Re: [PATCH] net: fix incorrect original ingress device index in PKTINFO

[David Miller]

From: Wei Zhang <asuka.com@xxxxxxx>
Date: Tue, 27 Dec 2016 17:52:24 +0800

> When we send a packet for our own local address on a non-loopback
> interface (e.g. eth0), due to the change had been introduced from
> commit 0b922b7a829c ("net: original ingress device index in PKTINFO"), the
> original ingress device index would be set as the loopback interface.
> However, the packet should be considered as if it is being arrived via the
> sending interface (eth0), otherwise it would break the expectation of the
> userspace application (e.g. the DHCPRELEASE message from dhcp_release
> binary would be ignored by the dnsmasq daemon, since it come from lo which
> is not the interface dnsmasq bind to)
> 
> Signed-off-by: Wei Zhang <asuka.com@xxxxxxx>

When you are fixing a problem introduced by another change, always CC:
the author of that change as I have done so here.

David, please take a look at this, thanks.

> ---
>  net/ipv4/ip_sockglue.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
> index b8a2d63..76d78a7 100644
> --- a/net/ipv4/ip_sockglue.c
> +++ b/net/ipv4/ip_sockglue.c
> @@ -1202,8 +1202,14 @@ void ipv4_pktinfo_prepare(const struct sock *sk, struct sk_buff *skb)
>  		 * which has interface index (iif) as the first member of the
>  		 * underlying inet{6}_skb_parm struct. This code then overlays
>  		 * PKTINFO_SKB_CB and in_pktinfo also has iif as the first
> -		 * element so the iif is picked up from the prior IPCB
> +		 * element so the iif is picked up from the prior IPCB except
> +		 * iif is loopback interface which the packet should be
> +		 * considered as if it is being arrived via the sending
> +		 * interface
>  		 */
> +		if (pktinfo->ipi_ifindex == LOOPBACK_IFINDEX)
> +			pktinfo->ipi_ifindex = inet_iif(skb);
> +
>  		pktinfo->ipi_spec_dst.s_addr = fib_compute_spec_dst(skb);
>  	} else {
>  		pktinfo->ipi_ifindex = 0;
> -- 
> 1.8.3.1
> 
>