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 net-next v4] skb_expand_head() adjust skb->truesize
 incorrectly
To:     Vasily Averin <vvs@virtuozzo.com>,
        Christoph Paasch <christoph.paasch@gmail.com>,
        "David S. Miller" <davem@davemloft.net>
Cc:     Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>,
        Jakub Kicinski <kuba@kernel.org>,
        netdev <netdev@vger.kernel.org>, linux-kernel@vger.kernel.org,
        kernel@openvz.org, Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        Julian Wiedmann <jwi@linux.ibm.com>
References: <b653692b-1550-e17a-6c51-894832c56065@virtuozzo.com>
 <ee5b763a-c39d-80fd-3dd4-bca159b5f5ac@virtuozzo.com>
 <ce783b33-c81f-4760-1f9e-90b7d8c51fd7@gmail.com>
 <b7c2cb05-7307-f04e-530e-89fc466aa83f@virtuozzo.com>
 <ef7ccff8-700b-79c2-9a82-199b9ed3d95b@gmail.com>
From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <67740366-7f1b-c953-dfe1-d2085297bdf3@gmail.com>
Date:   Wed, 1 Sep 2021 21:48:29 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.12.0
MIME-Version: 1.0
In-Reply-To: <ef7ccff8-700b-79c2-9a82-199b9ed3d95b@gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk


On 9/1/21 9:32 PM, Eric Dumazet wrote:

> 
> I think you missed netem case, in particular
> skb_orphan_partial() which I already pointed out.
> 
> You can setup a stack of virtual devices (tunnels),
> with a qdisc on them, before ip6_xmit() is finally called...
> 
> Socket might have been closed already.
> 
> To test your patch, you could force a skb_orphan_partial() at the beginning
> of skb_expand_head() (extending code coverage)
> 

To clarify :

It is ok to 'downgrade' an skb->destructor having a ref on sk->sk_wmem_alloc to
something owning a ref on sk->refcnt.

But the opposite operation (ref on sk->sk_refcnt -->  ref on sk->sk_wmem_alloc) is not safe.