netdev - Re: [MPTCP] Re: [PATCH net-next 09/11] tcp: Check for filled TCP option space before SACK

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Date:   Mon, 16 Dec 2019 08:55:34 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Mat Martineau <mathew.j.martineau@...ux.intel.com>,
        netdev@...r.kernel.org, mptcp@...ts.01.org
Subject: Re: [MPTCP] Re: [PATCH net-next 09/11] tcp: Check for filled TCP
 option space before SACK



On 12/16/19 4:52 AM, Paolo Abeni wrote:

> Thank you for the feedback!
> 
> Unfortunatelly, the above commit is not enough when MPTCP is enabled,
> as, without this patch, we can reach the following code:
> 
> 		const unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
> 		opts->num_sack_blocks =
> 			min_t(unsigned int, eff_sacks,
> 			      (remaining - TCPOLEN_SACK_BASE_ALIGNED) /
> 			      TCPOLEN_SACK_PERBLOCK);
> 
> with 'size == MAX_TCP_OPTION_SPACE' and num_sack_blocks will be
> miscalculated. So we need 'fix' but only for MPTCP/when MPTCP is
> enabled. Still ok for a -net commit?
> 

Does it means MPTCP flows can not use SACK at all ? That would be very bad.

What is the size of MPTCP options ?