netdev - Re: [PATCH net 2/4] tcp: tcp_fragment() should apply sane memory limits

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Date:   Thu, 11 Jul 2019 12:04:48 -0700
From:   "Jonathan Lemon" <jonathan.lemon@...il.com>
To:     "Eric Dumazet" <eric.dumazet@...il.com>
Cc:     "Prout, Andrew - LLSC - MITLL" <aprout@...mit.edu>,
        "Christoph Paasch" <christoph.paasch@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Jonathan Looney" <jtl@...flix.com>,
        "Neal Cardwell" <ncardwell@...gle.com>,
        "Tyler Hicks" <tyhicks@...onical.com>,
        "Yuchung Cheng" <ycheng@...gle.com>,
        "Bruce Curtis" <brucec@...flix.com>,
        "Dustin Marquess" <dmarquess@...le.com>
Subject: Re: [PATCH net 2/4] tcp: tcp_fragment() should apply sane memory
 limits



On 11 Jul 2019, at 11:28, Eric Dumazet wrote:

> On 7/11/19 7:14 PM, Prout, Andrew - LLSC - MITLL wrote:
>>
>> In my opinion, if a small SO_SNDBUF below a certain value is no 
>> longer supported, then SOCK_MIN_SNDBUF should be adjusted to reflect 
>> this. The RCVBUF/SNDBUF sizes are supposed to be hints, no error is 
>> returned if they are not honored. The kernel should continue to 
>> function regardless of what userspace requests for their values.
>>
>
> It is supported to set whatever SO_SNDBUF value and get terrible 
> performance.
>
> It always has been.
>
> The only difference is that we no longer allow an attacker to fool TCP 
> stack
> and consume up to 2 GB per socket while SO_SNDBUF was set to 128 KB.
>
> The side effect is that in some cases, the workload can appear to have 
> the signature of the attack.
>
> The solution is to increase your SO_SNDBUF, or even better let TCP 
> stack autotune it.
> nobody forced you to set very small values for it.

I discovered we have some production services that set SO_SNDBUF to
very small values (~4k), as they are essentially doing interactive
communications, not bulk transfers.  But there's a difference between
"terrible performance" and "TCP stops working".
-- 
Jonathan