[lisa] lisafs package POC.

This change mainly aims to define the semantics of communication for the LISAFS
(LInux SAndbox Filesystem) protocol. This protocol aims to replace 9P and
intends to bring some performance benefits with it.

Some of the notable differences from the p9 package are:
- Now the server implementations own the handlers.
- As a result, there is no verbose interface like `p9.File` that all servers
  need to implement. Different implementations can extend their File
  implementations to varying degrees without imposing those extensions to other
  server implementations that might not have anything to do with those features.
- If a server implementation adds a new RPC message, other implementations are
  not compelled to support it.

I wrote a benchmark `BenchmarkSendRecv` in connection_test.go which competes
with p9's `BenchmarkSendRecvChannel`. Running these on an AMD Milan machine
shows that lisafs is **45%** faster.

**With 9P**
goos: linux
goarch: amd64
pkg: gvisor/pkg/p9/p9
cpu: AMD EPYC 7B13 64-Core Processor
BenchmarkSendRecvLegacy-256     82830     14053 ns/op     633 B/op     23 allocs/op
BenchmarkSendRecvChannel-256     776971     1551 ns/op     184 B/op     6 allocs/op

**With lisafs**
goos: linux
goarch: amd64
pkg: pkg/lisafs/connection_test
cpu: AMD EPYC 7B13 64-Core Processor
BenchmarkSendRecv-256     1399610     853.5 ns/op     48 B/op     2 allocs/op

