Re: Buggy rhashtable walking

On Thu, Aug 04, 2016 at 03:18:46PM +0800, Herbert Xu wrote:

So the question is can wireless handle seeing an entry multiple
times? In particular, __ieee80211_rx_handle_packet would appear
to process the same packet multiple times if this were to happen.

It's worse than I thought.  In fact it's not walking the table
at all, rather it's doing a hash lookup by hand!

This cannot possibly work given that rhashtable makes use of
multiple hash tables.

In fact this also demonstrates why putting multiple identical
objects into the same table is crap.  Because there is no sane
way of returning all objects corresponding to a single key, given
that they may be spread over multiple tables.

So I'm going to fix this by consolidating identical objects into
a single rhashtable entry which also lets us get rid of the
insecure_elasticity setting.

So the next time someone comes along and wants to add multiple
objects with the same key to one table, please just say no.

Thanks,
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Email: Herbert Xu [off-list ref]
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So I'm going to fix this by consolidating identical objects into
a single rhashtable entry which also lets us get rid of the
insecure_elasticity setting.

Hm. Would you rather allocate a separate head entry for the hashtable,
or chain the entries?

(Luckily) the colliding key case practically never happens, and some
drivers don't even allow it, so that's perhaps something to keep in
mind for this. Perhaps we should just generally disallow it - but a few
people (hi Ben) would be really unhappy about that I guess.

I think this might affect more than one use of rhashtable in mac80211
now, since the mesh paths also use it.

johannes