netdev - Re: [PATCH net-next v3 RFC] skb_expand_head() adjust skb->truesize incorrectly

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Date:   Wed, 1 Sep 2021 09:20:27 +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>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>
Subject: Re: [PATCH net-next v3 RFC] skb_expand_head() adjust skb->truesize
 incorrectly

On 8/31/21 10:38 PM, Eric Dumazet wrote:
> On 8/31/21 7:34 AM, Vasily Averin wrote:
>> RFC because it have an extra changes:
>> new is_skb_wmem() helper can be called
>>  - either before pskb_expand_head(), to create skb clones 
>>     for skb with destructors that does not change sk->sk_wmem_alloc
>>  - or after pskb_expand_head(), to change owner in skb_set_owner_w()
>>
>> In current patch I've added both these ways,
>> we need to keep one of them.

If nobody object I vote for 2nd way:

>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>> index f931176..3ce33f2 100644
>> --- a/net/core/skbuff.c
>> +++ b/net/core/skbuff.c
>> @@ -1804,30 +1804,47 @@ struct sk_buff *skb_realloc_headroom(struct sk_buff *skb, unsigned int headroom)
>>  struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom)
>>  {
... skipped ...
>> -	return skb;
>> +	if (oskb) {
>> +		if (sk)
>> +			skb_set_owner_w(skb, sk);
> 
> Broken for non full sockets.
> Calling skb_set_owner_w(skb, sk) for them is a bug.

I think you're wrong here.
It is 100% equivalent of old code, 
skb_set_owner_w() handles sk_fullsock(sk) inside and does not adjust sk->sk_wmem_alloc.
Please explain if I'm wrong.

>> +		consume_skb(oskb);
>> +	} else if (sk) {
>> +		delta = osize - skb_end_offset(skb);
>> +		if (!is_skb_wmem(skb))
>> +			skb_set_owner_w(skb, sk);
> 
> This would be broken for non full sockets.
> Calling skb_set_owner_w(skb, sk) for them is a bug.
See my comment above.

>> +		skb->truesize += delta;
>> +		if (sk_fullsock(sk))
>> +			refcount_add(delta, &sk->sk_wmem_alloc);
> 
> 
>> +	}	return skb;
Strange line, will fix it.

>>  }
>>  EXPORT_SYMBOL(skb_expand_head);