Re: [PATCH bpf-next v5 4/6] bpf: Add helpers to issue and check SYN cookies in XDP

[Daniel Borkmann <daniel@xxxxxxxxxxxxx>]

On 4/13/22 3:41 PM, Maxim Mikityanskiy wrote:
[...]
>   /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 7446b0ba4e38..428cc63ecdf7 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -7425,6 +7425,124 @@ static const struct bpf_func_proto bpf_skb_set_tstamp_proto = {
>   	.arg3_type      = ARG_ANYTHING,
>   };
>   
> +BPF_CALL_3(bpf_tcp_raw_gen_syncookie_ipv4, struct iphdr *, iph,
> +	   struct tcphdr *, th, u32, th_len)
> +{
> +#ifdef CONFIG_SYN_COOKIES
> +	u32 cookie;
> +	u16 mss;
> +
> +	if (unlikely(th_len < sizeof(*th) || th_len != th->doff * 4))
> +		return -EINVAL;
> +
> +	mss = tcp_parse_mss_option(th, 0) ?: TCP_MSS_DEFAULT;
> +	cookie = __cookie_v4_init_sequence(iph, th, &mss);
> +
> +	return cookie | ((u64)mss << 32);
> +#else
> +	return -EOPNOTSUPP;
> +#endif /* CONFIG_SYN_COOKIES */

This (and for other added helpers below) will be rather tricky to probe for availability
e.g. via `bpftool feature probe [...]`. Much better if you wrap the ifdef CONFIG_SYN_COOKIES
around the {xdp,tc_cls_act}_func_proto() instead as we do elsewhere.

> +}
> +
> +static const struct bpf_func_proto bpf_tcp_raw_gen_syncookie_ipv4_proto = {
> +	.func		= bpf_tcp_raw_gen_syncookie_ipv4,
> +	.gpl_only	= true, /* __cookie_v4_init_sequence() is GPL */
> +	.pkt_access	= true,
> +	.ret_type	= RET_INTEGER,
> +	.arg1_type	= ARG_PTR_TO_MEM,
> +	.arg1_size	= sizeof(struct iphdr),
> +	.arg2_type	= ARG_PTR_TO_MEM,
> +	.arg3_type	= ARG_CONST_SIZE,
> +};
> +
> +BPF_CALL_3(bpf_tcp_raw_gen_syncookie_ipv6, struct ipv6hdr *, iph,
> +	   struct tcphdr *, th, u32, th_len)
> +{
> +#ifndef CONFIG_SYN_COOKIES
> +	return -EOPNOTSUPP;
> +#elif !IS_BUILTIN(CONFIG_IPV6)
> +	return -EPROTONOSUPPORT;
> +#else
> +	const u16 mss_clamp = IPV6_MIN_MTU - sizeof(struct tcphdr) -
> +		sizeof(struct ipv6hdr);
> +	u32 cookie;
> +	u16 mss;
> +
> +	if (unlikely(th_len < sizeof(*th) || th_len != th->doff * 4))
> +		return -EINVAL;
> +
> +	mss = tcp_parse_mss_option(th, 0) ?: mss_clamp;
> +	cookie = __cookie_v6_init_sequence(iph, th, &mss);
> +
> +	return cookie | ((u64)mss << 32);
> +#endif
> +}
> +
> +static const struct bpf_func_proto bpf_tcp_raw_gen_syncookie_ipv6_proto = {
> +	.func		= bpf_tcp_raw_gen_syncookie_ipv6,
> +	.gpl_only	= true, /* __cookie_v6_init_sequence() is GPL */
> +	.pkt_access	= true,
> +	.ret_type	= RET_INTEGER,
> +	.arg1_type	= ARG_PTR_TO_MEM,
> +	.arg1_size	= sizeof(struct ipv6hdr),
> +	.arg2_type	= ARG_PTR_TO_MEM,
> +	.arg3_type	= ARG_CONST_SIZE,
> +};
[...]
>   
>   bool bpf_helper_changes_pkt_data(void *func)
> @@ -7837,6 +7955,14 @@ xdp_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>   		return &bpf_tcp_check_syncookie_proto;
>   	case BPF_FUNC_tcp_gen_syncookie:
>   		return &bpf_tcp_gen_syncookie_proto;
> +	case BPF_FUNC_tcp_raw_gen_syncookie_ipv4:
> +		return &bpf_tcp_raw_gen_syncookie_ipv4_proto;
> +	case BPF_FUNC_tcp_raw_gen_syncookie_ipv6:
> +		return &bpf_tcp_raw_gen_syncookie_ipv6_proto;
> +	case BPF_FUNC_tcp_raw_check_syncookie_ipv4:
> +		return &bpf_tcp_raw_check_syncookie_ipv4_proto;
> +	case BPF_FUNC_tcp_raw_check_syncookie_ipv6:
> +		return &bpf_tcp_raw_check_syncookie_ipv6_proto;
>   #endif
>   	default:
>   		return bpf_sk_base_func_proto(func_id);