the offset from the start of the MessageHeader64 struct to
// the NameLen field.
const nameLenOffset = 8

// controlLenOffset is the offset form the start of the MessageHeader64 struct
// to the ControlLen field.
const controlLenOffset = 40

// flagsOffset is the offset form the start of the MessageHeader64 struct
// to the Flags field.
const flagsOffset = 48

const sizeOfInt32 = 4

// messageHeader64Len is the length of a MessageHeader64 struct.
var messageHeader64Len = uint64((*MessageHeader64)(nil).SizeBytes())

// multipleMessageHeader64Len is the length of a multipeMessageHeader64 struct.
var multipleMessageHeader64Len = uint64((*multipleMessageHeader64)(nil).SizeBytes())

// baseRecvFlags are the flags that are accepted across recvmsg(2),
// recvmmsg(2), and recvfrom(2).
const baseRecvFlags = linux.MSG_OOB | linux.MSG_DONTROUTE | linux.MSG_DONTWAIT | linux.MSG_NOSIGNAL | linux.MSG_WAITALL | linux.MSG_TRUNC | linux.MSG_CTRUNC

// MessageHeader64 is the 64-bit representation of the msghdr struct used in
// the recvmsg and sendmsg syscalls.
//
// +marshal
type MessageHeader64 struct {
	// Name is the optional pointer to a network address buffer.
	Name uint64

	// NameLen is the length of the buffer pointed to by Name.
	NameLen uint32
	_       uint32

	// Iov is a pointer to an array of io vectors that describe the memory
	// locations involved in the io operation.
	Iov uint64

	// IovLen is the length of the array pointed to by Iov.
	IovLen uint64

	// Control is the optional pointer to ancillary control data.
	Control uint64

	// ControlLen is the length of the data pointed to by Control.
	ControlLen uint64

	// Flags on the sent/received message.
	Flags int32
	_     int32
}

// multipleMessageHeader64 is the 64-bit representation of the mmsghdr struct used in
// the recvmmsg and sendmmsg syscalls.
//
// +marshal
type multipleMessageHeader64 struct {
	msgHdr MessageHeader64
	msgLen uint32
	_      int32
}

// CaptureAddress allocates memory for and copies a socket address structure
// from the untrusted address space range.
func CaptureAddress(t *kernel.Task, addr hostarch.Addr, addrlen uint32) ([]byte, error) {
	if addrlen > maxAddrLen {
		return nil, linuxerr.EINVAL
	}

	addrBuf := make([]byte, addrlen)
	if _, err := t.CopyInBytes(addr, addrBuf); err != nil {
		return nil, err
	}

	return addrBuf, nil
}

// writeAddress writes a sockaddr structure and its length to an output buffer
// in the unstrusted address space range. If the address is bigger than the
// buffer, it is truncated.
func writeAddress(t *kernel.Task, addr linux.SockAddr, addrLen uint32, addrPtr hostarch.Addr, addrLenPtr hostarch.Addr) error {
	// Get the buffer length.
	var bufLen uint32
	if _, err := primitive.CopyUint32In(t, addrLenPtr, &bufLen); err != nil {
		return err
	}

	if int32(bufLen) < 0 {
		return linuxerr.EINVAL
	}

	// Write the length unconditionally.
	if _, err := primitive.CopyUint32Out(t, addrLenPtr, addrLen); err != nil {
		return err
	}

	if addr == nil {
		return nil
	}

	if bufLen > addrLen {
		bufLen = addrLen
	}

	// Copy as much of the address as will fit in the buffer.
	encodedAddr := t.CopyScratchBuffer(addr.SizeBytes())
	addr.MarshalUnsafe(encodedAddr)
	if bufLen > uint32(len(encodedAddr)) {
		bufLen = uint32(len(encodedAddr))
	}
	_, err := t.CopyOutBytes(addrPtr, encodedAddr[:int(bufLen)])
	return err
}

// Socket implements the linux syscall socket(2).
func Socket(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	domain := int(args[0].Int())
	stype := args[1].Int()
	protocol := int(args[2].Int())

	// Check and initialize the flags.
	if stype & ^(0xf|linux.SOCK_NONBLOCK|linux.SOCK_CLOEXEC) != 0 {
		return 0, nil, linuxerr.EINVAL
	}

