Re: MSS clamping with XDP/eBPF

Answering my own question, parse_tcp_ts()[0] can be  easily adapted to
find any TCP option and return a pointer to its position so it can be
updated using bpf_skb_store_bytes().

Cheers,
Valentín Gutiérrez

[0] https://github.com/xdp-project/bpf-examples/blob/63cd4007b16b49b44241f17a38f26aea926f8afc/pping/pping_kern.c#L424

On Mon, Nov 6, 2023 at 10:57 AM Valentín Gutierrez
<[email protected]> wrote:
>
> Hi,
>
> I'm trying to perform some basic TCP MSS clamping with eBPF and it's
> being harder than expected:
>
> * Parsing TCP options can be challenging for the eBPF verifier as it's
> been shown in [1] by Christian Deacon.
> * bpf_store_hdr_opt() introduced in [2] doesn't seem to be useful here
> cause it refuses to overwrite an existing TCP option and MSS is
> already set when BPF_SOCK_OPS_WRITE_HDR_OPT_CB is triggered.
> * bpf_setsockopt() isn't able to set TCP_MAXSEG per [3].
>
> Am I missing something?
>
> Thanks,
> Valentin Gutierrez
>
> [1] https://github.com/gamemann/XDP-TCP-Header-Options/
> [2] https://lore.kernel.org/bpf/[email protected]/T/
> [3] https://github.com/torvalds/linux/blob/d2f51b3516dade79269ff45eae2a7668ae711b25/net/core/filter.c#L5151