Re: [PATCH] Packet mmap : allow the user to choose the offset of the tx payload.

[Daniel Borkmann <danborkmann@iogearbox.net>]

On Fri, Oct 5, 2012 at 9:21 PM, pchavent <Paul.Chavent@onera.fr> wrote:
> On Fri, 5 Oct 2012 16:17:12 +0200, Daniel Borkmann wrote:
>> On Fri, Oct 5, 2012 at 3:10 PM, Paul Chavent <Paul.Chavent@onera.fr>
>> wrote:
>>>
>>> The tx offset of packet mmap tx ring used to be :
>>> (TPACKET2_HDRLEN - sizeof(struct sockaddr_ll))
>>>
>>> The problem is that depending on the usage of SOCK_DGRAM or
>>> SOCK_RAW, the payload could be aligned or not.
>>>
>>> This patch allow to let the user give an offset for it's tx
>>> payload if he desires.
>>>
>>> Signed-off-by: Paul Chavent <paul.chavent@onera.fr>
>>
>> Can you provide an example when it doesn't hit TPACKET_ALIGNMENT?
>
> When we use tx ring, the user have to write at (TPACKET_HDRLEN -
> sizeof(struct sockaddr_ll))
>
> This adress is aligned on TPACKET_ALIGNMENT since
> TPACKET_HDRLEN = (TPACKET_ALIGN(sizeof(struct tpacket_hdr)) + sizeof(struct
> sockaddr_ll))
>
> When we use the tx ring with SOCK_RAW option, the mac header is aligned on
> TPACKET_ALIGNMENT, but not the payload (14 bytes away).

Okay, I'm confused about your intentions, maybe I'm missing something.
The man-page of packet(7) clearly says:

The socket_type is either SOCK_RAW for raw packets *including* the
link level header or SOCK_DGRAM for cooked packets with  the link
level header *removed*.

So this is perfectly intended behavior of PF_PACKET.

Cheers,

Daniel