netdev - Re: [PATCHv2] ibmveth: Fix off-by-one error in ibmveth_change

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Message-Id: <20150423.114245.1826557896621397900.davem@davemloft.net>
Date:	Thu, 23 Apr 2015 11:42:45 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	david@...son.dropbear.id.au
Cc:	anton@....ibm.com, tlfalcon@...ux.vnet.ibm.com,
	michael@...erman.id.au, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org
Subject: Re: [PATCHv2] ibmveth: Fix off-by-one error in ibmveth_change_mtu()

From: David Gibson <david@...son.dropbear.id.au>
Date: Thu, 23 Apr 2015 14:43:05 +1000

> AFAIK the PAPR document which defines the virtual device interface used by
> the ibmveth driver doesn't specify a specific maximum MTU.  So, in the
> ibmveth driver, the maximum allowed MTU is determined by the maximum
> allocated buffer size of 64k (corresponding to one page in the common case)
> minus the per-buffer overhead IBMVETH_BUFF_OH (which has value 22 for 14
> bytes of ethernet header, plus 8 bytes for an opaque handle).
> 
> This suggests a maximum allowable MTU of 65514 bytes, but in fact the
> driver only permits a maximum MTU of 65513.  This is because there is a <
> instead of an <= in ibmveth_change_mtu(), which only permits an MTU which
> is strictly smaller than the buffer size, rather than allowing the buffer
> to be completely filled.
> 
> This patch fixes the buglet.
> 
> Signed-off-by: David Gibson <david@...son.dropbear.id.au>
> Acked-by: Thomas Falcon <tlfalcon@...ux.vnet.ibm.com>

Applied, thank you.
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