netdev - Re: [PATCH net-next v4] skb_expand_head() adjust skb->truesize incorrectly

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Date:   Wed, 1 Sep 2021 21:48:29 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Vasily Averin <vvs@...tuozzo.com>,
        Christoph Paasch <christoph.paasch@...il.com>,
        "David S. Miller" <davem@...emloft.net>
Cc:     Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>, linux-kernel@...r.kernel.org,
        kernel@...nvz.org, Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Julian Wiedmann <jwi@...ux.ibm.com>
Subject: Re: [PATCH net-next v4] skb_expand_head() adjust skb->truesize
 incorrectly



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.