	// Create the new socket.
	s, e := socket.New(t, domain, linux.SockType(stype&0xf), protocol)
	if e != nil {
		return 0, nil, e.ToError()
	}
	defer s.DecRef(t)

	if err := s.SetStatusFlags(t, t.Credentials(), uint32(stype&linux.SOCK_NONBLOCK)); err != nil {
		return 0, nil, err
	}

	fd, err := t.NewFDFrom(0, s, kernel.FDFlags{
		CloseOnExec: stype&linux.SOCK_CLOEXEC != 0,
	})
	if err != nil {
		return 0, nil, err
	}

	return uintptr(fd), nil, nil
}

// SocketPair implements the linux syscall socketpair(2).
func SocketPair(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	domain := int(args[0].Int())
	stype := args[1].Int()
	protocol := int(args[2].Int())
	addr := args[3].Pointer()

	// Check and initialize the flags.
	if stype & ^(0xf|linux.SOCK_NONBLOCK|linux.SOCK_CLOEXEC) != 0 {
		return 0, nil, linuxerr.EINVAL
	}

	// Create the socket pair.
	s1, s2, e := socket.Pair(t, domain, linux.SockType(stype&0xf), protocol)
	if e != nil {
		return 0, nil, e.ToError()
	}
	// Adding to the FD table will cause an extra reference to be acquired.
	defer s1.DecRef(t)
	defer s2.DecRef(t)

	nonblocking := uint32(stype & linux.SOCK_NONBLOCK)
	if err := s1.SetStatusFlags(t, t.Credentials(), nonblocking); err != nil {
		return 0, nil, err
	}
	if err := s2.SetStatusFlags(t, t.Credentials(), nonblocking); err != nil {
		return 0, nil, err
	}

	// Create the FDs for the sockets.
	flags := kernel.FDFlags{
		CloseOnExec: stype&linux.SOCK_CLOEXEC != 0,
	}
	fds, err := t.NewFDs(0, []*vfs.FileDescription{s1, s2}, flags)
	if err != nil {
		return 0, nil, err
	}

	if _, err := primitive.CopyInt32SliceOut(t, addr, fds); err != nil {
		for _, fd := range fds {
			if file := t.FDTable().Remove(t, fd); file != nil {
				file.DecRef(t)
			}
		}
		return 0, nil, err
	}

	return 0, nil, nil
}

// Connect implements the linux syscall connect(2).
func Connect(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	fd := args[0].Int()
	addr := args[1].Pointer()
	addrlen := args[2].Uint()

	// Get socket from the file descriptor.
	file := t.GetFile(fd)
	if file == nil {
		return 0, nil, linuxerr.EBADF
	}
	defer file.DecRef(t)

	// Extract the socket.
	s, ok := file.Impl().(socket.Socket)
	if !ok {
		return 0, nil, linuxerr.ENOTSOCK
	}

	// Capture address and call syscall implementation.
	a, err := CaptureAddress(t, addr, addrlen)
	if err != nil {
		return 0, nil, err
	}

	blocking := (file.StatusFlags() & linux.SOCK_NONBLOCK) == 0
	return 0, nil, linuxerr.ConvertIntr(s.Connect(t, a, blocking).ToError(), linuxerr.ERESTARTSYS)
}

// accept is the implementation of the accept syscall. It is called by accept
// and accept4 syscall handlers.
func accept(t *kernel.Task, fd int32, addr hostarch.Addr, addrLen hostarch.Addr, flags int) (uintptr, error) {
	// Check that no unsupported flags are passed in.
	if flags & ^(linux.SOCK_NONBLOCK|linux.SOCK_CLOEXEC) != 0 {
		return 0, linuxerr.EINVAL
	}

	// Get socket from the file descriptor.
	file := t.GetFile(fd)
	if file == nil {
		return 0, linuxerr.EBADF
	}
	defer file.DecRef(t)

	// Extract the socket.
	s, ok := file.Impl().(socket.Socket)
	if !ok {
		return 0, linuxerr.ENOTSOCK
	}

	// Call the syscall implementation for this socket, then copy the
	// output address if one is specified.
	blocking := (file.StatusFlags() & linux.SOCK_NONBLOCK) == 0

	peerRequested := addrLen != 0
	nfd, peer, peerLen, e := s.Accept(t, peerRequested, flags, blocking)
	if e != nil {
		return 0, linuxerr.ConvertIntr(e.ToError(), linuxerr.ERESTARTSYS)
	}
	if peerRequested {
		// NOTE(magi): Linux does not give you an error if it can't
		// write the data back out so neither do we.
		if err := writeAddress(t, peer, peerLen, addr, addrLen); linuxerr.Equals(linuxerr.EINVAL, err) {
			return 0, err
		}
	}
	return uintptr(nfd), nil
}

