Subject: Re: [RFC bpf-next v5] bpf: Force to MPTCP

From: Stanislav Fomichev <[email-masked]>
Date: Thu, 27 Jul 2023 11:01:47 -0700

On 07/27, Matthieu Baerts wrote:
> Hi Paul, Stanislav,
> 
> On 18/07/2023 18:14, Paul Moore wrote:
> > On Tue, Jul 18, 2023 at 11:21 AM Geliang Tang <[email-masked]> wrote:
> >>
> >> As is described in the "How to use MPTCP?" section in MPTCP wiki [1]:
> >>
> >> "Your app can create sockets with IPPROTO_MPTCP as the proto:
> >> ( socket(AF_INET, SOCK_STREAM, IPPROTO_MPTCP); ). Legacy apps can be
> >> forced to create and use MPTCP sockets instead of TCP ones via the
> >> mptcpize command bundled with the mptcpd daemon."
> >>
> >> But the mptcpize (LD_PRELOAD technique) command has some limitations
> >> [2]:
> >>
> >>  - it doesn't work if the application is not using libc (e.g. GoLang
> >> apps)
> >>  - in some envs, it might not be easy to set env vars / change the way
> >> apps are launched, e.g. on Android
> >>  - mptcpize needs to be launched with all apps that want MPTCP: we could
> >> have more control from BPF to enable MPTCP only for some apps or all the
> >> ones of a netns or a cgroup, etc.
> >>  - it is not in BPF, we cannot talk about it at netdev conf.
> >>
> >> So this patchset attempts to use BPF to implement functions similer to
> >> mptcpize.
> >>
> >> The main idea is add a hook in sys_socket() to change the protocol id
> >> from IPPROTO_TCP (or 0) to IPPROTO_MPTCP.
> >>
> >> [1]
> >> https://github.com/multipath-tcp/mptcp_net-next/wiki
> >> [2]
> >> https://github.com/multipath-tcp/mptcp_net-next/issues/79
> >>
> >> v5:
> >>  - add bpf_mptcpify helper.
> >>
> >> v4:
> >>  - use lsm_cgroup/socket_create
> >>
> >> v3:
> >>  - patch 8: char cmd[128]; -> char cmd[256];
> >>
> >> v2:
> >>  - Fix build selftests errors reported by CI
> >>
> >> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/79
> >> Signed-off-by: Geliang Tang <[email-masked]>
> >> ---
> >>  include/linux/bpf.h                           |   1 +
> >>  include/linux/lsm_hook_defs.h                 |   2 +-
> >>  include/linux/security.h                      |   6 +-
> >>  include/uapi/linux/bpf.h                      |   7 +
> >>  kernel/bpf/bpf_lsm.c                          |   2 +
> >>  net/mptcp/bpf.c                               |  20 +++
> >>  net/socket.c                                  |   4 +-
> >>  security/apparmor/lsm.c                       |   8 +-
> >>  security/security.c                           |   2 +-
> >>  security/selinux/hooks.c                      |   6 +-
> >>  tools/include/uapi/linux/bpf.h                |   7 +
> >>  .../testing/selftests/bpf/prog_tests/mptcp.c  | 128 ++++++++++++++++--
> >>  tools/testing/selftests/bpf/progs/mptcpify.c  |  17 +++
> >>  13 files changed, 187 insertions(+), 23 deletions(-)
> >>  create mode 100644 tools/testing/selftests/bpf/progs/mptcpify.c
> > 
> > ...
> > 
> >> diff --git a/security/security.c b/security/security.c
> >> index b720424ca37d..bbebcddce420 100644
> >> --- a/security/security.c
> >> +++ b/security/security.c
> >> @@ -4078,7 +4078,7 @@ EXPORT_SYMBOL(security_unix_may_send);
> >>   *
> >>   * Return: Returns 0 if permission is granted.
> >>   */
> >> -int security_socket_create(int family, int type, int protocol, int kern)
> >> +int security_socket_create(int *family, int *type, int *protocol, int kern)
> >>  {
> >>         return call_int_hook(socket_create, 0, family, type, protocol, kern);
> >>  }
> > 
> > Using the LSM to change the protocol family is not something we want
> > to allow.  I'm sorry, but you will need to take a different approach.
> 
> @Paul: Thank you for your feedback. It makes sense and I understand.
> 
> @Stanislav: Despite the fact the implementation was smaller and reusing
> more code, it looks like we cannot go in the direction you suggested. Do
> you think what Geliang suggested before in his v3 [1] can be accepted?
> 
> (Note that the v3 is the same as the v1, only some fixes in the selftests.)

We have too many hooks in networking, so something that doesn't add
a new one is preferable :-( Moreover, we already have a 'socket init'
hook, but it runs a bit late.

Is existing cgroup/sock completely unworkable? Is it possible to
expose some new bpf_upgrade_socket_to(IPPROTO_MPTCP) kfunc which would
call some new net_proto_family->upgrade_to(IPPROTO_MPTCP) to do the surgery?
Or is it too hacky?

Another option Alexei suggested is to add some fentry-like thing:

noinline int update_socket_protocol(int protocol)
{
	return protocol;
}
/* TODO: ^^^ add the above to mod_ret set */

int __sys_socket(int family, int type, int protocol)
{
	...

	protocol = update_socket_protocol(protocol);

	...
}

But it's also too problem specific it seems? And it's not cgroup-aware.