linux-kernel - Re: Question on rhashtable in worst-case scenario.

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20160330140933.GA19606@gondor.apana.org.au>
Date:	Wed, 30 Mar 2016 22:09:33 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	Ben Greear <greearb@...delatech.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
	netdev <netdev@...r.kernel.org>, Thomas Graf <tgraf@...g.ch>
Subject: Re: Question on rhashtable in worst-case scenario.

On Wed, Mar 30, 2016 at 04:03:08PM +0200, Johannes Berg wrote:
> 
> But we really don't want that either - in the normal case where you
> don't create all these virtual interfaces for testing, you have a
> certain number of peers that can vary a lot (zero to hundreds, in
> theory thousands) where you *don't* have the same key, so we still want
> to have the rehashing if the chains get longer in that case.

insecure_elasticity only disables rehashing without growing, it
does not inhibit table expansion which is driven by the number of
objects in the whole table.

> It's really just the degenerate case that Ben is creating locally
> that's causing a problem, afaict, though it's a bit disconcerting that
> rhashtable in general can cause strange failures at delete time.

The failure is simple, rhashtable will rehash the table if a given
chain becomes too long.  This simply doesn't work if you hash many
objects with the same key since the chain will never get shorter
even after a rehash (or expansion).

Therefore if your hashtable has to do this then you have to disable
the rehash logic using the insecure_elasticity flag.

Alternatively you can construct your own linked list for objects
with the same key outside of rhashtable and hash the list instead.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt