netdev - Re: [PATCH net-next v2] mptcp: be careful on MPTCP-level ack.

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Date:   Wed, 02 Dec 2020 16:37:21 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org
Cc:     Jakub Kicinski <kuba@...nel.org>, mptcp@...ts.01.org
Subject: Re: [PATCH net-next v2] mptcp: be careful on MPTCP-level ack.

On Wed, 2020-12-02 at 14:18 +0100, Eric Dumazet wrote:
> 
> On 11/24/20 10:51 PM, Paolo Abeni wrote:
> > We can enter the main mptcp_recvmsg() loop even when
> > no subflows are connected. As note by Eric, that would
> > result in a divide by zero oops on ack generation.
> > 
> > Address the issue by checking the subflow status before
> > sending the ack.
> > 
> > Additionally protect mptcp_recvmsg() against invocation
> > with weird socket states.
> > 
> > v1 -> v2:
> >  - removed unneeded inline keyword - Jakub
> > 
> > Reported-and-suggested-by: Eric Dumazet <eric.dumazet@...il.com>
> > Fixes: ea4ca586b16f ("mptcp: refine MPTCP-level ack scheduling")
> > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> > ---
> >  net/mptcp/protocol.c | 67 ++++++++++++++++++++++++++++++++------------
> >  1 file changed, 49 insertions(+), 18 deletions(-)
> > 
> 
> Looking at mptcp recvmsg(), it seems that a read(fd, ..., 0) will
> trigger an infinite loop if there is available data in receive queue ?

Thank you for looking into this!

I can't reproduce the issue with the following packetdrill ?!?

+0.0  connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
+0.1   > S 0:0(0) <mss 1460,sackOK,TS val 100 ecr 0,nop,wscale 8,mpcapable v1 fflags[flag_h] nokey>
+0.1   < S. 0:0(0) ack 1 win 65535 <mss 1460,sackOK,TS val 700 ecr 100,nop,wscaale 8,mpcapable v1 flags[flag_h] key[skey=2] >
+0.1  > . 1:1(0) ack 1 <nop, nop, TS val 100 ecr 700,mpcapable v1 flags[flag_h]] key[ckey,skey]>
+0.1 fcntl(3, F_SETFL, O_RDWR) = 0
+0.1   < .  1:201(200) ack 1 win 225 <dss dack8=1 dsn8=1 ssn=1 dll=200 nocs,  nop, nop>
+0.1   > .  1:1(0) ack 201 <nop, nop, TS val 100 ecr 700, dss dack8=201 dll=00 nocs>
+0.1 read(3, ..., 0) = 0

The main recvmsg() loop is interrupted by the following check:

                if (copied >= target)
                        break;

I guess we could loop while the msk has available rcv space and some
subflow is feeding new data. If so, I think moving:

	if (skb_queue_empty(&msk->receive_queue) &&
                    __mptcp_move_skbs(msk, len - copied))
                        continue;

after the above check should address the issue, and will make the
common case faster. Let me test the above - unless I underlooked
something relevant!

Thanks,

Paolo