Subject: Re: Single socket with TX_RING and RX_RING

From: Daniel Borkmann <[email-masked]>
Date: Wed, 15 May 2013 16:47:42 +0200

On 05/15/2013 03:32 PM, Ricardo Tubío wrote:
> Daniel Borkmann <dborkman <at> redhat.com> writes:
>> On 05/15/2013 02:53 PM, Ricardo Tubío wrote:
>>> Once I tell kernel to export the TX_RING through setsockopt() (see code
>>> below) I always get an error (EBUSY) if i try to tell kernel to export the
>>> RX_RING with the same socket descriptor. Therefore, I have to open an
>>> additional socket for the RX_RING and I require of two sockets when I though
>>> that I would only require of one socket for both TX and RX using mmap()ed
>>> memory.
>>>
>>> Do I need both sockets or am I doing something wrong?
>>
>> The second time you call init_ring() in your code e.g. with TX_RING, where
>> you have previously set it up for the RX_RING. The kernel will give you
>> -EBUSY because the packet socket is already mmap(2)'ed.

(if you need an answer, then please do not drop the CC, otherwise it could be
  that I might not read it)

> Ok, so if I make the following system calls:
>
> void *ring=NULL;
> setsockopt(socket_fd, SOL_PACKET, PACKET_RX_RING, p, LEN__TPACKET_REQ);
> ring = mmap(NULL, ring_len, ring_access_flags, MAP_SHARED, socket_fd, 0);
>
> Would I be permitted to use the ring map obtained both for RX and for TX? If
> so, for me it is confusing to use PACKET_RX_RING if I can also TX data
> through that ring...

I haven't tried it out yet, and currently also do not really have time to. But
looking at the mmap code, it seems that the size of the mmap area is accumulated
for rx and tx ring. However, the header status bits are not really interoperable
with each other. So looks you will need to have two sockets ...