Prevent interleaving in sniffer pcap output

Remove "partial write" handling as io.Writer.Write is not permitted to
return a nil error on partial writes, and this code was already
panicking on non-nil errors.

PiperOrigin-RevId: 384289970

pkg/tcpip/link/sniffer/pcap.go

@@ -39,8 +39,6 @@ type pcapHeader struct {
 	Network uint32
 }
 
-const pcapPacketHeaderLen = 16
-
 type pcapPacketHeader struct {
 	// Seconds is the timestamp seconds.
 	Seconds uint32

pkg/tcpip/link/sniffer/sniffer.go

@@ -113,8 +113,9 @@ func writePCAPHeader(w io.Writer, maxLen uint32) error {
 // NewWithWriter creates a new sniffer link-layer endpoint. It wraps around
 // another endpoint and logs packets as they traverse the endpoint.
 //
-// Packets are logged to writer in the pcap format. A sniffer created with this
-// function will not emit packets using the standard log package.
+// Each packet is written to writer in the pcap format in a single Write call
+// without synchronization. A sniffer created with this function will not emit
+// packets using the standard log package.
 //
 // snapLen is the maximum amount of a packet to be saved. Packets with a length
 // less than or equal to snapLen will be saved in their entirety. Longer
@@ -155,27 +156,29 @@ func (e *endpoint) dumpPacket(dir direction, protocol tcpip.NetworkProtocolNumbe
 		if max := int(e.maxPCAPLen); length > max {
 			length = max
 		}
-		if err := binary.Write(writer, binary.BigEndian, newPCAPPacketHeader(time.Now(), uint32(length), uint32(totalLength))); err != nil {
-			panic(err)
-		}
-		write := func(b []byte) {
-			if len(b) > length {
-				b = b[:length]
+		packetHeader := newPCAPPacketHeader(time.Now(), uint32(length), uint32(totalLength))
+		packet := make([]byte, binary.Size(packetHeader)+length)
+		{
+			writer := tcpip.SliceWriter(packet)
+			if err := binary.Write(&writer, binary.BigEndian, packetHeader); err != nil {
+				panic(err)
 			}
-			for len(b) != 0 {
+			for _, b := range pkt.Views() {
+				if length == 0 {
+					break
+				}
+				if len(b) > length {
+					b = b[:length]
+				}
 				n, err := writer.Write(b)
 				if err != nil {
 					panic(err)
 				}
-				b = b[n:]
 				length -= n
 			}
 		}
-		for _, v := range pkt.Views() {
-			if length == 0 {
-				break
-			}
-			write(v)
+		if _, err := writer.Write(packet); err != nil {
+			panic(err)
 		}
 	}
 }