On Wed, Aug 1, 2018 at 12:35 PM, Florian Westphal <fw@xxxxxxxxx> wrote: > Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: >> Still can't grasp all details. >> There is state that we read without taking ct->ct_general.use ref >> first, namely ct->state and what's used by nf_ct_key_equal. >> So let's say the entry we want to find is in the list, but >> ____nf_conntrack_find finds a wrong entry earlier because all state it >> looks at is random garbage, so it returns the wrong entry to >> __nf_conntrack_find_get. > > If an entry can be found, it can't be random garbage. > We never link entries into global table until state has been set up. But... we don't hold a reference to the entry. So say it's in the table with valid state, now ____nf_conntrack_find discovers it, now the entry is removed and reused a dozen of times will all associated state reinitialization. And nf_ct_key_equal looks at it concurrently and decides if it's the entry we are looking for or now. I think unless we hold a ref to the entry, it's state needs to be considered random garbage for correctness reasoning. >> Now (nf_ct_is_dying(ct) || !atomic_inc_not_zero(&ct->ct_general.use)) >> check in __nf_conntrack_find_get passes, and it returns NULL to the >> caller (which means entry is not present). > > So entry is going away or marked as dead which for us is same as > 'not present', we need to allocate a new entry. > >> While in reality the entry >> is present, but we were just looking at the wrong one. > > We never add tuples that are identical to the global table. > > If N cores receive identical packets at same time with no prior state, all > will allocate a new conntrack, but we notice this when we try to insert the > nf_conn entries into the table. > > Only one will succeed, other cpus have to cope with this. > (worst case: all raced packets are dropped along with their conntrack > object). > > For lookup, we have following scenarios: > > 1. It doesn't exist -> new allocation needed > 2. It exists, not dead, has nonzero refount -> use it > 3. It exists, but marked as dying -> new allocation needed > 4. It exists but has 0 reference count -> new allocation needed > 5. It exists, we get reference, but 2nd nf_ct_key_equal check > fails. We saw a matching 'old incarnation' that just got > re-used on other core. -> retry lookup > >> Also I am not sure about order of checks in (nf_ct_is_dying(ct) || >> !atomic_inc_not_zero(&ct->ct_general.use)), because checking state >> before taking the ref is only a best-effort hint, so it can actually >> be a dying entry when we take a ref. > > Yes, it can also become a dying entry after we took the reference. > >> So shouldn't it read something like the following? >> >> rcu_read_lock(); >> begin: >> h = ____nf_conntrack_find(net, zone, tuple, hash); >> if (h) { >> ct = nf_ct_tuplehash_to_ctrack(h); >> if (!atomic_inc_not_zero(&ct->ct_general.use)) >> goto begin; >> if (unlikely(nf_ct_is_dying(ct)) || >> unlikely(!nf_ct_key_equal(h, tuple, zone, net))) { >> nf_ct_put(ct); > > It would be ok to make this change, but dying bit can be set > at any time e.g. because userspace tells kernel to flush the conntrack table. > So refcount is always > 0 when the DYING bit is set. > > I don't see why it would be a problem. > > nf_conn struct will stay valid until all cpus have dropped references. > The check in lookup function only serves to hide the known-to-go-away entry. -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html