netstack: Refactor tcpip.Endpoint.Read

Read now takes a destination io.Writer, count, options. Keeping the method name
Read, in contrast to the Write method.

This enables:
* direct transfer of views under VV
* zero copy

It also eliminates the need for sentry to keep a slice of view because
userspace had requested a read that is smaller than the view returned, removing
the complexity there.

Read/Peek/ReadPacket are now consolidated together and some duplicate code is
removed.

PiperOrigin-RevId: 350636322

3 files changed, 48 insertions, 27 deletions

diff --git a/pkg/tcpip/transport/udp/BUILD b/pkg/tcpip/transport/udp/BUILD
index 7ebae63d8..153e8c950 100644
--- a/pkg/tcpip/transport/udp/BUILD
+++ b/pkg/tcpip/transport/udp/BUILD
@@ -58,5 +58,6 @@ go_test(
         "//pkg/tcpip/stack",
         "//pkg/tcpip/transport/icmp",
         "//pkg/waiter",
+        "@com_github_google_go_cmp//cmp:go_default_library",
     ],
 )
diff --git a/pkg/tcpip/transport/udp/endpoint.go b/pkg/tcpip/transport/udp/endpoint.go
index 4e8bd8b04..075de1db0 100644
--- a/pkg/tcpip/transport/udp/endpoint.go
+++ b/pkg/tcpip/transport/udp/endpoint.go
@@ -16,6 +16,7 @@ package udp
 
 import (
 	"fmt"
+	"io"
 	"sync/atomic"
 
 	"gvisor.dev/gvisor/pkg/sync"
@@ -282,11 +283,10 @@ func (e *endpoint) Close() {
 // ModerateRecvBuf implements tcpip.Endpoint.ModerateRecvBuf.
 func (e *endpoint) ModerateRecvBuf(copied int) {}
 
-// Read reads data from the endpoint. This method does not block if
-// there is no data pending.
-func (e *endpoint) Read(addr *tcpip.FullAddress) (buffer.View, tcpip.ControlMessages, *tcpip.Error) {
+// Read implements tcpip.Endpoint.Read.
+func (e *endpoint) Read(dst io.Writer, count int, opts tcpip.ReadOptions) (tcpip.ReadResult, *tcpip.Error) {
 	if err := e.LastError(); err != nil {
-		return buffer.View{}, tcpip.ControlMessages{}, err
+		return tcpip.ReadResult{}, err
 	}
 
 	e.rcvMu.Lock()
@@ -298,18 +298,17 @@ func (e *endpoint) Read(addr *tcpip.FullAddress) (buffer.View, tcpip.ControlMess
 			err = tcpip.ErrClosedForReceive
 		}
 		e.rcvMu.Unlock()
-		return buffer.View{}, tcpip.ControlMessages{}, err
+		return tcpip.ReadResult{}, err
 	}
 
 	p := e.rcvList.Front()
-	e.rcvList.Remove(p)
-	e.rcvBufSize -= p.data.Size()
-	e.rcvMu.Unlock()
-
-	if addr != nil {
-		*addr = p.senderAddress
+	if !opts.Peek {
+		e.rcvList.Remove(p)
+		e.rcvBufSize -= p.data.Size()
 	}
+	e.rcvMu.Unlock()
 
+	// Control Messages
 	cm := tcpip.ControlMessages{
 		HasTimestamp: true,
 		Timestamp:    p.timestamp,
@@ -331,7 +330,22 @@ func (e *endpoint) Read(addr *tcpip.FullAddress) (buffer.View, tcpip.ControlMess
 		cm.HasOriginalDstAddress = true
 		cm.OriginalDstAddress = p.destinationAddress
 	}
-	return p.data.ToView(), cm, nil
+
+	// Read Result
+	res := tcpip.ReadResult{
+		Total:           p.data.Size(),
+		ControlMessages: cm,
+	}
+	if opts.NeedRemoteAddr {
+		res.RemoteAddr = p.senderAddress
+	}
+
+	n, err := p.data.ReadTo(dst, count, opts.Peek)
+	if n == 0 && err != nil {
+		return res, tcpip.ErrBadBuffer
+	}
+	res.Count = n
+	return res, nil
 }
 
 // prepareForWrite prepares the endpoint for sending data. In particular, it
@@ -566,11 +580,6 @@ func (e *endpoint) write(p tcpip.Payloader, opts tcpip.WriteOptions) (int64, <-c
 	return int64(len(v)), nil, nil
 }
 