// Accept4 implements the linux syscall accept4(2).
func Accept4(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	fd := args[0].Int()
	addr := args[1].Pointer()
	addrlen := args[2].Pointer()
	flags := int(args[3].Int())

	n, err := accept(t, fd, addr, addrlen, flags)
	return n, nil, err
}

// Accept implements the linux syscall accept(2).
func Accept(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	fd := args[0].Int()
	addr := args[1].Pointer()
	addrlen := args[2].Pointer()

	n, err := accept(t, fd, addr, addrlen, 0)
	return n, nil, err
}

// Bind implements the linux syscall bind(2).
func Bind(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	fd := args[0].Int()
	addr := args[1].Pointer()
	addrlen := args[2].Uint()

	// Get socket from the file descriptor.
	file := t.GetFile(fd)
	if file == nil {
		return 0, nil, linuxerr.EBADF
	}
	defer file.DecRef(t)

	// Extract the socket.
	s, ok := file.Impl().(socket.Socket)
	if !ok {
		return 0, nil, linuxerr.ENOTSOCK
	}

	// Capture address and call syscall implementation.
	a, err := CaptureAddress(t, addr, addrlen)
	if err != nil {
		return 0, nil, err
	}

	return 0, nil, s.Bind(t, a).ToError()
}

// Listen implements the linux syscall listen(2).
func Listen(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	fd := args[0].Int()
	backlog := args[1].Uint()

	// Get socket from the file descriptor.
	file := t.GetFile(fd)
	if file == nil {
		return 0, nil, linuxerr.EBADF
	}
	defer file.DecRef(t)

	// Extract the socket.
	s, ok := file.Impl().(socket.Socket)
	if !ok {
		return 0, nil, linuxerr.ENOTSOCK
	}

	if backlog > maxListenBacklog {
		// Linux treats incoming backlog as uint with a limit defined by
		// sysctl_somaxconn.
		// https://github.com/torvalds/linux/blob/7acac4b3196/net/socket.c#L1666
		backlog = maxListenBacklog
	}

	// Accept one more than the configured listen backlog to keep in parity with
	// Linux. Ref, because of missing equality check here:
	// https://github.com/torvalds/linux/blob/7acac4b3196/include/net/sock.h#L937
	//
	// In case of unix domain sockets, the following check
	// https://github.com/torvalds/linux/blob/7d6beb71da3/net/unix/af_unix.c#L1293
	// will allow 1 connect through since it checks for a receive queue len >
	// backlog and not >=.
	backlog++

	return 0, nil, s.Listen(t, int(backlog)).ToError()
}

// Shutdown implements the linux syscall shutdown(2).
func Shutdown(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	fd := args[0].Int()
	how := args[1].Int()

	// Get socket from the file descriptor.
	file := t.GetFile(fd)
	if file == nil {
		return 0, nil, linuxerr.EBADF
	}
	defer file.DecRef(t)

	// Extract the socket.
	s, ok := file.Impl().(socket.Socket)
	if !ok {
		return 0, nil, linuxerr.ENOTSOCK
	}

	// Validate how, then call syscall implementation.
	switch how {
	case linux.SHUT_RD, linux.SHUT_WR, linux.SHUT_RDWR:
	default:
		return 0, nil, linuxerr.EINVAL
	}

	return 0, nil, s.Shutdown(t, int(how)).ToError()
}

// GetSockOpt implements the linux syscall getsockopt(2).
func GetSockOpt(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	fd := args[0].Int()
	level := args[1].Int()
	name := args[2].Int()
	optValAddr := args[3].Pointer()
	optLenAddr := args[4].Pointer()

	// Get socket from the file descriptor.
	file := t.GetFile(fd)
	if file == nil {
		return 0, nil, linuxerr.EBADF
	}
	defer file.DecRef(t)

	// Extract the socket.
	s, ok := file.Impl().(socket.Socket)
	if !ok {
		return 0, nil, linuxerr.ENOTSOCK
	}

	// Read the length. Reject negative values.
	var optLen int32
	if _, err := primitive.CopyInt32In(t, optLenAddr, &optLen); err != nil {
		return 0, nil, err
	}
	if optLen < 0 {
		return 0, nil, linuxerr.EINVAL
	}

	// Call syscall implementation then copy both value and value len out.
	v, e := getSockOpt(t, s, int(level), int(name), optValAddr, int(optLen))
	if e != nil {
		return 0, nil, e.ToError()
	}

	if _, err := primitive.CopyInt32Out(t, optLenAddr, int32(v.SizeBytes())); err != nil {
		return 0, nil, err
	}

	if v != nil {
		if _, err := v.CopyOut(t, optValAddr); err != nil {
			return 0, nil, err
		}
	}

	return 0, nil, nil
}

// getSockOpt tries to handle common socket options, or dispatches to a specific
// socket implementation.
func getSockOpt(t *kernel.Task, s socket.Socket, level, name int, optValAddr hostarch.Addr, len int) (marshal.Marshallable, *syserr.Error) {
	if level == linux.SOL_SOCKET {
		switch name {
		case linux.SO_TYPE, linux.SO_DOMAIN, linux.SO_PROTOCOL:
			if len < sizeOfInt32 {
				return nil, syserr.ErrInvalidArgument
			}
		}

