netdev - Re: [PATCH net-next 3/3] ip: Add support for IP

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Date:	Thu, 04 Dec 2014 17:17:49 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 3/3] ip: Add support for IP_CHECKSUM cmsg

On Wed, 2014-12-03 at 16:44 -0800, Tom Herbert wrote:

> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 221b53f..bba2e06 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -1315,8 +1315,16 @@ try_again:
>  		memset(sin->sin_zero, 0, sizeof(sin->sin_zero));
>  		*addr_len = sizeof(*sin);
>  	}
> -	if (inet->cmsg_flags)
> +	if (inet->cmsg_flags) {
> +		/* Pull checksum past UDP header in case we are providing
> +		 * checksum in cmsg.
> +		 */
> +		if (inet->cmsg_flags & IP_CMSG_CHECKSUM)
> +			skb_postpull_rcsum(skb, skb->data,
> +					   sizeof(struct udphdr));
> +
>  		ip_cmsg_recv(msg, skb);
> +	}


Have you tried MSG_PEEK ?

You can not modify the skb, you need to do something else.


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