-// Peek only returns data from a single datagram, so do nothing here.
-func (e *endpoint) Peek([][]byte) (int64, *tcpip.Error) {
-	return 0, nil
-}
-
 // OnReuseAddressSet implements tcpip.SocketOptionsHandler.OnReuseAddressSet.
 func (e *endpoint) OnReuseAddressSet(v bool) {
 	e.mu.Lock()
diff --git a/pkg/tcpip/transport/udp/udp_test.go b/pkg/tcpip/transport/udp/udp_test.go
index 8429f34b4..455b8c2aa 100644
--- a/pkg/tcpip/transport/udp/udp_test.go
+++ b/pkg/tcpip/transport/udp/udp_test.go
@@ -18,10 +18,12 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	"io/ioutil"
 	"math/rand"
 	"testing"
 	"time"
 
+	"github.com/google/go-cmp/cmp"
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/buffer"
 	"gvisor.dev/gvisor/pkg/tcpip/checker"
@@ -595,13 +597,13 @@ func testReadInternal(c *testContext, flow testFlow, packetShouldBeDropped, expe
 	// Take a snapshot of the stats to validate them at the end of the test.
 	epstats := c.ep.Stats().(*tcpip.TransportEndpointStats).Clone()
 
-	var addr tcpip.FullAddress
-	v, cm, err := c.ep.Read(&addr)
+	var buf bytes.Buffer
+	res, err := c.ep.Read(&buf, defaultMTU, tcpip.ReadOptions{NeedRemoteAddr: true})
 	if err == tcpip.ErrWouldBlock {
 		// Wait for data to become available.
 		select {
 		case <-ch:
-			v, cm, err = c.ep.Read(&addr)
+			res, err = c.ep.Read(&buf, defaultMTU, tcpip.ReadOptions{NeedRemoteAddr: true})
 
 		case <-time.After(300 * time.Millisecond):
 			if packetShouldBeDropped {
@@ -621,23 +623,32 @@ func testReadInternal(c *testContext, flow testFlow, packetShouldBeDropped, expe
 	}
 
 	if packetShouldBeDropped {
-		c.t.Fatalf("Read unexpectedly received data from %s", addr.Addr)
+		c.t.Fatalf("Read unexpectedly received data from %s", res.RemoteAddr.Addr)
 	}
 
-	// Check the peer address.
+	// Check the read result.
 	h := flow.header4Tuple(incoming)
-	if addr.Addr != h.srcAddr.Addr {
-		c.t.Fatalf("got address = %s, want = %s", addr.Addr, h.srcAddr.Addr)
+	if diff := cmp.Diff(tcpip.ReadResult{
+		Count:      buf.Len(),
+		Total:      buf.Len(),
+		RemoteAddr: tcpip.FullAddress{Addr: h.srcAddr.Addr},
+	}, res, checker.IgnoreCmpPath(
+		"ControlMessages", // ControlMessages will be checked later.
+		"RemoteAddr.NIC",
+		"RemoteAddr.Port",
+	)); diff != "" {
+		c.t.Fatalf("Read: unexpected result (-want +got):\n%s", diff)
 	}
 
 	// Check the payload.
+	v := buf.Bytes()
 	if !bytes.Equal(payload, v) {
 		c.t.Fatalf("got payload = %x, want = %x", v, payload)
 	}
 
 	// Run any checkers against the ControlMessages.
 	for _, f := range checkers {
-		f(c.t, cm)
+		f(c.t, res.ControlMessages)
 	}
 
 	c.checkEndpointReadStats(1, epstats, err)
@@ -828,8 +839,8 @@ func TestV4ReadSelfSource(t *testing.T) {
 				t.Errorf("c.s.Stats().IP.InvalidSourceAddressesReceived got %d, want %d", got, tt.wantInvalidSource)
 			}
 
-			if _, _, err := c.ep.Read(nil); err != tt.wantErr {
-				t.Errorf("got c.ep.Read(nil) = %s, want = %s", err, tt.wantErr)
+			if _, err := c.ep.Read(ioutil.Discard, defaultMTU, tcpip.ReadOptions{}); err != tt.wantErr {
+				t.Errorf("got c.ep.Read = %s, want = %s", err, tt.wantErr)
 			}
 		})
 	}