netdev - Re: [PATCH net] tcp: add missing tcp_skb_can_collapse() test in tcp_shift_skb

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Date:   Wed, 02 Feb 2022 10:38:19 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Mat Martineau <mathew.j.martineau@...ux.intel.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Talal Ahmad <talalahmad@...gle.com>,
        Arjun Roy <arjunroy@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Davide Caratti <dcaratti@...hat.com>
Subject: Re: [PATCH net] tcp: add missing tcp_skb_can_collapse() test in
 tcp_shift_skb_data()

On Tue, 2022-02-01 at 12:01 -0800, Mat Martineau wrote:
> On Tue, 1 Feb 2022, Soheil Hassas Yeganeh wrote:
> 
> > On Tue, Feb 1, 2022 at 1:46 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
> > > 
> > > From: Eric Dumazet <edumazet@...gle.com>
> > > 
> > > tcp_shift_skb_data() might collapse three packets into a larger one.
> > > 
> > > P_A, P_B, P_C  -> P_ABC
> > > 
> > > Historically, it used a single tcp_skb_can_collapse_to(P_A) call,
> > > because it was enough.
> > > 
> > > In commit 85712484110d ("tcp: coalesce/collapse must respect MPTCP extensions"),
> > > this call was replaced by a call to tcp_skb_can_collapse(P_A, P_B)
> > > 
> > > But the now needed test over P_C has been missed.
> > > 
> > > This probably broke MPTCP.

Indeed it looks like it could cause MPTCP data stream corruption, in
case of multiple substreams, if we hit this code-path. Thanks for
catching and fixing it!

> > > Then later, commit 9b65b17db723 ("net: avoid double accounting for pure zerocopy skbs")
> > > added an extra condition to tcp_skb_can_collapse(), but the missing call
> > > from tcp_shift_skb_data() is also breaking TCP zerocopy, because P_A and P_C
> > > might have different skb_zcopy_pure() status.
> > > 
> > > Fixes: 85712484110d ("tcp: coalesce/collapse must respect MPTCP extensions")
> > > Fixes: 9b65b17db723 ("net: avoid double accounting for pure zerocopy skbs")
> > > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > > Cc: Paolo Abeni <pabeni@...hat.com>
> > > Cc: Mat Martineau <mathew.j.martineau@...ux.intel.com>
> > > Cc: Talal Ahmad <talalahmad@...gle.com>
> > > Cc: Arjun Roy <arjunroy@...gle.com>
> > > Cc: Soheil Hassas Yeganeh <soheil@...gle.com>
> > > Cc: Willem de Bruijn <willemb@...gle.com>
> > 
> > Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>

Acked-by: Paolo Abeni <pabeni@...hat.com>
> > 
> > I wish there were some packetdrill tests for MPTCP. Thank you for the fix!

Do you have by chance a drill for the zero-copy case? it may help
creating the MPTCP one, too.

Thanks!

Paolo