Re: [PATCH IPV6 1/1] ipv6: allocate enough headroom in ip6_finish_output2()

[Eric Dumazet]

On 7/7/21 8:30 PM, Jakub Kicinski wrote:
> 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?
> 

Some of us try hard to remove skb_get() occurrences,
but they tend to re-appear fast :/

Refs: commit a516993f0ac1694673412eb2d16a091eafa77d2a
("net: fix wrong skb_get() usage / crash in IGMP/MLD parsing code")