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

[Vasily Averin]

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?

Thank you,
	Vasily Averin