Subject: Re: [PATCHv2] ibmveth: Fix off-by-one error in ibmveth_change_mtu()

From: Tyrel Datwyler <[email-masked]>
Date: Thu, 23 Apr 2015 16:12:48 -0700

On 04/22/2015 02:42 PM, Thomas Falcon wrote:
> On 04/20/2015 08:07 PM, David Gibson wrote:
>> 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

PAPR itself doesn't, but the max-frame-size property, which I understand
to be synonymous with max MTU, is required by PAPR to be present in the
Logical LAN OF node corresponding to the ibmveth device.

-Tyrel

>> 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.
> 
> Thanks!
> 
> Acked-by: Thomas Falcon <[email-masked]>
> 
>>
>> Signed-off-by: David Gibson <[email-masked]>1
>> ---
>>  drivers/net/ethernet/ibm/ibmveth.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> Changes since v1:
>>  * Fixed a second instance of the same off-by-one error.  Thanks to
>>    Thomas Falcon for spotting this.
>>
>> diff --git a/drivers/net/ethernet/ibm/ibmveth.c b/drivers/net/ethernet/ibm/ibmveth.c
>> index cd7675a..1813476 100644
>> --- a/drivers/net/ethernet/ibm/ibmveth.c
>> +++ b/drivers/net/ethernet/ibm/ibmveth.c
>> @@ -1238,7 +1238,7 @@ static int ibmveth_change_mtu(struct net_device *dev, int new_mtu)
>>  		return -EINVAL;
>>  
>>  	for (i = 0; i < IBMVETH_NUM_BUFF_POOLS; i++)
>> -		if (new_mtu_oh < adapter->rx_buff_pool[i].buff_size)
>> +		if (new_mtu_oh <= adapter->rx_buff_pool[i].buff_size)
>>  			break;
>>  
>>  	if (i == IBMVETH_NUM_BUFF_POOLS)
>> @@ -1257,7 +1257,7 @@ static int ibmveth_change_mtu(struct net_device *dev, int new_mtu)
>>  	for (i = 0; i < IBMVETH_NUM_BUFF_POOLS; i++) {
>>  		adapter->rx_buff_pool[i].active = 1;
>>  
>> -		if (new_mtu_oh < adapter->rx_buff_pool[i].buff_size) {
>> +		if (new_mtu_oh <= adapter->rx_buff_pool[i].buff_size) {
>>  			dev->mtu = new_mtu;
>>  			vio_cmo_set_dev_desired(viodev,
>>  						ibmveth_get_desired_dma
> 
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