Re: [PATCH bpf-next/net v3 2/5] bpf: Add mptcp_subflow bpf_iter

[Martin KaFai Lau <martin.lau@linux.dev>]

On 3/20/25 10:48 AM, Matthieu Baerts (NGI0) wrote:
> From: Geliang Tang <tanggeliang@kylinos.cn>
> 
> It's necessary to traverse all subflows on the conn_list of an MPTCP
> socket and then call kfunc to modify the fields of each subflow. In
> kernel space, mptcp_for_each_subflow() helper is used for this:
> 
> 	mptcp_for_each_subflow(msk, subflow)
> 		kfunc(subflow);
> 
> But in the MPTCP BPF program, this has not yet been implemented. As
> Martin suggested recently, this conn_list walking + modify-by-kfunc
> usage fits the bpf_iter use case.
> 
> So this patch adds a new bpf_iter type named "mptcp_subflow" to do
> this and implements its helpers bpf_iter_mptcp_subflow_new()/_next()/
> _destroy(). And register these bpf_iter mptcp_subflow into mptcp
> common kfunc set. Then bpf_for_each() for mptcp_subflow can be used
> in BPF program like this:
> 
> 	bpf_for_each(mptcp_subflow, subflow, msk)
> 		kfunc(subflow);
> 
> Suggested-by: Martin KaFai Lau <martin.lau@kernel.org>
> Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
> Reviewed-by: Mat Martineau <martineau@kernel.org>
> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
> ---
> Notes:
>   - v2:
>     - Add BUILD_BUG_ON() checks, similar to the ones done with other
>       bpf_iter_(...) helpers.
>     - Replace msk_owned_by_me() by sock_owned_by_user_nocheck() and
>       !spin_is_locked() (Martin).
>   - v3:
>     - Switch parameter from 'struct mptcp_sock' to 'struct sock' (Martin)
>     - Remove unneeded !msk check (Martin)
>     - Remove locks checks, add msk_owned_by_me for lockdep (Martin)
>     - The following note and 2 questions have been added below.
> 
> This new bpf_iter will be used by our future BPF packet schedulers and
> path managers. To see how we are going to use them, please check our
> export branch [1], especially these two commits:
> 
>   - "bpf: Add mptcp packet scheduler struct_ops": introduce a new
>     struct_ops.
>   - "selftests/bpf: Add bpf_burst scheduler & test": new test showing
>     how the new struct_ops and bpf_iter are being used.
> 
> [1] https://github.com/multipath-tcp/mptcp_net-next/commits/export
> 
> @BPF maintainers: we would like to allow this new mptcp_subflow bpf_iter
> to be used with struct_ops, but only with the two new ones we are going
> to introduce that are specific to MPTCP, and with not others struct_ops
> (TCP CC, sched_ext, etc.). We are not sure how to do that. By chance, do
> you have examples or doc you could point to us to have this restriction
> in place, please?

The bpf_qdisc.c has done that. Take a look at the "bpf_qdisc_kfunc_filter()".

It is in net-next and bpf-next/net.

> 
> Also, for one of the two future MPTCP struct_ops, not all callbacks
> should be allowed to use this new bpf_iter, because they are called from
> different contexts. How can we ensure such callbacks from a struct_ops
> cannot call mptcp_subflow bpf_iter without adding new dedicated checks
> looking if some locks are held for all callbacks? We understood that
> they wanted to have something similar with sched_ext, but we are not
> sure if this code is ready nor if it is going to be accepted.

Same. Take a look at "bpf_qdisc_kfunc_filter()".