netdev - Re: [PATCH net-next v2 1/2] udp: msg

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Message-ID: <CAF=yD-J-QP_zUFY9W7CMdjOPgAwkHGgSfVQfVb=EKTNOZB4P8Q@mail.gmail.com>
Date:   Mon, 26 Nov 2018 12:59:21 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next v2 1/2] udp: msg_zerocopy

On Mon, Nov 26, 2018 at 11:32 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> Hi,
>
> Sorry for the long delay...
>
> On Mon, 2018-11-26 at 10:29 -0500, Willem de Bruijn wrote:
> > @@ -1109,6 +1128,7 @@ static int __ip_append_data(struct sock *sk,
> >  error_efault:
> >       err = -EFAULT;
> >  error:
> > +     sock_zerocopy_put_abort(uarg);
> >       cork->length -= length;
> >       IP_INC_STATS(sock_net(sk), IPSTATS_MIB_OUTDISCARDS);
> >       refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc);
>
> Out of sheer ignorance on my side, don't we have a bad reference
> accounting if e.g.:
>
> - uarg is attached to multiple skbs, each holding a ref,
> - there is a failure on 'getfrag()'
>
> Such failure will release 2 references (1 kfree_skb(), and another in
> the above sock_zerocopy_put_abort(), as the count is still positive).

Thanks Paolo. Indeed, I had not anticipated the case of partial failure,
where more than one skb is allocated (or converted to zerocopy) and
only the last one is freed on error inside __ip_append_data.

The callers of this function do flush the queue of the other skbs on
error, but only after the call to sock_zerocopy_put_abort.

sock_zerocopy_put_abort depends on total rollback to revert the
sk_zckey increment and suppress the completion notification (which
must not happen on return with error).

I don't immediately have a fix. Need to think about this some more..