Messages in this thread | | | From | Willem de Bruijn <> | Date | Sun, 17 Mar 2019 12:42:01 -0400 | Subject | Re: [PATCH net] packets: Always register packet sk in the same order |
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On Sat, Mar 16, 2019 at 9:21 PM David Miller <davem@davemloft.net> wrote: > > From: Willem de Bruijn <willemdebruijn.kernel@gmail.com> > Date: Sat, 16 Mar 2019 14:09:33 -0400 > > > Note that another consequence of this patch is that insertion on > > packet create is now O(N) with the number of active packet sockets, > > due to sklist being an hlist. > > Exploitable...
With root in userns? The running time is limited by open file rlimit. This pattern is already used in a sk_add_node_rcu in some way, so important to be sure.
In practice I see no significant wall clock time difference when inserting to a fairly standard default limit of 16K. Regardless of insertion order, running time is dominated by cleanup on process exit (synchronize_net barriers?). At higher rlimit it does become problematic.
The packet socket sklist is not easily converted from hlist to a regular list, due to the use of seq_hlist_next_rcu in packet_seq_ops. There is no equivalent seq_list_next_rcu. One option might be instead to leave insertion order as is, but traverse the list in reverse in packet_notifier on NETDEV_DOWN. That would require an sk_for_each_reverse_rcu and hlist_for_each_entry_reverse_rcu. These do not exist, but since an hlist_pprev_rcu does exist, it is probably feasible. Though not a trivial change.
Another more narrow option may be to work around the ordering in fanout itself, e.g., record in the socket the initially assigned location in the fanout array and try to reclaim this spot on re-insertion.
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