net/tstun: fix TUN log spam when ACLs drop a packet

Whenever we dropped a packet due to ACLs, wireguard-go was logging:

Failed to write packet to TUN device: packet dropped by filter

Instead, just lie to wireguard-go and pretend everything is okay.

Fixes #1229

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tstun/wrap.go

@@ -457,13 +457,22 @@ func (t *Wrapper) filterIn(buf []byte) filter.Response {
 // like wireguard-go/tun.Device.Write.
 func (t *Wrapper) Write(buf []byte, offset int) (int, error) {
 	if !t.disableFilter {
-		res := t.filterIn(buf[offset:])
-		if res == filter.DropSilently {
+		if t.filterIn(buf[offset:]) != filter.Accept {
+			// If we're not accepting the packet, lie to wireguard-go and pretend
+			// that everything is okay with a nil error, so wireguard-go
+			// doesn't log about this Write "failure".
+			//
+			// We return len(buf), but the ill-defined wireguard-go/tun.Device.Write
+			// method doesn't specify how the offset affects the return value.
+			// In fact, the Linux implementation does one of two different things depending
+			// on how the /dev/net/tun was created. But fortunately the wireguard-go
+			// code ignores the int return and only looks at the error:
+			//
+			//     device/receive.go: _, err = device.tun.device.Write(....)
+			//
+			// TODO(bradfitz): fix upstream interface docs, implementation.
 			return len(buf), nil
 		}
-		if res != filter.Accept {
-			return 0, ErrFiltered
-		}
 	}
 
 	t.noteActivity()

net/tstun/wrap_test.go

@@ -329,11 +329,14 @@ func TestFilter(t *testing.T) {
 			var filtered bool
 
 			if tt.dir == in {
+				// Use the side effect of updating the last
+				// activity atomic to determine whether the
+				// data was actually filtered.
+				// If it stays zero, nothing made it through
+				// to the wrapped TUN.
+				atomic.StoreInt64(&tun.lastActivityAtomic, 0)
 				_, err = tun.Write(tt.data, 0)
-				if err == ErrFiltered {
-					filtered = true
-					err = nil
-				}
+				filtered = atomic.LoadInt64(&tun.lastActivityAtomic) == 0
 			} else {
 				chtun.Outbound <- tt.data
 				n, err = tun.Read(buf[:], 0)