linux-kernel - Re: [PATCH v2] skb_expand_head() adjust skb->truesize incorrectly

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Message-ID: <c4373bb7-bb4f-2895-c692-e61a1a89e21f@virtuozzo.com>
Date:   Mon, 30 Aug 2021 21:09:08 +0300
From:   Vasily Averin <vvs@...tuozzo.com>
To:     Eric Dumazet <eric.dumazet@...il.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, Julian Wiedmann <jwi@...ux.ibm.com>
Subject: Re: [PATCH v2] skb_expand_head() adjust skb->truesize incorrectly

On 8/30/21 7:01 PM, Eric Dumazet wrote:
> On 8/29/21 5:59 AM, Vasily Averin wrote:
>> Christoph Paasch reports [1] about incorrect skb->truesize
>> after skb_expand_head() call in ip6_xmit.
>> This may happen because of two reasons:
>> - skb_set_owner_w() for newly cloned skb is called too early,
>> before pskb_expand_head() where truesize is adjusted for (!skb-sk) case.
>> - pskb_expand_head() does not adjust truesize in (skb->sk) case.
>> In this case sk->sk_wmem_alloc should be adjusted too.
>>
>> [1] https://lkml.org/lkml/2021/8/20/1082
>> @@ -1756,9 +1756,13 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
>>  	 * For the moment, we really care of rx path, or
>>  	 * when skb is orphaned (not attached to a socket).
>>  	 */
>> -	if (!skb->sk || skb->destructor == sock_edemux)
>> -		skb->truesize += size - osize;
>> -
>> +	delta = size - osize;
>> +	if (!skb->sk || skb->destructor == sock_edemux) {
>> +		skb->truesize += delta;
>> +	} else if (update_truesize) {
> 
> Unfortunately we can not always do this sk_wmem_alloc change here.
> 
> Some skb have skb->sk set, but the 'reference on socket' is not through sk_wmem_alloc

Could you please provide some example?
In past in all handeled cases we have cloned original skb and then unconditionally assigned skb sock_wfree destructor.
Do you want to say that it worked correctly somehow?

I expected if we set sock_wfree, we have guarantee that old skb adjusted sk_wmem_alloc.
Am I wrong?
Could you please point on such case?

> It seems you need a helper to make sure skb->destructor is one of
> the destructors that use skb->truesize and sk->sk_wmem_alloc
> 
> For instance, skb_orphan_partial() could have been used.

Thank you, will investigate.
	Vasily Averin