		switch name {
		case linux.SO_TYPE:
			_, skType, _ := s.Type()
			v := primitive.Int32(skType)
			return &v, nil
		case linux.SO_DOMAIN:
			family, _, _ := s.Type()
			v := primitive.Int32(family)
			return &v, nil
		case linux.SO_PROTOCOL:
			_, _, protocol := s.Type()
			v := primitive.Int32(protocol)
			return &v, nil
		}
	}

	return s.GetSockOpt(t, level, name, optValAddr, len)
}

// SetSockOpt implements the linux syscall setsockopt(2).
//
// Note that unlike Linux, enabling SO_PASSCRED does not autobind the socket.
func SetSockOpt(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	fd := args[0].Int()
	level := args[1].Int()
	name := args[2].Int()
	optValAddr := args[3].Pointer()
	optLen := args[4].Int()

	// Get socket from the file descriptor.
	file := t.GetFile(fd)
	if file == nil {
		return 0, nil, linuxerr.EBADF
	}
	defer file.DecRef(t)

	// Extract the socket.
	s, ok := file.Impl().(socket.Socket)
	if !ok {
		return 0, nil, linuxerr.ENOTSOCK
	}

	if optLen < 0 {
		return 0, nil, linuxerr.EINVAL
	}
	if optLen > maxOptLen {
		return 0, nil, linuxerr.EINVAL
	}
	buf := t.CopyScratchBuffer(int(optLen))
	if _, err := t.CopyInBytes(optValAddr, buf); err != nil {
		return 0, nil, err
	}

	// Call syscall implementation.
	if err := s.SetSockOpt(t, int(level), int(name), buf); err != nil {
		return 0, nil, err.ToError()
	}

	return 0, nil, nil
}

// GetSockName implements the linux syscall getsockname(2).
func GetSockName(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	fd := args[0].Int()
	addr := args[1].Pointer()
	addrlen := args[2].Pointer()

	// Get socket from the file descriptor.
	file := t.GetFile(fd)
	if file == nil {
		return 0, nil, linuxerr.EBADF
	}
	defer file.DecRef(t)

	// Extract the socket.
	s, ok := file.Impl().(socket.Socket)
	if !ok {
		return 0, nil, linuxerr.ENOTSOCK
	}

	// Get the socket name and copy it to the caller.
	v, vl, err := s.GetSockName(t)
	if err != nil {
		return 0, nil, err.ToError()
	}

	return 0, nil, writeAddress(t, v, vl, addr, addrlen)
}

// GetPeerName implements the linux syscall getpeername(2).
func GetPeerName(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	fd := args[0].Int()
	addr := args[1].Pointer()
	addrlen := args[2].Pointer()

	// Get socket from the file descriptor.
	file := t.GetFile(fd)
	if file == nil {
		return 0, nil, linuxerr.EBADF
	}
	defer file.DecRef(t)

	// Extract the socket.
	s, ok := file.Impl().(socket.Socket)
	if !ok {
		return 0, nil, linuxerr.ENOTSOCK
	}

	// Get the socket peer name and copy it to the caller.
	v, vl, err := s.GetPeerName(t)
	if err != nil {
		return 0, nil, err.ToError()
	}

	return 0, nil, writeAddress(t, v, vl, addr, addrlen)
}

// RecvMsg implements the linux syscall recvmsg(2).
func RecvMsg(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	fd := args[0].Int()
	msgPtr := args[1].Pointer()
	flags := args[2].Int()

	if t.Arch().Width() != 8 {
		// We only handle 64-bit for now.
		return 0, nil, linuxerr.EINVAL
	}

	// Get socket from the file descriptor.
	file := t.GetFile(fd)
	if file == nil {
		return 0, nil, linuxerr.EBADF
	}
	defer file.DecRef(t)

	// Extract the socket.
	s, ok := file.Impl().(socket.Socket)
	if !ok {
		return 0, nil, linuxerr.ENOTSOCK
	}

	// Reject flags that we don't handle yet.
	if flags & ^(baseRecvFlags|linux.MSG_PEEK|linux.MSG_CMSG_CLOEXEC|linux.MSG_ERRQUEUE) != 0 {
		return 0, nil, linuxerr.EINVAL
	}

	if (file.StatusFlags() & linux.SOCK_NONBLOCK) != 0 {
		flags |= linux.MSG_DONTWAIT
	}

	var haveDeadline bool
	var deadline ktime.Time
	if dl := s.RecvTimeout(); dl > 0 {
		deadline = t.Kernel().MonotonicClock().Now().Add(time.Duration(dl) * time.Nanosecond)
		haveDeadline = true
	} else if dl < 0 {
		flags |= linux.MSG_DONTWAIT
	}

	n, err := recvSingleMsg(t, s, msgPtr, flags, haveDeadline, deadline)
	return n, nil, err
}

