Jakub Sitnicki wrote:
> On Sun, Jan 12, 2020 at 12:14 AM CET, John Fastabend wrote:
> > Jakub Sitnicki wrote:
> >> sk_msg and ULP frameworks override protocol callbacks pointer in
> >> sk->sk_prot, while TCP accesses it locklessly when cloning the listening
> >> socket.
> >>
> >> Once we enable use of listening sockets with sockmap (and hence sk_msg),
> >> there can be shared access to sk->sk_prot if socket is getting cloned while
> >> being inserted/deleted to/from the sockmap from another CPU. Mark the
> >> shared access with READ_ONCE/WRITE_ONCE annotations.
> >>
> >> Signed-off-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
> >
> > In sockmap side I fixed this by wrapping the access in a lock_sock[0]. So
> > Do you think this is still needed with that in mind? The bpf_clone call
> > is using sk_prot_creater and also setting the newsk's proto field. Even
> > if the listening parent sock was being deleted in parallel would that be
> > a problem? We don't touch sk_prot_creator from the tear down path. I've
> > only scanned the 3..11 patches so maybe the answer is below. If that is
> > the case probably an improved commit message would be helpful.
>
> I think it is needed. Not because of tcp_bpf_clone or that we access
> listener's sk_prot_creator from there, if I'm grasping your question.
>
> Either way I'm glad this came up. Let's go though my reasoning and
> verify it. tcp stack accesses the listener sk_prot while cloning it:
>
> tcp_v4_rcv
> sk = __inet_lookup_skb(...)
> tcp_check_req(sk)
> inet_csk(sk)->icsk_af_ops->syn_recv_sock
> tcp_v4_syn_recv_sock
> tcp_create_openreq_child
> inet_csk_clone_lock
> sk_clone_lock
> READ_ONCE(sk->sk_prot)
>
> It grabs a reference to the listener, but doesn't grab the sk_lock.
>
> On another CPU we can be inserting/removing the listener socket from the
> sockmap and writing to its sk_prot. We have the update and the remove
> path:
>
> sock_map_ops->map_update_elem
> sock_map_update_elem
> sock_map_update_common
> sock_map_link_no_progs
> tcp_bpf_init
> tcp_bpf_update_sk_prot
> sk_psock_update_proto
> WRITE_ONCE(sk->sk_prot, ops)
>
> sock_map_ops->map_delete_elem
> sock_map_delete_elem
> __sock_map_delete
> sock_map_unref
> sk_psock_put
> sk_psock_drop
> sk_psock_restore_proto
> tcp_update_ulp
> WRITE_ONCE(sk->sk_prot, proto)
>
> Following the guidelines from KTSAN project [0], sk_prot looks like a
> candidate for annotating it. At least on these 3 call paths.
>
> If that sounds correct, I can add it to the patch description.
>
Logic looks correct to me thanks for the details, please put those in
the commit so we don't lose them. Can you also add a comment where it
makes most sense in the code? This is a bit subtle and we don't want
to miss it later. Probably in tcp_update_ulp near that WRITE_ONCE would
do. It doesn't need to be too verbose but something as simple as,
"{WRITE|READ}_ONCE wrappers needed around sk_prot to protect unlocked
reads in sk_clone_lock"
> Thanks,
> -jkbs
>
> [0] https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE
