Re: [PATCH mptcp-next 1/4] bpf: Add mptcp_subflow bpf_iter

[Geliang Tang <geliang@kernel.org>]

Hi Martin,

On Thu, 2024-09-05 at 11:24 -0700, Martin KaFai Lau wrote:
> On 9/5/24 6:52 AM, Geliang Tang 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.
> > 
> > This patch adds a new bpf_iter type named "mptcp_subflow" to do
> > this.
> > 
> > Suggested-by: Martin KaFai Lau <martin.lau@kernel.org>
> > Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
> > ---
> >   kernel/bpf/helpers.c |  3 +++
> >   net/mptcp/bpf.c      | 57
> > ++++++++++++++++++++++++++++++++++++++++++++
> >   2 files changed, 60 insertions(+)
> > 
> > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > index b5f0adae8293..2340ba967444 100644
> > --- a/kernel/bpf/helpers.c
> > +++ b/kernel/bpf/helpers.c
> > @@ -3023,6 +3023,9 @@ BTF_ID_FLAGS(func, bpf_preempt_enable)
> >   BTF_ID_FLAGS(func, bpf_iter_bits_new, KF_ITER_NEW)
> >   BTF_ID_FLAGS(func, bpf_iter_bits_next, KF_ITER_NEXT |
> > KF_RET_NULL)
> >   BTF_ID_FLAGS(func, bpf_iter_bits_destroy, KF_ITER_DESTROY)
> > +BTF_ID_FLAGS(func, bpf_iter_mptcp_subflow_new)
> > +BTF_ID_FLAGS(func, bpf_iter_mptcp_subflow_next)
> > +BTF_ID_FLAGS(func, bpf_iter_mptcp_subflow_destroy)
> >   BTF_KFUNCS_END(common_btf_ids)
> >   
> >   static const struct btf_kfunc_id_set common_kfunc_set = {
> > diff --git a/net/mptcp/bpf.c b/net/mptcp/bpf.c
> > index 9672a70c24b0..cda09bbfd617 100644
> > --- a/net/mptcp/bpf.c
> > +++ b/net/mptcp/bpf.c
> > @@ -204,6 +204,63 @@ static const struct btf_kfunc_id_set
> > bpf_mptcp_fmodret_set = {
> >    .set   = &bpf_mptcp_fmodret_ids,
> >   };
> >   
> > +struct bpf_iter__mptcp_subflow {
> > + __bpf_md_ptr(struct bpf_iter_meta *, meta);
> > + __bpf_md_ptr(struct mptcp_sock *, msk);
> > + __bpf_md_ptr(struct list_head *, pos);
> > +};
> > +
> > +DEFINE_BPF_ITER_FUNC(mptcp_subflow, struct bpf_iter_meta *meta,
> > +      struct mptcp_sock *msk, struct list_head *pos)
> > +
> > +struct bpf_iter_mptcp_subflow {
> > + __u64 __opaque[3];
> > +} __attribute__((aligned(8)));
> > +
> > +struct bpf_iter_mptcp_subflow_kern {
> > + struct mptcp_sock *msk;
> > + struct list_head *pos;
> > +} __attribute__((aligned(8)));
> > +
> > +__bpf_kfunc_start_defs();
> > +
> > +__bpf_kfunc 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;
> > +
> > + kit->msk = msk;
> > + kit->pos = &msk->conn_list;
> > + spin_lock_bh(&msk->pm.lock);
> 
> I don't think spin_lock here without unlock can be used. e.g. What if
> bpf_iter_mptcp_subflow_new() is called twice back-to-back.
> 
> I haven't looked at the mptcp details, some questions:
> The list is protected by msk->pm.lock?
> What happen to the sk_lock of the msk?
> Can this be rcu-ify? or it needs some cares when walking the
> established TCP 
> subflow?

Thank you for your review. msk->pm.lock shouldn't be used here. The
conn_list is not protected by msk->pm.lock. I will remove it in v2.

> 
> 
> [ Please cc the bpf list. Helping to review patches is a good way to
> contribute 
> back to the mailing list. ]

This patch is for "mptcp-next", it depends on the "new MPTCP subflow
subtest" which is under review on the bpf list. We will send it to the
bpf list very soon.

Thanks,
-Geliang

> 
> > +
> > + return 0;
> > +}
> > +
> > +__bpf_kfunc struct mptcp_subflow_context *
> > +bpf_iter_mptcp_subflow_next(struct bpf_iter_mptcp_subflow *it)
> > +{
> > + struct bpf_iter_mptcp_subflow_kern *kit = (void *)it;
> > + struct mptcp_subflow_context *subflow;
> > + struct mptcp_sock *msk = kit->msk;
> > +
> > + subflow = list_entry((kit->pos)->next, struct
> > mptcp_subflow_context, node);
> > + if (list_entry_is_head(subflow, &msk->conn_list, node))
> > + return NULL;
> > +
> > + kit->pos = &subflow->node;
> > + return subflow;
> > +}
> > +
> > +__bpf_kfunc void bpf_iter_mptcp_subflow_destroy(struct
> > bpf_iter_mptcp_subflow *it)
> > +{
> > + struct bpf_iter_mptcp_subflow_kern *kit = (void *)it;
> > + struct mptcp_sock *msk = kit->msk;
> > +
> > + spin_unlock_bh(&msk->pm.lock);
> > +}
> > +
> > +__bpf_kfunc_end_defs();
> > +
> >   __diag_push();
> >   __diag_ignore_all("-Wmissing-prototypes",
> >      "kfuncs which will be used in BPF programs");
>