Fix packetcapture bpf filter issue (#6815) (#6821)

In PacketCapture, packets which don’t match the target BPF can be
received after the socket is created and before the bpf filter is
applied. This patch uses a zero bpf filter (matches no packet), then
empties out any packets that arrived before the "zero-BPF" filter was
applied. At this point the socket is definitely empty and it can’t
fill up with junk because the zero-BPF is in place. Then we replace
the zero-BPF with the real BPF we want.

Signed-off-by: Hang Yan <[email protected]>
Co-authored-by: Antonin Bas <[email protected]>

pkg/agent/packetcapture/capture/pcap_linux.go

@@ -17,6 +17,7 @@ package capture
 import (
 	"context"
 	"net"
+	"time"
 
 	"github.com/gopacket/gopacket"
 	"github.com/gopacket/gopacket/layers"
@@ -39,6 +40,12 @@ func NewPcapCapture() (*pcapCapture, error) {
 	return &pcapCapture{}, nil
 }
 
+// zeroFilter is a filter that will drop all packets.
+// see: https://github.com/antrea-io/antrea/issues/6815 for the user case.
+func zeroFilter() []bpf.Instruction {
+	return []bpf.Instruction{returnDrop}
+}
+
 func (p *pcapCapture) Capture(ctx context.Context, device string, srcIP, dstIP net.IP, packet *crdv1alpha1.Packet) (chan gopacket.Packet, error) {
 	// Compile the BPF filter in advance to reduce the time window between starting the capture and applying the filter.
 	inst := compilePacketFilter(packet, srcIP, dstIP)
@@ -48,21 +55,52 @@ func (p *pcapCapture) Capture(ctx context.Context, device string, srcIP, dstIP n
 		return nil, err
 	}
 
+	zeroRawInst, err := bpf.Assemble(zeroFilter())
+	if err != nil {
+		return nil, err
+	}
+
 	eth, err := pcapgo.NewEthernetHandle(device)
 	if err != nil {
 		return nil, err
 	}
 	if err = eth.SetPromiscuous(false); err != nil {
 		return nil, err
 	}
-	if err = eth.SetBPF(rawInst); err != nil {
+	// Install a BPF filter that won't match any packets
+	// see: https://natanyellin.com/posts/ebpf-filtering-done-right/.
+	// Packets which don’t match the target BPF can be received after the socket
+	// is created and before setsockopt is called. Those packets will remain
+	// in the socket’s buffer even after the BPF is applied and will later
+	// be transferred to the application via recv. Here we use a zero
+	// bpf filter(match no packet), then empty out any packets that arrived
+	// before the “zero-BPF” filter was applied. At this point the socket is
+	// definitely empty and it can’t fill up with junk because the zero-BPF
+	// is in place. Then we replace the zero-BPF with the real BPF we want.
+	if err = eth.SetBPF(zeroRawInst); err != nil {
 		return nil, err
 	}
 	if err = eth.SetCaptureLength(maxSnapshotBytes); err != nil {
 		return nil, err
 	}
 
 	packetSource := gopacket.NewPacketSource(eth, layers.LinkTypeEthernet, gopacket.WithNoCopy(true))
-	return packetSource.PacketsCtx(ctx), nil
-
+	packetCh := packetSource.PacketsCtx(ctx)
+	// Drain the channel
+	for {
+		select {
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		case <-packetCh:
+			klog.V(5).InfoS("Found irrelevant packet, discard it", "device", device)
+			break
+		case <-time.After(50 * time.Millisecond):
+			// timeout: channel is drained so socket is drained
+			// install the correct BPF filter
+			if err := eth.SetBPF(rawInst); err != nil {
+				return nil, err
+			}
+			return packetCh, nil
+		}
+	}
 }