netdev - Re: [PATCH IPV6 1/1] ipv6: allocate enough headroom in ip6_finish

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Date:   Wed, 7 Jul 2021 11:30:27 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Ahern <dsahern@...il.com>, Vasily Averin <vvs@...tuozzo.com>,
        "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH IPV6 1/1] ipv6: allocate enough headroom in
 ip6_finish_output2()

On Wed, 7 Jul 2021 19:41:44 +0200 Eric Dumazet wrote:
> On 7/7/21 6:42 PM, Jakub Kicinski wrote:
> > On Wed, 7 Jul 2021 08:45:13 -0600 David Ahern wrote:  
> >> why not use hh_len here?  
> > 
> > Is there a reason for the new skb? Why not pskb_expand_head()?  
> 
> 
> pskb_expand_head() might crash, if skb is shared.
> 
> We possibly can add a helper, factorizing all this,
> and eventually use pskb_expand_head() if safe.

Is there a strategically placed skb_share_check() somewhere further
down? Otherwise there seems to be a lot of questionable skb_cow*()
calls, also __skb_linearize() and skb_pad() are risky, no?
Or is it that shared skbs are uncommon and syzbot doesn't hit them?