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    SubjectRe: [bug] __nf_ct_refresh_acct(): WARNING: at lib/list_debug.c:30 __list_add+0x7d/0xad()
    Patrick McHardy wrote:
    > Eric Dumazet wrote:
    >> In my own analysis, I found death_by_timeout() might be problematic,
    >> with RCU and lockless lookups.
    >>
    >> static void death_by_timeout(unsigned long ul_conntrack)
    >> {
    >> struct nf_conn *ct = (void *)ul_conntrack;
    >>
    >> if (!test_bit(IPS_DYING_BIT, &ct->status) &&
    >> unlikely(nf_conntrack_event(IPCT_DESTROY, ct) < 0)) {
    >> /* destroy event was not delivered */
    >> nf_ct_delete_from_lists(ct);
    >> << HERE >>
    >>
    >> nf_ct_insert_dying_list(ct);
    >> return;
    >> }
    >> set_bit(IPS_DYING_BIT, &ct->status);
    >> nf_ct_delete_from_lists(ct);
    >> nf_ct_put(ct);
    >> }
    >>
    >>
    >> We delete ct from a list and insert it in a new list.
    >>
    >> I believe a reader could "*catch*" ct while doing a lookup and miss
    >> the end
    >> of its chain. (nulls algo check the null value at the end of lookup
    >> and can
    >> decide to restart the lookup if the null value is not the expected one)
    >>
    >> We need to change nf_conntrack_init_net() and use a different "null"
    >> value,
    >> guaranteed not being used in regular lists
    >
    > Good catch. This is a new bug, but it shouldn't matter in this case
    > since nf_conntrack_event() can't fail unless you have a userspace
    > listener that makes use of reliable delivery, which I think hasn't
    > even been released yet.

    Indeed. I didn't include user-space support for this yet in my tree, so
    this should not be the problem. Thanks for the catch anyway!


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