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SubjectRe: [PATCH] ipheth.c: Enable IP header alignment
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On Mon, 2011-05-02 at 21:35 +0200, L. Alberto Giménez wrote:
> Since commit ea812ca1b06113597adcd8e70c0f84a413d97544 (x86: Align skb w/ start
> of cacheline on newer core 2/Xeon Arch), NET_IP_ALIGN changed from 2 to 0, and
> the constant is used to reserve more room for the socket buffer.
>
> Some people have reported that tethering stopped working and David Hill
> submited a patch that redefined NET_IP_ALIGN. Pointed by Ben Hutchings, the
> patch has been reworked to use a private constant.
>
> I have no more an iPhone device to test it, so it is only compile-tested.
>
> Signed-off-by: L. Alberto Giménez <agimenez@sysvalve.es>
> ---
> drivers/net/usb/ipheth.c | 8 +++++---
> 1 files changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/usb/ipheth.c b/drivers/net/usb/ipheth.c
> index 7d42f9a..8f1ffc7 100644
> --- a/drivers/net/usb/ipheth.c
> +++ b/drivers/net/usb/ipheth.c
> @@ -80,6 +80,8 @@
> #define IPHETH_CARRIER_CHECK_TIMEOUT round_jiffies_relative(1 * HZ)
> #define IPHETH_CARRIER_ON 0x04
>
> +#define IPHETH_IP_ALIGN 2
> +
> static struct usb_device_id ipheth_table[] = {
> { USB_DEVICE_AND_INTERFACE_INFO(
> USB_VENDOR_APPLE, USB_PRODUCT_IPHONE,
> @@ -205,15 +207,15 @@ static void ipheth_rcvbulk_callback(struct urb *urb)
> len = urb->actual_length;
> buf = urb->transfer_buffer;
>
> - skb = dev_alloc_skb(NET_IP_ALIGN + len);
> + skb = dev_alloc_skb(IPHETH_IP_ALIGN + len);
> if (!skb) {
> err("%s: dev_alloc_skb: -ENOMEM", __func__);
> dev->net->stats.rx_dropped++;
> return;
> }
>
> - skb_reserve(skb, NET_IP_ALIGN);
> - memcpy(skb_put(skb, len), buf + NET_IP_ALIGN, len - NET_IP_ALIGN);
> + skb_reserve(skb, IPHETH_IP_ALIGN);
> + memcpy(skb_put(skb, len), buf + IPHETH_IP_ALIGN, len - IPHETH_IP_ALIGN);
[...]

So this was using NET_IP_ALIGN as an offset into the URB. Which was
totally bogus, as its value has long been architecture-dependent. The
code is also claiming to put len bytes but only copying len - delta.

The correct code would be something like:

if (urb->actual_length <= IPHETH_IP_ALIGN) {
dev->net->stats.rx_length_errors++;
return;
}
len = urb->actual_length - IPHETH_IP_ALIGN;
buf = urb->transfer_buffer + IPHETH_IP_ALIGN;

dev_alloc_skb(len);
...
memcpy(skb_put(skb, len), buf, len);

Ben.

> skb->dev = dev->net;
> skb->protocol = eth_type_trans(skb, dev->net);
>

--
Ben Hutchings, Senior Software Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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