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

[John Fastabend <john.fastabend@gmail.com>]

John Fastabend wrote:
> Maxim Mikityanskiy wrote:
> > 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.
> 
> I'm not sure you would need the len checks in helper, they provide
> some guarantees I guess, but the void * is just memory I don't see
> any checks on its size. It could be the last byte of a value for
> example?

I suspect we need to add verifier checks here anyways to ensure we don't
walk off the end of a value unless something else is ensuring the iph
is inside a valid memory block.

> 
> > 
> > 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? 
> 
> The only benefit would be to shave some instructions off the program.
> XDP is about performance so I figure we shouldn't be adding arbitrary
> stuff here. OTOH you're already jumping into a helper so it might
> not matter at all.
> 
> >  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).
> 
> Just not sure you need the checks either, can you just assume the user
> gives good data?
> 
> > 
> > > 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?
> 
> Just that the XDP code I maintain has a if ipv4 {...} else ipv6{...}
> structure in it so could use a v4_check... and v6_check... then call
> the correct version directly, removing the switch from the helper.
> 
> Do you think there could be a performance reason to drop out those
> instructions or is it just hid by the hash itself. Also it seems
> a bit annoying if user is calling multiple helpers and they keep
> doing the same checks over and over.