Re: [PATCH bpf-next v2 2/3] bpf: Add helpers to issue and check SYN cookies in XDP

[Maxim Mikityanskiy <maximmi@nvidia.com>]

On 2022-01-25 09:54, John Fastabend wrote:
> Maxim Mikityanskiy wrote:
>> The new helpers bpf_tcp_raw_{gen,check}_syncookie allow an XDP program
>> to generate SYN cookies in response to TCP SYN packets and to check
>> those cookies upon receiving the first ACK packet (the final packet of
>> the TCP handshake).
>>
>> Unlike bpf_tcp_{gen,check}_syncookie these new helpers don't need a
>> listening socket on the local machine, which allows to use them together
>> with synproxy to accelerate SYN cookie generation.
>>
>> Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
>> Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
>> ---
> 
> [...]
> 
>> +
>> +BPF_CALL_4(bpf_tcp_raw_check_syncookie, void *, iph, u32, iph_len,
>> +	   struct tcphdr *, th, u32, th_len)
>> +{
>> +#ifdef CONFIG_SYN_COOKIES
>> +	u32 cookie;
>> +	int ret;
>> +
>> +	if (unlikely(th_len < sizeof(*th)))
>> +		return -EINVAL;
>> +
>> +	if (!th->ack || th->rst || th->syn)
>> +		return -EINVAL;
>> +
>> +	if (unlikely(iph_len < sizeof(struct iphdr)))
>> +		return -EINVAL;
>> +
>> +	cookie = ntohl(th->ack_seq) - 1;
>> +
>> +	/* Both struct iphdr and struct ipv6hdr have the version field at the
>> +	 * same offset so we can cast to the shorter header (struct iphdr).
>> +	 */
>> +	switch (((struct iphdr *)iph)->version) {
>> +	case 4:
> 
> Did you consider just exposing __cookie_v4_check() and __cookie_v6_check()?

No, I didn't, I just implemented it consistently with 
bpf_tcp_check_syncookie, but let's consider it.

I can't just pass a pointer from BPF without passing the size, so I 
would need some wrappers around __cookie_v{4,6}_check anyway. The checks 
for th_len and iph_len would have to stay in the helpers. The check for 
TCP flags (ACK, !RST, !SYN) could be either in the helper or in BPF. The 
switch would obviously be gone.

The bottom line is that it would be the same code, but without the 
switch, and repeated twice. What benefit do you see in this approach? 
 From my side, I only see the ability to drop one branch at the expense 
of duplicating the code above the switch (th_len and iph_len checks).

> My code at least has already run the code above before it would ever call
> this helper so all the other bits are duplicate.

Sorry, I didn't quite understand this part. What "your code" are you 
referring to?

> The only reason to build
> it this way, as I see it, is either code can call it blindly without doing
> 4/v6 switch. or to make it look and feel like 'tc' world, but its already
> dropped the ok so its a bit different already and ifdef TC/XDP could
> hanlde the different parts.
> 
> 
>> +		ret = __cookie_v4_check((struct iphdr *)iph, th, cookie);
>> +		break;
>> +
>> +#if IS_BUILTIN(CONFIG_IPV6)
>> +	case 6:
>> +		if (unlikely(iph_len < sizeof(struct ipv6hdr)))
>> +			return -EINVAL;
>> +
>> +		ret = __cookie_v6_check((struct ipv6hdr *)iph, th, cookie);
>> +		break;
>> +#endif /* CONFIG_IPV6 */
>> +
>> +	default:
>> +		return -EPROTONOSUPPORT;
>> +	}
>> +
>> +	if (ret > 0)
>> +		return 0;
>> +
>> +	return -EACCES;
>> +#else
>> +	return -EOPNOTSUPP;
>> +#endif
>> +}