netdev - Re: [PATCH] net: unix socket code abuses csum

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Date:	Tue, 04 Mar 2014 13:41:41 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	anton@...ba.org
Cc:	edumazet@...gle.com, gustavold@...ux.vnet.ibm.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH] net: unix socket code abuses csum_partial

From: Anton Blanchard <anton@...ba.org>
Date: Tue, 4 Mar 2014 21:01:04 +1100

> The unix socket code is using the result of csum_partial to
> hash into a lookup table:
> 
> 	unix_hash_fold(csum_partial(sunaddr, len, 0));
> 
> csum_partial is only guaranteed to produce something that can be
> folded into a checksum, as its prototype explains:
> 
>  * returns a 32-bit number suitable for feeding into itself
>  * or csum_tcpudp_magic
> 
> The 32bit value should not be used directly.
> 
> Depending on the alignment the ppc64 csum_partial will return
> different 32bit values that will fold into the same checksum.
> 
> This difference causes the following testcase (courtesy of
> Gustavo) to sometimes fail:

Alternatively you could just make unix_hash_fold() use csum_fold(),
right?

static inline unsigned int unix_hash_fold(__wsum n)
{
	unsigned int hash = (__force unsigned int) csum_fold(n);

	hash ^= hash>>8;
	return hash&(UNIX_HASH_SIZE-1);
}
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