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List:       linux-sctp
Subject:    Re: [PATCHv2] sctp: Fix port hash table size computation
From:       David Miller <davem () davemloft ! net>
Date:       2016-02-22 2:53:46
Message-ID: 20160221.215346.18865067787438743.davem () davemloft ! net
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From: Neil Horman <nhorman@tuxdriver.com>
Date: Thu, 18 Feb 2016 16:10:57 -0500

> Dmitry Vyukov noted recently that the sctp_port_hashtable had an error in
> its size computation, observing that the current method never guaranteed
> that the hashsize (measured in number of entries) would be a power of two,
> which the input hash function for that table requires.  The root cause of
> the problem is that two values need to be computed (one, the allocation
> order of the storage requries, as passed to __get_free_pages, and two the
> number of entries for the hash table).  Both need to be ^2, but for
> different reasons, and the existing code is simply computing one order
> value, and using it as the basis for both, which is wrong (i.e. it assumes
> that ((1<<order)*PAGE_SIZE)/sizeof(bucket) is still ^2 when its not).
> 
> To fix this, we change the logic slightly.  We start by computing a goal
> allocation order (which is limited by the maximum size hash table we want
> to support.  Then we attempt to allocate that size table, decreasing the
> order until a successful allocation is made.  Then, with the resultant
> successful order we compute the number of buckets that hash table supports,
> which we then round down to the nearest power of two, giving us the number
> of entries the table actually supports.
> 
> I've tested this locally here, using non-debug and spinlock-debug kernels,
> and the number of entries in the hashtable consistently work out to be
> powers of two in all cases.
> 
> Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
> Reported-by: Dmitry Vyukov <dvyukov@google.com>

Applied and queued up for -stable, thanks Neil.
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