[PATCH net-next] net: usb: smsc95xx: stop lying about skb->truesize

[PATCH net-next] net: usb: smsc95xx: stop lying about skb->truesize

[Eric Dumazet]

Some usb drivers try to set small skb->truesize and break
core networking stacks.

In this patch, I removed one of the skb->truesize override.

I also replaced one skb_clone() by an allocation of a fresh
and small skb, to get minimally sized skbs, like we did
in commit 1e2c61172342 ("net: cdc_ncm: reduce skb truesize
in rx path") and 4ce62d5b2f7a ("net: usb: ax88179_178a:
stop lying about skb->truesize")

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Steve Glendinning <steve.glendinning@shawell.net>
Cc: UNGLinuxDriver@microchip.com
---
 drivers/net/usb/smsc95xx.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/drivers/net/usb/smsc95xx.c b/drivers/net/usb/smsc95xx.c
index 2fa46baa589e5e87e12e145fe46268bdaf9fc219..54499c1a27f31805932cda9ed5ab5847f9c4fca1 100644
--- a/drivers/net/usb/smsc95xx.c
+++ b/drivers/net/usb/smsc95xx.c
@@ -1870,25 +1870,22 @@ static int smsc95xx_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
 				if (dev->net->features & NETIF_F_RXCSUM)
 					smsc95xx_rx_csum_offload(skb);
 				skb_trim(skb, skb->len - 4); /* remove fcs */
-				skb->truesize = size + sizeof(struct sk_buff);
 
 				return 1;
 			}
 
-			ax_skb = skb_clone(skb, GFP_ATOMIC);
+			/* Use "size - 4" to remove fcs */
+			ax_skb = netdev_alloc_skb_ip_align(dev->net, size - 4);
 			if (unlikely(!ax_skb)) {
 				netdev_warn(dev->net, "Error allocating skb\n");
 				return 0;
 			}
 
-			ax_skb->len = size;
-			ax_skb->data = packet;
-			skb_set_tail_pointer(ax_skb, size);
+			skb_put(ax_skb, size - 4);
+			memcpy(ax_skb->data, packet, size - 4);
 
 			if (dev->net->features & NETIF_F_RXCSUM)
 				smsc95xx_rx_csum_offload(ax_skb);
-			skb_trim(ax_skb, ax_skb->len - 4); /* remove fcs */
-			ax_skb->truesize = size + sizeof(struct sk_buff);
 
 			usbnet_skb_return(dev, ax_skb);
 		}
-- 
2.45.0.rc1.225.g2a3ae87e7f-goog

Re: [PATCH net-next] net: usb: smsc95xx: stop lying about skb->truesize

[Jakub Kicinski]

On Mon,  6 May 2024 14:28:35 +0000 Eric Dumazet wrote:
> -			ax_skb->len = size;
> -			ax_skb->data = packet;
> -			skb_set_tail_pointer(ax_skb, size);
> +			skb_put(ax_skb, size - 4);
> +			memcpy(ax_skb->data, packet, size - 4);
>  
>  			if (dev->net->features & NETIF_F_RXCSUM)
>  				smsc95xx_rx_csum_offload(ax_skb);
> -			skb_trim(ax_skb, ax_skb->len - 4); /* remove fcs */
> -			ax_skb->truesize = size + sizeof(struct sk_buff);

I think this one's off:

static void smsc95xx_rx_csum_offload(struct sk_buff *skb)
{
     skb->csum = *(u16 *)(skb_tail_pointer(skb) - 2);
     skb->ip_summed = CHECKSUM_COMPLETE;
     skb_trim(skb, skb->len - 2);
}

Re: [PATCH net-next] net: usb: smsc95xx: stop lying about skb->truesize

[Eric Dumazet]

On Wed, May 8, 2024 at 2:41 AM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Mon,  6 May 2024 14:28:35 +0000 Eric Dumazet wrote:
> > -                     ax_skb->len = size;
> > -                     ax_skb->data = packet;
> > -                     skb_set_tail_pointer(ax_skb, size);
> > +                     skb_put(ax_skb, size - 4);
> > +                     memcpy(ax_skb->data, packet, size - 4);
> >
> >                       if (dev->net->features & NETIF_F_RXCSUM)
> >                               smsc95xx_rx_csum_offload(ax_skb);
> > -                     skb_trim(ax_skb, ax_skb->len - 4); /* remove fcs */
> > -                     ax_skb->truesize = size + sizeof(struct sk_buff);
>
> I think this one's off:
>
> static void smsc95xx_rx_csum_offload(struct sk_buff *skb)
> {
>      skb->csum = *(u16 *)(skb_tail_pointer(skb) - 2);
>      skb->ip_summed = CHECKSUM_COMPLETE;
>      skb_trim(skb, skb->len - 2);
> }

Good catch, I will revise this patch accordingly, thanks !