Fix test data and improve /proc/net/tcp header parsing

The header checking code was unsafe because:

1. It was accessing the byteslice at [2] without ensuring a length >= 3
2. It was assuming that the indentation of the 'sl' header is always 2 (which seems to be the case in recent kernels 8f18e4d03e/net/ipv4/tcp_ipv4.c (L2304) and 8f18e4d03e/net/ipv6/tcp_ipv6.c (L1831) ) but it's more robust to simply trim the byteslice.

probe/endpoint/procspy/procnet.go

@@ -5,6 +5,9 @@ import (
 	"net"
 )
 
+// Used to check whether we are parsing a header line
+var slHeader = []byte("sl")
+
 // ProcNet is an iterator to parse /proc/net/tcp{,6} files.
 type ProcNet struct {
 	b                       []byte
@@ -31,16 +34,16 @@ again:
 	}
 	b := p.b
 
-	if p.b[2] == 's' {
+	var (
+		sl, local, remote, inode []byte
+	)
+
+	sl, b = nextField(b) // 'sl' column
+	if bytes.Equal(sl, slHeader) {
 		// Skip header
 		p.b = nextLine(b)
 		goto again
 	}
-
-	var (
-		local, remote, inode []byte
-	)
-	_, b = nextField(b) // 'sl' column
 	local, b = nextField(b)
 	remote, b = nextField(b)
 	_, b = nextField(b) // 'st' column

probe/endpoint/procspy/procnet_internal_test.go

@@ -59,7 +59,7 @@ func TestProcNet(t *testing.T) {
 
 func TestTransport6(t *testing.T) {
 	// Abridged copy of my /proc/net/tcp6
-	testString := ` sl  local_address                         remote_address                        st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout Inode
+	testString := `  sl  local_address                         remote_address                        st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout Inode
    0: 00000000000000000000000000000000:19C8 00000000000000000000000000000000:0000 01 00000000:00000000 00:00000000 00000000     0        0 23661201 1 ffff880103fb4800 100 0 0 10 -1
    8: 4500032000BE692B8AE31EBD919D9D10:D61C 5014002A080805400000000015100000:01BB 01 00000000:00000000 02:00000045 00000000  1000        0 36856710 2 ffff88010b796080 22 4 30 8 7
 `