netdev - Re: [RFC PATCH] IP: do not modify ingress packet IP option in ip_options

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Date:   Mon, 24 Jul 2017 14:12:53 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     pabeni@...hat.com
Cc:     netdev@...r.kernel.org, edumazet@...gle.com,
        hannes@...essinduktion.org
Subject: Re: [RFC PATCH] IP: do not modify ingress packet IP option in
 ip_options_echo()

From: Paolo Abeni <pabeni@...hat.com>
Date: Fri, 21 Jul 2017 15:55:18 +0200

> diff --git a/net/ipv4/ip_options.c b/net/ipv4/ip_options.c
> index 93157f2..fdda973 100644
> --- a/net/ipv4/ip_options.c
> +++ b/net/ipv4/ip_options.c
> @@ -174,9 +174,6 @@ int __ip_options_echo(struct ip_options *dopt, struct sk_buff *skb,
>  				doffset -= 4;
>  		}
>  		if (doffset > 3) {
> -			__be32 daddr = fib_compute_spec_dst(skb);
> -
> -			memcpy(&start[doffset-1], &daddr, 4);
>  			dopt->faddr = faddr;

This transformation is required, but in the destination not the source.

The red flag is that we are indexing 'start' with 'doffset' instead of
'soffset'.