netdev - Re: getsockopt(TCP_DEFER

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Date:	Tue, 05 Jan 2010 16:45:57 +0100
From:	Daniel Lezcano <daniel.lezcano@...e.fr>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	Julian Anastasov <ja@....bg>,
	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: getsockopt(TCP_DEFER_ACCEPT) value change

Eric Dumazet wrote:
> Le 05/01/2010 14:40, Ilpo Järvinen a écrit :
>   
>> On Tue, 5 Jan 2010, Daniel Lezcano wrote:
>>
>>     
>>> I noticed a change in the value returned by the getsockopt for the
>>> TCP_DEFER_ACCEPT option with a 2.6.32 kernel. The value retrieved with the
>>> getsockopt is different from the one specified with the setsockopt. Is it an
>>> expected behaviour ?
>>>
>>> I saw there were  changes around the TCP_DEFER_ACCEPT option with the number
>>> of attempts converted to a number of seconds.
>>>
>>> The following program is working fine with a 2.6.31 but fails with a 2.6.32
>>> kernel.
>>>
>>> Thanks
>>>  -- Daniel
>>>
>>> #include <stdio.h>
>>> #include <sys/socket.h>
>>> #include <netinet/in.h>
>>> #include <netinet/tcp.h>
>>>
>>> int main(int argc, char *argv[])
>>> {
>>>    int val1 = 12, val2;
>>>    socklen_t len = sizeof(val2);
>>>    int fd;
>>>      fd = socket(PF_INET, SOCK_STREAM, 0);
>>>    if (fd < 0) {
>>>        perror("socket");
>>>        return -1;
>>>    }
>>>      if (setsockopt(fd, SOL_TCP, TCP_DEFER_ACCEPT, &val1, sizeof(val1))) {
>>>        perror("setsockopt");
>>>        return -1;
>>>    }
>>>      if (getsockopt(fd, SOL_TCP, TCP_DEFER_ACCEPT, &val2, &len)) {
>>>        perror("getsockopt");
>>>        return -1;
>>>    }
>>>
>>>    if (val1 != val2) {
>>>        fprintf(stderr, "error %d != %d\n", val1, val2);
>>>        return -1;
>>>    }
>>>
>>>    return 0;
>>> }
>>>       
>> Added Cc.
>>
>> I don't think this change was intentional. ...However, is this difference 
>> particularly significant besides failing such a test program? The actual 
>> value now returned by the getsockopt is more accurate than what the
>> userspace initially provided.
>>
>> In general, I wonder if there's something that mandates that a set/get 
>> pair of value should be equal?
>>
>>     
>
> Nothing... really... we can round the value, and we indeed round it in 2.6.32
>
> defer value is given in second by user, and converted to number of retransmits by kernel.
>
> Program assumption is wrong.
>   
It's not problem if the set / get values are not same, but I was asking 
because I am working with a test suite checking if a checkpoint / 
restart solution is correct.  One of these tests, sets the 
TCP_DEFER_ACCEPT value to 12, checkpoints / restarts, and reads the 
value in order to check if it was correctly restored. The value 12 was 
chosen because it is not rounded, so we were able to safely do the test. 
But with the 2.6.32, the behaviour changed, so I preferred to report it 
in case that is something not expected.


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