linux-kernel - Re: [PATCH] ibmveth: Fix off-by-one error in ibmveth_change

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Date:	Tue, 14 Apr 2015 10:33:18 -0500
From:	Thomas Falcon <tlfalcon@...ux.vnet.ibm.com>
To:	David Gibson <david@...son.dropbear.id.au>, anton@....ibm.com,
	benh@...nel.crashing.org, michael@...erman.id.au
CC:	linux-kernel@...r.kernel.org, linuxppc-dev@...r.kernel.org
Subject: Re: [PATCH] ibmveth: Fix off-by-one error in ibmveth_change_mtu()

On 04/13/2015 12:39 AM, 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.


The same expression is made using < just a few lines above.  Shouldn't this be changed to <= too?

@@ -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)

Thanks,

Tom
> Signed-off-by: David Gibson <david@...son.dropbear.id.au>
> ---
>  drivers/net/ethernet/ibm/ibmveth.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/ibm/ibmveth.c b/drivers/net/ethernet/ibm/ibmveth.c
> index cd7675a..3aa280a 100644
> --- a/drivers/net/ethernet/ibm/ibmveth.c
> +++ b/drivers/net/ethernet/ibm/ibmveth.c
> @@ -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


-- 
Tom Falcon
Software Engineer
IBM LTC Austin

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