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

[Vasily Averin]

On 9/2/21 10:13 AM, Vasily Averin wrote:
> On 9/2/21 7:48 AM, Eric Dumazet wrote:
>> 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.
> 
> Could you please explain in more details, since I stil have a completely opposite point of view?
> 
> Every sk referenced in skb have sk_wmem_alloc > 9 
> It is assigned to 1 in sk_alloc and decremented right before last __sk_free(),
> inside  both sk_free() sock_wfree() and __sock_wfree()
> 
> So it is safe to adjust skb->sk->sk_wmem_alloc, 
> because alive skb keeps reference to alive sk and last one keeps sk_wmem_alloc > 0
> 
> So any destructor used sk->sk_refcnt will already have sk_wmem_alloc > 0, 
> because last sock_put() calls sk_free().
> 
> However now I'm not sure in reversed direction.
> skb_set_owner_w() check !sk_fullsock(sk) and call sock_hold(sk);
> If sk->sk_refcnt can be 0 here (i.e. after execution of old destructor inside skb_orphan) 
> -- it can be trigger pointed problem:
> "refcount_add() will trigger a warning (panic under KASAN)".
> 
> Could you please explain where I'm wrong?

To clarify:
I'm agree it is unsafe  to call on alive skb:
skb_orphan(skb)
adjust(skb_>sk->sk_wmem_alloc)

becasue 2 reasone:
1) old destructor can decrease sk_vmem_alloc to zero and free sk
2) becasue old destructor if !sk_fullsock(sk) can call sock_out and release last sk->sk_refcnt reference.
  in this case sock_hold() will trigger warning.

1) can be handled, we can adjust(sk_wmem_alloc) before skb_orphan()
but I badly understand how to handle 2nd case.

Thank you,
	Vasily Averin