Fixes #5464

PiperOrigin-RevId: 397803163

1 files changed, 190 insertions, 0 deletions

diff --git a/pkg/lisafs/channel.go b/pkg/lisafs/channel.go
new file mode 100644
index 000000000..301212e51
--- /dev/null
+++ b/pkg/lisafs/channel.go
@@ -0,0 +1,190 @@
+// Copyright 2021 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 lisafs
+
+import (
+	"math"
+	"runtime"
+
+	"golang.org/x/sys/unix"
+	"gvisor.dev/gvisor/pkg/fdchannel"
+	"gvisor.dev/gvisor/pkg/flipcall"
+	"gvisor.dev/gvisor/pkg/log"
+)
+
+var (
+	chanHeaderLen = uint32((*channelHeader)(nil).SizeBytes())
+)
+
+// maxChannels returns the number of channels a client can create.
+//
+// The server will reject channel creation requests beyond this (per client).
+// Note that we don't want the number of channels to be too large, because each
+// accounts for a large region of shared memory.
+// TODO(gvisor.dev/issue/6313): Tune the number of channels.
+func maxChannels() int {
+	maxChans := runtime.GOMAXPROCS(0)
+	if maxChans < 2 {
+		maxChans = 2
+	}
+	if maxChans > 4 {
+		maxChans = 4
+	}
+	return maxChans
+}
+
+// channel implements Communicator and represents the communication endpoint
+// for the client and server and is used to perform fast IPC. Apart from
+// communicating data, a channel is also capable of donating file descriptors.
+type channel struct {
+	fdTracker
+	dead   bool
+	data   flipcall.Endpoint
+	fdChan fdchannel.Endpoint
+}
+
+var _ Communicator = (*channel)(nil)
+
+// PayloadBuf implements Communicator.PayloadBuf.
+func (ch *channel) PayloadBuf(size uint32) []byte {
+	return ch.data.Data()[chanHeaderLen : chanHeaderLen+size]
+}
+
+// SndRcvMessage implements Communicator.SndRcvMessage.
+func (ch *channel) SndRcvMessage(m MID, payloadLen uint32, wantFDs uint8) (MID, uint32, error) {
+	// Write header. Requests can not donate FDs.
+	ch.marshalHdr(m, 0 /* numFDs */)
+
+	// One-shot communication. RPCs are expected to be quick rather than block.
+	rcvDataLen, err := ch.data.SendRecvFast(chanHeaderLen + payloadLen)
+	if err != nil {
+		// This channel is now unusable.
+		ch.dead = true
+		// Map the transport errors to EIO, but also log the real error.
+		log.Warningf("lisafs.sndRcvMessage: flipcall.Endpoint.SendRecv: %v", err)
+		return 0, 0, unix.EIO
+	}
+
+	return ch.rcvMsg(rcvDataLen)
+}
+
+func (ch *channel) shutdown() {
+	ch.data.Shutdown()
+}
+
+func (ch *channel) destroy() {
+	ch.dead = true
+	ch.fdChan.Destroy()
+	ch.data.Destroy()
+}
+
+// createChannel creates a server side channel. It returns a packet window
+// descriptor (for the data channel) and an open socket for the FD channel.
+func (c *Connection) createChannel(maxMessageSize uint32) (*channel, flipcall.PacketWindowDescriptor, int, error) {
+	c.channelsMu.Lock()
+	defer c.channelsMu.Unlock()
+	// If c.channels is nil, the connection has closed.
+	if c.channels == nil || len(c.channels) >= maxChannels() {
+		return nil, flipcall.PacketWindowDescriptor{}, -1, unix.ENOSYS
+	}
+	ch := &channel{}
+
+	// Set up data channel.
+	desc, err := c.channelAlloc.Allocate(flipcall.PacketHeaderBytes + int(chanHeaderLen+maxMessageSize))
+	if err != nil {
+		return nil, flipcall.PacketWindowDescriptor{}, -1, err
+	}
+	if err := ch.data.Init(flipcall.ServerSide, desc); err != nil {
+		return nil, flipcall.PacketWindowDescriptor{}, -1, err
+	}
+
+	// Set up FD channel.
+	fdSocks, err := fdchannel.NewConnectedSockets()
+	if err != nil {
+		ch.data.Destroy()
+		return nil, flipcall.PacketWindowDescriptor{}, -1, err
+	}
+	ch.fdChan.Init(fdSocks[0])
+	clientFDSock := fdSocks[1]
+
+	c.channels = append(c.channels, ch)
+	return ch, desc, clientFDSock, nil
+}
+
+// sendFDs sends as many FDs as it can. The failure to send an FD does not
+// cause an error and fail the entire RPC. FDs are considered supplementary
+// responses that are not critical to the RPC response itself. The failure to
+// send the (i)th FD will cause all the following FDs to not be sent as well
+// because the order in which FDs are donated is important.
+func (ch *channel) sendFDs(fds []int) uint8 {
+	numFDs := len(fds)
+	if numFDs == 0 {
+		return 0
+	}
+
+	if numFDs > math.MaxUint8 {
+		log.Warningf("dropping all FDs because too many FDs to donate: %v", numFDs)
+		return 0
+	}
+
+	for i, fd := range fds {
+		if err := ch.fdChan.SendFD(fd); err != nil {
+			log.Warningf("error occurred while sending (%d/%d)th FD on channel(%p): %v", i+1, numFDs, ch, err)
+			return uint8(i)
+		}
+	}
+	return uint8(numFDs)
+}
+
+// channelHeader is the header present in front of each message received on
+// flipcall endpoint when the protocol version being used is 1.
+//
+// +marshal
+type channelHeader struct {
+	message MID
+	numFDs  uint8
+	_       uint8 // Need to make struct packed.
+}
+
+func (ch *channel) marshalHdr(m MID, numFDs uint8) {
+	header := &channelHeader{
+		message: m,
+		numFDs:  numFDs,
+	}
+	header.MarshalUnsafe(ch.data.Data())
+}
+
+func (ch *channel) rcvMsg(dataLen uint32) (MID, uint32, error) {
+	if dataLen < chanHeaderLen {
+		log.Warningf("received data has size smaller than header length: %d", dataLen)
+		return 0, 0, unix.EIO
+	}
+
+	// Read header first.
+	var header channelHeader
+	header.UnmarshalUnsafe(ch.data.Data())
+
+	// Read any FDs.
+	for i := 0; i < int(header.numFDs); i++ {
+		fd, err := ch.fdChan.RecvFDNonblock()
+		if err != nil {
+			log.Warningf("expected %d FDs, received %d successfully, got err after that: %v", header.numFDs, i, err)
+			break
+		}
+		ch.TrackFD(fd)
+	}
+
+	return header.message, dataLen - chanHeaderLen, nil
+}