Subject: Re: [PATCH bpf-next/net v2 4/7] bpf: Add mptcp_subflow bpf_iter

From: Martin KaFai Lau <[email-masked]>
Date: Thu, 23 Jan 2025 16:47:41 -0800

On 12/19/24 7:46 AM, Matthieu Baerts (NGI0) wrote:
> From: Geliang Tang <[email-masked]>
> 
> 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 <[email-masked]>
> Signed-off-by: Geliang Tang <[email-masked]>
> Reviewed-by: Mat Martineau <[email-masked]>
> Signed-off-by: Matthieu Baerts (NGI0) <[email-masked]>
> ---
> 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).
> 
> A few versions of this single patch have been previously posted to the
> BPF mailing list by Geliang, before continuing to the MPTCP mailing list
> only, with other patches of this series. The version of the whole series
> has been reset to 1, but here is the ChangeLog for the previous ones:
>   - v2: remove msk->pm.lock in _new() and _destroy() (Martin)
>         drop DEFINE_BPF_ITER_FUNC, change opaque[3] to opaque[2] (Andrii)
>   - v3: drop bpf_iter__mptcp_subflow
>   - v4: if msk is NULL, initialize kit->msk to NULL in _new() and check
>         it in _next() (Andrii)
>   - v5: use list_is_last() instead of list_entry_is_head() add
>         KF_ITER_NEW/NEXT/DESTROY flags add msk_owned_by_me in _new()
>   - v6: add KF_TRUSTED_ARGS flag (Andrii, Martin)
> ---
>   net/mptcp/bpf.c | 53 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 53 insertions(+)
> 
> diff --git a/net/mptcp/bpf.c b/net/mptcp/bpf.c
> index c5bfd84c16c43230d9d8e1fd8ff781a767e647b5..e39f0e4fb683c1aa31ee075281daee218dac5878 100644
> --- a/net/mptcp/bpf.c
> +++ b/net/mptcp/bpf.c
> @@ -35,6 +35,15 @@ static const struct btf_kfunc_id_set bpf_mptcp_fmodret_set = {
>   	.set   = &bpf_mptcp_fmodret_ids,
>   };
>   
> +struct bpf_iter_mptcp_subflow {
> +	__u64 __opaque[2];
> +} __aligned(8);
> +
> +struct bpf_iter_mptcp_subflow_kern {
> +	struct mptcp_sock *msk;
> +	struct list_head *pos;
> +} __aligned(8);
> +
>   __bpf_kfunc_start_defs();
>   
>   __bpf_kfunc static struct mptcp_subflow_context *
> @@ -47,10 +56,54 @@ bpf_mptcp_subflow_ctx(const struct sock *sk)
>   	return NULL;
>   }
>   
> +__bpf_kfunc static int
> +bpf_iter_mptcp_subflow_new(struct bpf_iter_mptcp_subflow *it,
> +			   struct mptcp_sock *msk)
> +{
> +	struct bpf_iter_mptcp_subflow_kern *kit = (void *)it;
> +	struct sock *sk = (struct sock *)msk;
> +
> +	BUILD_BUG_ON(sizeof(struct bpf_iter_mptcp_subflow_kern) >
> +		     sizeof(struct bpf_iter_mptcp_subflow));
> +	BUILD_BUG_ON(__alignof__(struct bpf_iter_mptcp_subflow_kern) !=
> +		     __alignof__(struct bpf_iter_mptcp_subflow));
> +
> +	kit->msk = msk;
> +	if (!msk)

NULL check is not needed. verifier should have rejected it for KF_TRUSTED_ARGS.

> +		return -EINVAL;
> +
> +	if (!sock_owned_by_user_nocheck(sk) &&
> +	    !spin_is_locked(&sk->sk_lock.slock))

I could have missed something. If it is to catch bug, should it be 
sock_owned_by_me() that has the lockdep splat? For the cg get/setsockopt hook 
here, the lock should have already been held earlier in the kernel.

This set is only showing the cg sockopt bpf prog but missing the major 
struct_ops piece. It is hard to comment. I assumed the lock situation is the 
same for the struct_ops where the lock will be held before calling the 
struct_ops prog?

> +		return -EINVAL;
> +
> +	kit->pos = &msk->conn_list;
> +	return 0;
> +}
> +
> +__bpf_kfunc static struct mptcp_subflow_context *
> +bpf_iter_mptcp_subflow_next(struct bpf_iter_mptcp_subflow *it)
> +{
> +	struct bpf_iter_mptcp_subflow_kern *kit = (void *)it;
> +
> +	if (!kit->msk || list_is_last(kit->pos, &kit->msk->conn_list))
> +		return NULL;
> +
> +	kit->pos = kit->pos->next;
> +	return list_entry(kit->pos, struct mptcp_subflow_context, node);
> +}
> +
> +__bpf_kfunc static void
> +bpf_iter_mptcp_subflow_destroy(struct bpf_iter_mptcp_subflow *it)
> +{
> +}
> +
>   __bpf_kfunc_end_defs();
>   
>   BTF_KFUNCS_START(bpf_mptcp_common_kfunc_ids)
>   BTF_ID_FLAGS(func, bpf_mptcp_subflow_ctx, KF_RET_NULL)
> +BTF_ID_FLAGS(func, bpf_iter_mptcp_subflow_new, KF_ITER_NEW | KF_TRUSTED_ARGS)
> +BTF_ID_FLAGS(func, bpf_iter_mptcp_subflow_next, KF_ITER_NEXT | KF_RET_NULL)
> +BTF_ID_FLAGS(func, bpf_iter_mptcp_subflow_destroy, KF_ITER_DESTROY)
>   BTF_KFUNCS_END(bpf_mptcp_common_kfunc_ids)
>   
>   static const struct btf_kfunc_id_set bpf_mptcp_common_kfunc_set = {
>