// RecvMMsg implements the linux syscall recvmmsg(2).
func RecvMMsg(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	fd := args[0].Int()
	msgPtr := args[1].Pointer()
	vlen := args[2].Uint()
	flags := args[3].Int()
	toPtr := args[4].Pointer()

	if t.Arch().Width() != 8 {
		// We only handle 64-bit for now.
		return 0, nil, linuxerr.EINVAL
	}

	if vlen > linux.UIO_MAXIOV {
		vlen = linux.UIO_MAXIOV
	}

	// Reject flags that we don't handle yet.
	if flags & ^(baseRecvFlags|linux.MSG_CMSG_CLOEXEC|linux.MSG_ERRQUEUE) != 0 {
		return 0, nil, linuxerr.EINVAL
	}

	// Get socket from the file descriptor.
	file := t.GetFile(fd)
	if file == nil {
		return 0, nil, linuxerr.EBADF
	}
	defer file.DecRef(t)

	// Extract the socket.
	s, ok := file.Impl().(socket.Socket)
	if !ok {
		return 0, nil, linuxerr.ENOTSOCK
	}

	if (file.StatusFlags() & linux.SOCK_NONBLOCK) != 0 {
		flags |= linux.MSG_DONTWAIT
	}

	var haveDeadline bool
	var deadline ktime.Time
	if toPtr != 0 {
		var ts linux.Timespec
		if _, err := ts.CopyIn(t, toPtr); err != nil {
			return 0, nil, err
		}
		if !ts.Valid() {
			return 0, nil, linuxerr.EINVAL
		}
		deadline = t.Kernel().MonotonicClock().Now().Add(ts.ToDuration())
		haveDeadline = true
	}

	if !haveDeadline {
		if dl := s.RecvTimeout(); dl > 0 {
			deadline = t.Kernel().MonotonicClock().Now().Add(time.Duration(dl) * time.Nanosecond)
			haveDeadline = true
		} else if dl < 0 {
			flags |= linux.MSG_DONTWAIT
		}
	}

	var count uint32
	var err error
	for i := uint64(0); i < uint64(vlen); i++ {
		mp, ok := msgPtr.AddLength(i * multipleMessageHeader64Len)
		if !ok {
			return 0, nil, linuxerr.EFAULT
		}
		var n uintptr
		if n, err = recvSingleMsg(t, s, mp, flags, haveDeadline, deadline); err != nil {
			break
		}

		// Copy the received length to the caller.
		lp, ok := mp.AddLength(messageHeader64Len)
		if !ok {
			return 0, nil, linuxerr.EFAULT
		}
		if _, err = primitive.CopyUint32Out(t, lp, uint32(n)); err != nil {
			break
		}
		count++
	}

	if count == 0 {
		return 0, nil, err
	}
	return uintptr(count), nil, nil
}

func getSCMRights(t *kernel.Task, rights transport.RightsControlMessage) control.SCMRights {
	switch v := rights.(type) {
	case control.SCMRights:
		return v
	case *transport.SCMRights:
		rf := control.RightsFiles(fdsToHostFiles(t, v.FDs))
		return &rf
	default:
		panic(fmt.Sprintf("rights of type %T must be *transport.SCMRights or implement SCMRights", rights))
	}
}

// If an error is encountered, only files created before the error will be
// returned. This is what Linux does.
func fdsToHostFiles(ctx context.Context, fds []int) []*vfs.FileDescription {
	files := make([]*vfs.FileDescription, 0, len(fds))
	for _, fd := range fds {
		// Get flags. We do it here because they may be modified
		// by subsequent functions.
		fileFlags, _, errno := unix.Syscall(unix.SYS_FCNTL, uintptr(fd), unix.F_GETFL, 0)
		if errno != 0 {
			ctx.Warningf("Error retrieving host FD flags: %v", error(errno))
			break
		}

