Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH IPV6 1/1] ipv6: allocate enough headroom in
 ip6_finish_output2()
To:     Jakub Kicinski <kuba@kernel.org>,
        Eric Dumazet <eric.dumazet@gmail.com>
Cc:     David Ahern <dsahern@gmail.com>, Vasily Averin <vvs@virtuozzo.com>,
        "David S. Miller" <davem@davemloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <cover.1625665132.git.vvs@virtuozzo.com>
 <3cb5a2e5-4e4c-728a-252d-4757b6c9612d@virtuozzo.com>
 <8996db63-5554-d3dc-cd36-94570ade6d18@gmail.com>
 <20210707094218.0e9b6ffc@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
 <1cbf3c7b-455e-f3a5-cc2c-c18ce8be4ce1@gmail.com>
 <20210707113027.4077e544@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <0f2c9281-a5fa-f129-ffef-7add5b1d7d02@gmail.com>
Date:   Wed, 7 Jul 2021 20:50:22 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.11.0
MIME-Version: 1.0
In-Reply-To: <20210707113027.4077e544@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk


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")