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

From: Thomas Falcon <[email-masked]>
Date: Wed, 22 Apr 2015 16:42:37 -0500

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
> 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