netdev - Re: [PATCH] net: add missing skb_uarg refcount increment in pskb_carve_inside

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Date:   Fri, 14 Aug 2020 15:45:58 +0200
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     linmiaohe <linmiaohe@...wei.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Florian Westphal <fw@...len.de>,
        "martin.varghese@...ia.com" <martin.varghese@...ia.com>,
        "pshelar@....org" <pshelar@....org>,
        "dcaratti@...hat.com" <dcaratti@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Shmulik Ladkani <shmulik@...anetworks.com>,
        Yadu Kishore <kyk.segfault@...il.com>,
        "sowmini.varadhan@...cle.com" <sowmini.varadhan@...cle.com>,
        Network Development <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: add missing skb_uarg refcount increment in pskb_carve_inside_header()

On Fri, Aug 14, 2020 at 9:20 AM linmiaohe <linmiaohe@...wei.com> wrote:
>
> Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:
> >On Thu, Aug 13, 2020 at 2:16 PM Miaohe Lin <linmiaohe@...wei.com> wrote:
> >>
> >> If the skb is zcopied, we should increase the skb_uarg refcount before
> >> we involve skb_release_data(). See pskb_expand_head() as a reference.
> >
> >Did you manage to observe a bug through this datapath in practice?
> >
> >pskb_carve_inside_header is called
> >  from pskb_carve
> >    from pskb_extract
> >      from rds_tcp_data_recv
> >
> >That receive path should not see any packets with zerocopy state associated.
> >
>
> This works fine yet as its caller is limited. But we should take care of the skb_uarg refcount for future use.

If a new application of this interface is proposed, the author will
have to make sure that it is exercised correctly.

> On the other hand, because this codepath should not see any packets with zerocopy state associated, then we
> should not call skb_orphan_frags here.

I'm also not convinced that the skb_orphan_frags here are needed,
given the only path is from tcp_read_sock.