Re: [PATCH] net: correct zerocopy refcnt with newly allocated UDP or RAW uarg

[linmiaohe]

Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx> wrote:
>On Thu, Aug 13, 2020 at 1:59 PM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>>
>> The var extra_uref is introduced to pass the initial reference taken 
>> in sock_zerocopy_alloc to the first generated skb. But now we may fail 
>> to pass the initial reference with newly allocated UDP or RAW uarg 
>> when the skb is zcopied.
>
>extra_uref is true if there is no previous skb to append to or there is a previous skb, but that does not have zerocopy data associated yet (because the previous call(s) did not set MSG_ZEROCOPY).
>
>In other words, when first (allocating and) associating a zerocopy struct with the skb.

Many thanks for your explaination. The var extra_uref plays the role as you say. I just borrowed the description of var extra_uref from previous commit log here.

>
>> -               extra_uref = !skb_zcopy(skb);   /* only ref on new uarg */
>> +               /* Only ref on newly allocated uarg. */
>> +               if (!skb_zcopy(skb) || (sk->sk_type != SOCK_STREAM && skb_zcopy(skb) != uarg))
>> +                       extra_uref = true;
>
>SOCK_STREAM does not use __ip_append_data.
>
>This leaves as new branch skb_zcopy(skb) && skb_zcopy(skb) != uarg.
>
>This function can only acquire a uarg through sock_zerocopy_realloc, which on skb_zcopy(skb) only returns the existing uarg or NULL (for not SOCK_STREAM).
>
>So I don't see when that condition can happen.
>

On skb_zcopy(skb), we returns the existing uarg iff (uarg->id + uarg->len == atomic_read(&sk->sk_zckey)) in sock_zerocopy_realloc. So we may get a newly allocated
uarg via sock_zerocopy_alloc(). Though we may not trigger this codepath now, it's still a potential problem that we may missed the right trace to uarg.

Or am I still miss anything? Thanks.