		// Create the file backed by hostFD.
		file, err := host.NewFD(ctx, kernel.KernelFromContext(ctx).HostMount(), fd, &host.NewFDOptions{})
		if err != nil {
			ctx.Warningf("Error creating file from host FD: %v", err)
			break
		}

		if err := file.SetStatusFlags(ctx, auth.CredentialsFromContext(ctx), uint32(fileFlags&linux.O_NONBLOCK)); err != nil {
			ctx.Warningf("Error setting flags on host FD file: %v", err)
			break
		}

		files = append(files, file)
	}
	return files
}

func recvSingleMsg(t *kernel.Task, s socket.Socket, msgPtr hostarch.Addr, flags int32, haveDeadline bool, deadline ktime.Time) (uintptr, error) {
	// Capture the message header and io vectors.
	var msg MessageHeader64
	if _, err := msg.CopyIn(t, msgPtr); err != nil {
		return 0, err
	}

	if msg.IovLen > linux.UIO_MAXIOV {
		return 0, linuxerr.EMSGSIZE
	}
	dst, err := t.IovecsIOSequence(hostarch.Addr(msg.Iov), int(msg.IovLen), usermem.IOOpts{
		AddressSpaceActive: true,
	})
	if err != nil {
		return 0, err
	}

	// Fast path when no control message nor name buffers are provided.
	if msg.ControlLen == 0 && msg.NameLen == 0 {
		n, mflags, _, _, cms, err := s.RecvMsg(t, dst, int(flags), haveDeadline, deadline, false, 0)
		if err != nil {
			return 0, linuxerr.ConvertIntr(err.ToError(), linuxerr.ERESTARTSYS)
		}
		if !cms.Unix.Empty() {
			mflags |= linux.MSG_CTRUNC
			cms.Release(t)
		}

		if int(msg.Flags) != mflags {
			// Copy out the flags to the caller.
			if _, err := primitive.CopyInt32Out(t, msgPtr+flagsOffset, int32(mflags)); err != nil {
				return 0, err
			}
		}

		return uintptr(n), nil
	}

	if msg.ControlLen > maxControlLen {
		return 0, linuxerr.ENOBUFS
	}
	n, mflags, sender, senderLen, cms, e := s.RecvMsg(t, dst, int(flags), haveDeadline, deadline, msg.NameLen != 0, msg.ControlLen)
	if e != nil {
		return 0, linuxerr.ConvertIntr(e.ToError(), linuxerr.ERESTARTSYS)
	}
	defer cms.Release(t)

	controlData := make([]byte, 0, msg.ControlLen)
	controlData = control.PackControlMessages(t, cms, controlData)

	if cr, ok := s.(transport.Credentialer); ok && cr.Passcred() {
		creds, _ := cms.Unix.Credentials.(control.SCMCredentials)
		controlData, mflags = control.PackCredentials(t, creds, controlData, mflags)
	}

	if cms.Unix.Rights != nil {
		cms.Unix.Rights = getSCMRights(t, cms.Unix.Rights)
		controlData, mflags = control.PackRights(t, cms.Unix.Rights.(control.SCMRights), flags&linux.MSG_CMSG_CLOEXEC != 0, controlData, mflags)
	}

	// Copy the address to the caller.
	if msg.NameLen != 0 {
		if err := writeAddress(t, sender, senderLen, hostarch.Addr(msg.Name), hostarch.Addr(msgPtr+nameLenOffset)); err != nil {
			return 0, err
		}
	}

	// Copy the control data to the caller.
	if _, err := primitive.CopyUint64Out(t, msgPtr+controlLenOffset, uint64(len(controlData))); err != nil {
		return 0, err
	}
	if len(controlData) > 0 {
		if _, err := t.CopyOutBytes(hostarch.Addr(msg.Control), controlData); err != nil {
			return 0, err
		}
	}

	// Copy out the flags to the caller.
	if _, err := primitive.CopyInt32Out(t, msgPtr+flagsOffset, int32(mflags)); err != nil {
		return 0, err
	}

	return uintptr(n), nil
}

// recvFrom is the implementation of the recvfrom syscall. It is called by
// recvfrom and recv syscall handlers.
func recvFrom(t *kernel.Task, fd int32, bufPtr hostarch.Addr, bufLen uint64, flags int32, namePtr hostarch.Addr, nameLenPtr hostarch.Addr) (uintptr, error) {
	if int(bufLen) < 0 {
		return 0, linuxerr.EINVAL
	}

	// Reject flags that we don't handle yet.
	if flags & ^(baseRecvFlags|linux.MSG_PEEK|linux.MSG_CONFIRM) != 0 {
		return 0, linuxerr.EINVAL
	}

	// Get socket from the file descriptor.
	file := t.GetFile(fd)
	if file == nil {
		return 0, linuxerr.EBADF
	}
	defer file.DecRef(t)

