inet_csk_accept ebpf program doesn't work properly

[rectified95]

kretprobe/inet_csk_accept (reason TCP_ACCEPT_BASIC) has two big issues:

1. it doesn't track bytes dropped for any packet it processes (it calls get_packet_from_sock which doesn't populate the packet's skb_len, since that value is not available in the socket struct this ebpf hook receives as input; this means that the subsequent call to update_metrics_map doesn't increase the drop bytes metrics)
2. it misses some drops (it does a single rather than double pointer dereference of the value retrieved from accept_pids map; the rerieved pointer (and not underlying value) is assigned to int which given 2's complement notation means that anytime the memory address of the err value has the most significant bit == 0 (positive number), it returns early thinking the underlying function didn't indicate a drop)

Solutions:

1. We'd need to figure out how to get the sk_buff from the socket.
2. I tried 2 approaches unsuccessfully so far: doing the second pointer deref (ebpf verifier failure); storing the actual error value from inet_csk_accept rather than the pointer, as a signed int, which obviates the need for the double deref, but also got a verifier error.