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

[Eric Dumazet]

On 8/30/21 11:09 AM, Vasily Averin wrote:
> 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.

In the past we ignored old value of skb->destructor,
since the clone got a NULL destructor.

In your patch you assumes it is sock_wfree, or other destructors changing sk_wmem_alloc


You need to make sure skb->destructor is one of the known destructors which 
will basically remove skb->truesize from sk->sk_wmem_alloc.

This will also make sure skb->sk is a 'full socket'

If not, you should not change sk->sk_wmem_alloc

> Do you want to say that it worked correctly somehow?

I am simply saying your patch adds a wrong assumption.

> 
> 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
>