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

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Date:   Wed, 7 Jul 2021 19:41:44 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>, David Ahern <dsahern@...il.com>
Cc:     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 7/7/21 6:42 PM, Jakub Kicinski wrote:
> On Wed, 7 Jul 2021 08:45:13 -0600 David Ahern wrote:
>> On 7/7/21 8:04 AM, Vasily Averin wrote:
>>> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
>>> index ff4f9eb..e5af740 100644
>>> --- a/net/ipv6/ip6_output.c
>>> +++ b/net/ipv6/ip6_output.c
>>> @@ -61,9 +61,24 @@ static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *
>>>  	struct dst_entry *dst = skb_dst(skb);
>>>  	struct net_device *dev = dst->dev;
>>>  	const struct in6_addr *nexthop;
>>> +	unsigned int hh_len = LL_RESERVED_SPACE(dev);
>>>  	struct neighbour *neigh;
>>>  	int ret;
>>>  
>>> +	/* Be paranoid, rather than too clever. */
>>> +	if (unlikely(skb_headroom(skb) < hh_len && dev->header_ops)) {
>>> +		struct sk_buff *skb2;
>>> +
>>> +		skb2 = skb_realloc_headroom(skb, LL_RESERVED_SPACE(dev));  
>>
>> 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.