Re: [PATCH mptcp-next v3 1/2] mptcp: add bpf_iter_task for mptcp_sock

[Geliang Tang <geliang@kernel.org>]

Hi Matt,

On Fri, 2025-03-21 at 10:19 +0100, Matthieu Baerts wrote:
> Hi Geliang,
> 
> On 21/03/2025 05:14, Geliang Tang wrote:
> > On Tue, 2025-03-18 at 12:26 +0100, Matthieu Baerts wrote:
> 
> (...)
> 
> > > Mmh, all 8 callbacks from add_addr to here seem to be linked to
> > > Netlink
> > > commands, right? If yes, they should not be here: they don't make
> > > sense
> > > for a BPF PM, no? BPF PMs should be configured with BPF, and not
> > > via
> > 
> > Do you have any idea how we can pass commands like ADD_ADDR and
> > addresses from user space to BPF?
> 
> The userspace can inject any kind of BPF program. Additional
> addresses
> can then be hardcoded in this program, or it can load them from eBPF
> maps, or read skel->bss->(...), etc.
> 
> Netlink is then not needed.
> 
> > What I can think of is passing through sockopt, the user space
> > passes
> > the command and the address through setsockopt, and the BPF program
> > handles it through the custom "cgroup/setsockopt". I would like to
> > hear
> > your opinions. If you also agree to use sockopt to implement it, I
> > can
> > write a test program for this.

I just tested it and found it difficult to use setsockopt.

"cgroup/setsockopt" program doesn't work. We need to invoke sleepable
kfuncs in BPF PM:

BTF_ID_FLAGS(func, mptcp_pm_remove_addr_entry, KF_SLEEPABLE)
BTF_ID_FLAGS(func, mptcp_pm_addr_send_ack, KF_SLEEPABLE)
BTF_ID_FLAGS(func, mptcp_pm_mp_prio_send_ack, KF_SLEEPABLE)
BTF_ID_FLAGS(func, bpf_mptcp_subflow_connect, KF_SLEEPABLE)
BTF_ID_FLAGS(func, mptcp_subflow_shutdown, KF_SLEEPABLE)
BTF_ID_FLAGS(func, mptcp_close_ssk, KF_SLEEPABLE)

They can't be invoked in "cgroup/setsockopt" program. And
"cgroup/setsockopt" program can't be set with BPF_F_SLEEPABLE flag:

        err = err ?: bpf_program__set_flags(skel->progs.pm_setsockopt,
                                            BPF_F_SLEEPABLE);

With this error occurs:

test_bpf_hashmap_pm:PASS:set sleepable flags 0 nsec
libbpf: prog 'pm_setsockopt': BPF program load failed: -EINVAL
libbpf: prog 'pm_setsockopt': -- BEGIN PROG LOAD LOG --
Only fentry/fexit/fmod_ret, lsm, iter, uprobe, and struct_ops programs
can be sleepable
processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0
peak_states 0 mark_read 0

It seems that we can only choose one of fentry/fexit/fmod_ret, lsm,
iter, uprobe, or struct_ops type to implement BPF PM.

Thanks,
-Geliang

> That's a good idea for userspace apps which want to control how the
> MPTCP PM is acting. I guess MPTCP BPF PM will generally not be linked
> to
> network apps: a separate application dedicated to inject and
> configure
> the BPF PM, no?
> 
> Cheers,
> Matt