netdev - Re: [PATCH net] net: ip6_gre: access skb data after skb_cow

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Message-Id: <20190518.104512.963962673481938495.davem@davemloft.net>
Date:   Sat, 18 May 2019 10:45:12 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     u9012063@...il.com
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net] net: ip6_gre: access skb data after skb_cow_head()

From: William Tu <u9012063@...il.com>
Date: Thu, 16 May 2019 09:25:48 -0700

> When increases the headroom, skb's pointer might get re-allocated.
> Fix it by moving skb_cow_head before accessing the skb->data pointer.
> 
> Fixes: 01b8d064d58b4 ("net: ip6_gre: Request headroom in __gre6_xmit()")
> Reported-by: Haichao Ma <haichaom@...are.com>
> Signed-off-by: William Tu <u9012063@...il.com>

I don't understand the problem.

The fl6->daddr assignments are object copies, not pointer assignments.

So there are no dangling pointer references I can see.

Also, you need to explain exactly what dangling pointer is the problem
in your commit message.