Re: [Bug #13339] rtable leak in ipv4/route.c

From: Eric Dumazet
Date: Sun May 24 2009 - 18:35:24 EST

Rafael J. Wysocki a Ãcrit :
> This message has been generated automatically as a part of a report
> of recent regressions.
> The following bug entry is on the current list of known regressions
> from 2.6.29. Please verify if it still should be listed and let me know
> (either way).
> Bug-Entry :
> Subject : rtable leak in ipv4/route.c
> Submitter : Alexander V. Lukyanov <lav@xxxxxx>
> Date : 2009-05-18 14:10 (7 days old)

Bug was present in 2.6.29, so its a regression from 2.6.28

It is solved and available in David tree (net-2.6), and scheduled for stable submission

commit 1ddbcb005c395518c2cd0df504cff3d4b5c85853
net: fix rtable leak in net/ipv4/route.c

Alexander V. Lukyanov found a regression in 2.6.29 and made a complete
analysis found in
Quoted here because its a perfect one :

2.6.29 patch has introduced flexible route cache rebuilding. Unfortunately the
patch has at least one critical flaw, and another problem.

rt_intern_hash calculates rthi pointer, which is later used for new entry
insertion. The same loop calculates cand pointer which is used to clean the
list. If the pointers are the same, rtable leak occurs, as first the cand is
removed then the new entry is appended to it.

This leak leads to unregister_netdevice problem (usage count > 0).

Another problem of the patch is that it tries to insert the entries in certain
order, to facilitate counting of entries distinct by all but QoS parameters.
Unfortunately, referencing an existing rtable entry moves it to list beginning,
to speed up further lookups, so the carefully built order is destroyed.

For the first problem the simplest patch it to set rthi=0 when rthi==cand, but
it will also destroy the ordering.

Problematic commit is 1080d709fb9d8cd4392f93476ee46a9d6ea05a5b
(net: implement emergency route cache rebulds when gc_elasticity is exceeded)

Trying to keep dst_entries ordered is too complex and breaks the fact that
order should depend on the frequency of use for garbage collection.

A possible fix is to make rt_intern_hash() simpler, and only makes
rt_check_expire() a litle bit smarter, being able to cope with an arbitrary
entries order. The added loop is running on cache hot data, while cpu
is prefetching next object, so should be unnoticied.

Reported-and-analyzed-by: Alexander V. Lukyanov <lav@xxxxxx>
Signed-off-by: Eric Dumazet <dada1@xxxxxxxxxxxxx>
Acked-by: Neil Horman <nhorman@xxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>


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