	// Extract the socket.
	s, ok := file.Impl().(socket.Socket)
	if !ok {
		return 0, linuxerr.ENOTSOCK
	}

	if (file.StatusFlags() & linux.SOCK_NONBLOCK) != 0 {
		flags |= linux.MSG_DONTWAIT
	}

	dst, err := t.SingleIOSequence(bufPtr, int(bufLen), usermem.IOOpts{
		AddressSpaceActive: true,
	})
	if err != nil {
		return 0, err
	}

	var haveDeadline bool
	var deadline ktime.Time
	if dl := s.RecvTimeout(); dl > 0 {
		deadline = t.Kernel().MonotonicClock().Now().Add(time.Duration(dl) * time.Nanosecond)
		haveDeadline = true
	} else if dl < 0 {
		flags |= linux.MSG_DONTWAIT
	}

	n, _, sender, senderLen, cm, e := s.RecvMsg(t, dst, int(flags), haveDeadline, deadline, nameLenPtr != 0, 0)
	cm.Release(t)
	if e != nil {
		return 0, linuxerr.ConvertIntr(e.ToError(), linuxerr.ERESTARTSYS)
	}

	// Copy the address to the caller.
	if nameLenPtr != 0 {
		if err := writeAddress(t, sender, senderLen, namePtr, nameLenPtr); err != nil {
			return 0, err
		}
	}

	return uintptr(n), nil
}

// RecvFrom implements the linux syscall recvfrom(2).
func RecvFrom(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	fd := args[0].Int()
	bufPtr := args[1].Pointer()
	bufLen := args[2].Uint64()
	flags := args[3].Int()
	namePtr := args[4].Pointer()
	nameLenPtr := args[5].Pointer()

	n, err := recvFrom(t, fd, bufPtr, bufLen, flags, namePtr, nameLenPtr)
	return n, nil, err
}

// SendMsg implements the linux syscall sendmsg(2).
func SendMsg(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	fd := args[0].Int()
	msgPtr := args[1].Pointer()
	flags := args[2].Int()

	if t.Arch().Width() != 8 {
		// We only handle 64-bit for now.
		return 0, nil, linuxerr.EINVAL
	}

	// Get socket from the file descriptor.
	file := t.GetFile(fd)
	if file == nil {
		return 0, nil, linuxerr.EBADF
	}
	defer file.DecRef(t)

	// Extract the socket.
	s, ok := file.Impl().(socket.Socket)
	if !ok {
		return 0, nil, linuxerr.ENOTSOCK
	}

	// Reject flags that we don't handle yet.
	if flags & ^(linux.MSG_DONTWAIT|linux.MSG_EOR|linux.MSG_MORE|linux.MSG_NOSIGNAL) != 0 {
		return 0, nil, linuxerr.EINVAL
	}

	if (file.StatusFlags() & linux.SOCK_NONBLOCK) != 0 {
		flags |= linux.MSG_DONTWAIT
	}

	n, err := sendSingleMsg(t, s, file, msgPtr, flags)
	return n, nil, err
}

// SendMMsg implements the linux syscall sendmmsg(2).
func SendMMsg(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	fd := args[0].Int()
	msgPtr := args[1].Pointer()
	vlen := args[2].Uint()
	flags := args[3].Int()

	if t.Arch().Width() != 8 {
		// We only handle 64-bit for now.
		return 0, nil, linuxerr.EINVAL
	}

	if vlen > linux.UIO_MAXIOV {
		vlen = linux.UIO_MAXIOV
	}

	// Get socket from the file descriptor.
	file := t.GetFile(fd)
	if file == nil {
		return 0, nil, linuxerr.EBADF
	}
	defer file.DecRef(t)

	// Extract the socket.
	s, ok := file.Impl().(socket.Socket)
	if !ok {
		return 0, nil, linuxerr.ENOTSOCK
	}

	// Reject flags that we don't handle yet.
	if flags & ^(linux.MSG_DONTWAIT|linux.MSG_EOR|linux.MSG_MORE|linux.MSG_NOSIGNAL) != 0 {
		return 0, nil, linuxerr.EINVAL
	}

	if (file.StatusFlags() & linux.SOCK_NONBLOCK) != 0 {
		flags |= linux.MSG_DONTWAIT
	}

	var count uint32
	var err error
	for i := uint64(0); i < uint64(vlen); i++ {
		mp, ok := msgPtr.AddLength(i * multipleMessageHeader64Len)
		if !ok {
			return 0, nil, linuxerr.EFAULT
		}
		var n uintptr
		if n, err = sendSingleMsg(t, s, file, mp, flags); err != nil {
			break
		}

