bpftrace script for gathering stack traces for dropped packets

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) 2020 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();                                  
        }                                                                       
}