Re: status in PACKET_RX_RING is actually a bit mask

[Daniel Borkmann <dborkman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

Hi Michael et al,

On 04/24/2014 09:42 AM, Michael Kerrisk (man-pages) wrote:
> Willem, Daniel, Carsten, Neil,
>
> All of you have submitted patches in the recent past for packet.7, and
> are presumably somewhat knowledgeable. Could one or more of your Ack
> (or, if needed, improve) Stefan's proposed change?

Sorry for replying so late as I was just way to busy with other stuff.
Yes, I agree with you and Carsten that the man-page still need more
improvements, it's not perfect, and I'm glad that since a long time
Willem chipped in to at least bring the man page a bit more up to date.

I tried to update ...

  - http://lingrok.org/xref/linux-net-next/Documentation/networking/packet_mmap.txt
  - http://lingrok.org/xref/linux-net-next/Documentation/networking/filter.txt

... over time, both are somewhat tightly coupled as AF_PACKET users
heavily deal with BPF filters as well; so that at least it's a bit more
clear for other [kernel] developers as even the kernel documentation was
very terse for many years and without examples.

Now to your question. It can easily be seen from the if_packet.h header
file http://lingrok.org/xref/linux-net-next/include/uapi/linux/if_packet.h#93
that TP_STATUS_* are individual bits that are set in tp_status field.

TP_STATUS_USER simply means a frame was received in the ring and is
ready for user space to be processed. If the field also indicates
TP_STATUS_LOSING then this means that there were packet drops in the
rx ring itself so a user knows it didn't get all traffic. This bit
is being reset on getsockopt() when querying PACKET_STATISTICS,
otherwise it stays. Drops occur when the ring buffer was not emptied
fast enough by user space (so no free slot with TP_STATUS_KERNEL), e.g.
due to high incoming traffic load. However, the current frame you're
reading that has TP_STATUS_USER|TP_STATUS_LOSING is fine by itself.

Hope that clarifies it?

Thanks,

Daniel

> Thanks,
>
> Michael
>
>
> On Tue, Feb 25, 2014 at 10:24 PM, Stefan Puiu <stefan.puiu-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>> [re-sending since the original message was bounced back, something
>> about HTML subpart]
>>
>> Hi,
>>
>> I'm playing with the PACKET_RX_RING option to packet sockets and I
>> noticed that the status field from the tpacket_hdr structures inside
>> the mmaped buffer is actually a bit mask. I'm testing with 4Gbps
>> traffic rates and sometimes I get status=5. I'm using Ubuntu 12.04
>> with a 3.8.0-29 kernel, and at least from the code (I got it using
>> "sudo apt-get source linux-image-$(uname -r)") it seems the 5 means
>> TP_STATUS_USER | TP_STATUS_LOSING. The 3.8.0 kernel code has this line
>> in tpacket_rcv():
>>
>>          /*
>>           * LOSING will be reported till you read the stats,
>>           * because it's COR - Clear On Read.
>>           * Anyways, moving it for V1/V2 only as V3 doesn't need this
>>           * at packet level.
>>           */
>>                  if (po->stats.tp_drops)
>>                          status |= TP_STATUS_LOSING;
>>
>> Also, in some source code example I found using google
>> (http://www.google.ro/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&ved=0CEIQFjAE&url=http%3A%2F%2Fstuff.mit.edu%2Fafs%2Fsipb%2Fcontrib%2Flinux%2Ftools%2Ftesting%2Fselftests%2Fnet%2Fpsock_fanout.c&ei=lM8MU5yhB-iK7Aan7IGgAQ&usg=AFQjCNFtZBHSWgzHZkV3LNfLOSSTq0pV4Q&sig2=razCT0kUDv2-BIm_8gTE5g&bvm=bv.61725948,d.ZGU),
>> the status is checked like this:
>>
>> while (header->tp_status & TP_STATUS_USER && count < RING_NUM_FRAMES) {
>> [...]
>>
>> I haven't yet figured out how TP_STATUS_LOSING works, but I think it
>> would be worthwhile to at least document that the status is a bit mask
>> and not a value - that's what the patch at the end does.
>>
>> Michael, maybe you can Cc somebody more knowledgeable on this to
>> confirm? IIRC there were some message exchanges on this on the list
>> recently...
>>
>>
>> diff --git a/man7/packet.7 b/man7/packet.7
>> index 1d3f222..6bac465 100644
>> --- a/man7/packet.7
>> +++ b/man7/packet.7
>> @@ -353,9 +353,9 @@ The packet socket owns all slots with status
>>   .BR TP_STATUS_KERNEL .
>>   After filling a slot, it changes the status of the slot to transfer
>>   ownership to the application.
>> -During normal operation, the new status is
>> -.BR TP_STATUS_USER ,
>> -to signal that a correctly received packet has been stored.
>> +During normal operation, the new status has the
>> +.BR TP_STATUS_USER
>> +bit set to signal that a correctly received packet has been stored.
>>   When the application has finished processing a packet, it transfers
>>   ownership of the slot back to the socket by setting the status to
>> --
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