Messages in this thread | | | Date | Mon, 6 Jan 2014 22:23:26 +0100 | From | Florian Westphal <> | Subject | Re: [PATCH] netfilter: nf_conntrack: release conntrack from rcu callback |
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Cyrill Gorcunov <gorcunov@gmail.com> wrote: > On Mon, Jan 06, 2014 at 06:02:35PM +0100, Florian Westphal wrote: > > > > Can you elaborate? > > Yes, nf_ct_is_dying(ct) might be called for the wrong conntrack. > > > > But, in case we _think_ that its the right one we call > > nf_ct_tuple_equal() to verify we indeed found the right one: > > > > h = ____nf_conntrack_find(net, zone, tuple, hash); > > if (h) { // might be released right now, but page won't go away (SLAB_BY_RCU) > > ct = nf_ct_tuplehash_to_ctrack(h); > > if (unlikely(nf_ct_is_dying(ct) || > > !atomic_inc_not_zero(&ct->ct_general.use))) > > // which means we should hit this path (0 ref). > > h = NULL; > > else { > > // otherwise, it cannot go away from under us, since > > // we own a reference now. > > if (unlikely(!nf_ct_tuple_equal(tuple, &h->tuple) || > > nf_ct_zone(ct) != zone)) { > > // if we get here, the entry got recycled on other cpu > > // for a different tuple, we can bail out and drop > > // the reference safely and re-try the lookup > > nf_ct_put(ct); > > goto begin; > > } > > } > > I think tuple may match if > > task 1 task 2 task 3 > nf_conntrack_find_get > ____nf_conntrack_find > destroy_conntrack > hlist_nulls_del_rcu > nf_conntrack_free > kmem_cache_free > __nf_conntrack_alloc > kmem_cache_alloc > if (nf_ct_is_dying(ct)) > > data is not yet cleaned > > memset(&ct->tuplehash[IP_CT_DIR_MAX], > > No? Or there something obvious I'm missing?
IMHO this isn't obvious at all :-)
But, in the example above, the atomic_inc_not_zero() should fail until after __nf_conntrack_alloc() re-inits the refcount to 1.
The mb there should make sure ____nf_conntrack_find() doesn't find an outdated tuple before this.
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