[PATCH] ibmveth: Fix off-by-one error in ibmveth_change_mtu()

[PATCH] ibmveth: Fix off-by-one error in ibmveth_change_mtu()

[David Gibson]

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@gibson.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
-- 
2.1.0

[PATCH] ibmveth: Fix off-by-one error in ibmveth_change_mtu()

[David Gibson]

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@gibson.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
-- 
2.1.0

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

[Thomas Falcon]

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@gibson.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

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

[David Gibson]

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On Tue, Apr 14, 2015 at 10:33:18AM -0500, Thomas Falcon wrote:
> 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)

Yes, yes it should.

Good catch, thanks.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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