Re: [patch] packet.7: PACKET_LOSS has inverse meaning

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 04/22/2014 08:36 PM, Willem de Bruijn wrote:
> On Tue, Apr 22, 2014 at 2:23 PM, Michael Kerrisk (man-pages)
> <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>> Hi Daniel, Willem,
>>
>>
>> Daniel, your patch (dbb4f7516b) added the text on PACKET_LOSS.
>> Could you comment on the patch below please?
> 
> Carsten is correct. I made a mistake in that explanation, accidentally
> inverting the branch. My apologies and thanks for following up with a
> replacement text, Carsten. That looks good to me.

@Willem: thanks for the quick response. That helps so much.

@Carsten: I've applied your patch, but I have a question. You wrote:

[[
However if
.BR PACKET_LOSS
is set, malformed packets will be skipped, their status reset to
.BR TP_STATUS_AVAILABLE
]]

What does "their" in "their status" refer to? 

Cheers,

Michael



>> On 04/22/2014 06:01 PM, Carsten Andrich wrote:
>>> Hey there,
>>>
>>> I stumbled upon an error in packet.7 regarding the meaning of the
>>> PACKET_LOSS socket option. According to the current git version of
>>> linux-man setting PACKET_LOSS causes malformed packets to *not* be
>>> silently dropped.
>>>
>>> However it is the other way round. If PACKET_LOSS is *not* set,
>>> malformed packets will be marked TP_STATUS_WRONG_FORMAT and the
>>> transmission process aborted, leaving untransmitted packets in the ring.
>>> If it *is* set, malformed packets will be silently skipped, their status
>>> set to TP_STATUS_AVAILABLE and the transmission process continued with
>>> the following packet.
>>>
>>> This behaviour can be clearly seen in net/packet/af_packet.c:
>>> http://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/net/packet/af_packet.c#n2300
>>>
>>> The value accompanying TP_PACKET_LOSS translates into po->tp_loss:
>>> http://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/net/packet/af_packet.c#n3304
>>>
>>> I inverted the meaning of PACKET_LOSS and clarified the description in
>>> the attached patch.
>>
>> diff --git a/man7/packet.7 b/man7/packet.7
>> index d8257f9..5085814 100644
>> --- a/man7/packet.7
>> +++ b/man7/packet.7
>> @@ -319,9 +319,15 @@ original fanout algorithm selects a backlogged socket, the packet
>>  rolls over to the next available one.
>>  .TP
>>  .BR PACKET_LOSS " (with " PACKET_TX_RING )
>> -If set, do not silently drop a packet on transmission error, but
>> -return it with status set to
>> -.BR TP_STATUS_WRONG_FORMAT .
>> +When a malformed packet is encountered on a transmit ring, the default is to
>> +set its status to
>> +.BR TP_STATUS_WRONG_FORMAT
>> +and abort the transmission immediately (it and following packets are left
>> +lingering on the ring). However if
>> +.BR PACKET_LOSS
>> +is set, malformed packets will be skipped, their status reset to
>> +.BR TP_STATUS_AVAILABLE
>> +and the transmission process continued.
>>  .TP
>>  .BR PACKET_RESERVE " (with " PACKET_RX_RING )
>>  By default, a packet receive ring writes packets immediately following the
>> @@ -419,7 +425,7 @@ or
>>  then that overrides the socket default.
>>  On successful transmission, the socket resets the slot to
>>  .BR TP_STATUS_AVAILABLE .
>> -It discards packets silently on error unless
>> +It immediately aborts the transmission on error unless
>>  .BR PACKET_LOSS
>>  is set.
>>  .TP
>>
>>
>> --
>> Michael Kerrisk
>> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
>> Linux/UNIX System Programming Training: http://man7.org/training/
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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