Collect stack traces for dropped packets and print them after ^C

skb_free.bt

#!/usr/bin/env bpftrace
/*
 * skb_free.bt  Trace skb drops when the socket is associated with the skb.
 *              For Linux, uses bpftrace and eBPF.
 *
 * USAGE: skb_free.bt
 *
 * Copyright (c) 2019 Joe Stringer.
 * Licensed under the Apache License, Version 2.0 (the "License")
 *
 * 08-May-2019  Joe Stringer    created this.
 */

#include <linux/netdevice.h>
#include <linux/skbuff.h>

kprobe:kfree_skb {
        $skb = (struct sk_buff *)arg0;
        $sk = $skb->sk;
        if ($sk) {
                @ip4_stacks[kstack] = count();
        }
}