set bpf filter for pcap (#865)

* set bpf filter for pcap

* implement pod updating mechanism

* Update tap/source/netns_packet_source.go

* Update tap/source/netns_packet_source.go

* minor pr fixes

Co-authored-by: Nimrod Gilboa Markevich <59927337+nimrod-up9@users.noreply.github.com>

tap/source/netns_packet_source.go

@@ -0,0 +1,83 @@
+package source
+
+import (
+	"fmt"
+	"runtime"
+
+	"github.com/up9inc/mizu/shared/logger"
+	"github.com/vishvananda/netns"
+)
+
+func newNetnsPacketSource(procfs string, pid string,
+	interfaceName string, behaviour TcpPacketSourceBehaviour) (*tcpPacketSource, error) {
+	nsh, err := netns.GetFromPath(fmt.Sprintf("%s/%s/ns/net", procfs, pid))
+
+	if err != nil {
+		logger.Log.Errorf("Unable to get netns of pid %s - %w", pid, err)
+		return nil, err
+	}
+
+	src, err := newPacketSourceFromNetnsHandle(pid, nsh, interfaceName, behaviour)
+
+	if err != nil {
+		logger.Log.Errorf("Error starting netns packet source for %s - %w", pid, err)
+		return nil, err
+	}
+
+	return src, nil
+}
+
+func newPacketSourceFromNetnsHandle(pid string, nsh netns.NsHandle, interfaceName string,
+	behaviour TcpPacketSourceBehaviour) (*tcpPacketSource, error) {
+
+	done := make(chan *tcpPacketSource)
+	errors := make(chan error)
+
+	go func(done chan<- *tcpPacketSource) {
+		// Setting a netns should be done from a dedicated OS thread.
+		//
+		// goroutines are not really OS threads, we try to mimic the issue by
+		//	locking the OS thread to this goroutine
+		//
+		runtime.LockOSThread()
+		defer runtime.UnlockOSThread()
+
+		oldnetns, err := netns.Get()
+
+		if err != nil {
+			logger.Log.Errorf("Unable to get netns of current thread %w", err)
+			errors <- err
+			return
+		}
+
+		if err := netns.Set(nsh); err != nil {
+			logger.Log.Errorf("Unable to set netns of pid %s - %w", pid, err)
+			errors <- err
+			return
+		}
+
+		name := fmt.Sprintf("netns-%s-%s", pid, interfaceName)
+		src, err := newTcpPacketSource(name, "", interfaceName, behaviour)
+
+		if err != nil {
+			logger.Log.Errorf("Error listening to PID %s - %w", pid, err)
+			errors <- err
+			return
+		}
+
+		if err := netns.Set(oldnetns); err != nil {
+			logger.Log.Errorf("Unable to set back netns of current thread %w", err)
+			errors <- err
+			return
+		}
+
+		done <- src
+	}(done)
+
+	select {
+	case err := <-errors:
+		return nil, err
+	case source := <-done:
+		return source, nil
+	}
+}