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Subject: Re: [PATCH 3/4] usbnet: smsc95xx: check for csum being in last four
 bytes
To:     Ben Dooks <ben.dooks@codethink.co.uk>, netdev@vger.kernel.org
Cc:     oneukum@suse.com, davem@davemloft.net, linux-usb@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-kernel@lists.codethink.co.uk
References: <20181002092645.1115-1-ben.dooks@codethink.co.uk>
 <20181002092645.1115-4-ben.dooks@codethink.co.uk>
From:   Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Message-ID: <1aeea157-7890-42c2-1f84-85e9cc9d91a5@cogentembedded.com>
Date:   Tue, 2 Oct 2018 12:45:05 +0300
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Hello!

On 10/2/2018 12:26 PM, Ben Dooks wrote:

> The manual states that the checksum cannot lie in the last DWORD of the
> transmission, so add a basic check for this and fall back to software
> checksumming the packet.
> 
> This only seems to trigger for ACK packets with no options or data to
> return to the other end, and the use of the tx-alignment option makes
> it more likely to happen.
> 
> Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
> ---
>   drivers/net/usb/smsc95xx.c | 17 ++++++++++++++++-
>   1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/usb/smsc95xx.c b/drivers/net/usb/smsc95xx.c
> index d244357bf1ad..46385a4b8b98 100644
> --- a/drivers/net/usb/smsc95xx.c
> +++ b/drivers/net/usb/smsc95xx.c
> @@ -2003,6 +2003,20 @@ static u32 smsc95xx_calc_csum_preamble(struct sk_buff *skb)
>   	return (high_16 << 16) | low_16;
>   }
>   
> +/* The CSUM won't work if the checksum lies in the last 4 bytes of the
> + * transmission. This is fairly unlikely, only seems to trigger with some
> + * short TCP ACK packets sent.
> + *
> + * Note, this calculation should probably check for the alignment of the
> + * data as well, but a straight chec for csum being in the last four bytes

    s/chec/check/?

> + * of the packet should be ok for now.
> +*/
> +static bool smsc95xx_can_checksum(struct sk_buff *skb)
> +{
> +       unsigned int len = skb->len - skb_checksum_start_offset(skb);
> +       return skb->csum_offset < (len - (4 + 1));
> +}
> +
>   static struct sk_buff *smsc95xx_tx_fixup(struct usbnet *dev,
>   					 struct sk_buff *skb, gfp_t flags)
>   {
[...]

MBR, Sergei