		// Copy the received length to the caller.
		lp, ok := mp.AddLength(messageHeader64Len)
		if !ok {
			return 0, nil, linuxerr.EFAULT
		}
		if _, err = primitive.CopyUint32Out(t, lp, uint32(n)); err != nil {
			break
		}
		count++
	}

	if count == 0 {
		return 0, nil, err
	}
	return uintptr(count), nil, nil
}

func sendSingleMsg(t *kernel.Task, s socket.Socket, file *vfs.FileDescription, msgPtr hostarch.Addr, flags int32) (uintptr, error) {
	// Capture the message header.
	var msg MessageHeader64
	if _, err := msg.CopyIn(t, msgPtr); err != nil {
		return 0, err
	}

	var controlData []byte
	if msg.ControlLen > 0 {
		// Put an upper bound to prevent large allocations.
		if msg.ControlLen > maxControlLen {
			return 0, linuxerr.ENOBUFS
		}
		controlData = make([]byte, msg.ControlLen)
		if _, err := t.CopyInBytes(hostarch.Addr(msg.Control), controlData); err != nil {
			return 0, err
		}
	}

	// Read the destination address if one is specified.
	var to []byte
	if msg.NameLen != 0 {
		var err error
		to, err = CaptureAddress(t, hostarch.Addr(msg.Name), msg.NameLen)
		if err != nil {
			return 0, err
		}
	}

	// Read data then call the sendmsg implementation.
	if msg.IovLen > linux.UIO_MAXIOV {
		return 0, linuxerr.EMSGSIZE
	}
	src, err := t.IovecsIOSequence(hostarch.Addr(msg.Iov), int(msg.IovLen), usermem.IOOpts{
		AddressSpaceActive: true,
	})
	if err != nil {
		return 0, err
	}

	controlMessages, err := control.Parse(t, s, controlData, t.Arch().Width())
	if err != nil {
		return 0, err
	}

	var haveDeadline bool
	var deadline ktime.Time
	if dl := s.SendTimeout(); dl > 0 {
		deadline = t.Kernel().MonotonicClock().Now().Add(time.Duration(dl) * time.Nanosecond)
		haveDeadline = true
	} else if dl < 0 {
		flags |= linux.MSG_DONTWAIT
	}

	// Call the syscall implementation.
	n, e := s.SendMsg(t, src, to, int(flags), haveDeadline, deadline, controlMessages)
	err = HandleIOError(t, n != 0, e.ToError(), linuxerr.ERESTARTSYS, "sendmsg", file)
	// Control messages should be released on error as well as for zero-length
	// messages, which are discarded by the receiver.
	if n == 0 || err != nil {
		controlMessages.Release(t)
	}
	return uintptr(n), err
}

// sendTo is the implementation of the sendto syscall. It is called by sendto
// and send syscall handlers.
func sendTo(t *kernel.Task, fd int32, bufPtr hostarch.Addr, bufLen uint64, flags int32, namePtr hostarch.Addr, nameLen uint32) (uintptr, error) {
	bl := int(bufLen)
	if bl < 0 {
		return 0, linuxerr.EINVAL
	}

	// Get socket from the file descriptor.
	file := t.GetFile(fd)
	if file == nil {
		return 0, linuxerr.EBADF
	}
	defer file.DecRef(t)

	// Extract the socket.
	s, ok := file.Impl().(socket.Socket)
	if !ok {
		return 0, linuxerr.ENOTSOCK
	}

	if (file.StatusFlags() & linux.SOCK_NONBLOCK) != 0 {
		flags |= linux.MSG_DONTWAIT
	}

	// Read the destination address if one is specified.
	var to []byte
	var err error
	if namePtr != 0 {
		to, err = CaptureAddress(t, namePtr, nameLen)
		if err != nil {
			return 0, err
		}
	}

	src, err := t.SingleIOSequence(bufPtr, bl, usermem.IOOpts{
		AddressSpaceActive: true,
	})
	if err != nil {
		return 0, err
	}

	var haveDeadline bool
	var deadline ktime.Time
	if dl := s.SendTimeout(); dl > 0 {
		deadline = t.Kernel().MonotonicClock().Now().Add(time.Duration(dl) * time.Nanosecond)
		haveDeadline = true
	} else if dl < 0 {
		flags |= linux.MSG_DONTWAIT
	}

	// Call the syscall implementation.
	n, e := s.SendMsg(t, src, to, int(flags), haveDeadline, deadline, socket.ControlMessages{Unix: control.New(t, s)})
	return uintptr(n), HandleIOError(t, n != 0, e.ToError(), linuxerr.ERESTARTSYS, "sendto", file)
}

// SendTo implements the linux syscall sendto(2).
func SendTo(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	fd := args[0].Int()
	bufPtr := args[1].Pointer()
	bufLen := args[2].Uint64()
	flags := args[3].Int()
	namePtr := args[4].Pointer()
	nameLen := args[5].Uint()

	n, err := sendTo(t, fd, bufPtr, bufLen, flags, namePtr, nameLen)
	return n, nil, err
}