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

On Sat, Oct 6, 2012 at 9:43 AM, pchavent [off-list ref] wrote:

On Fri, 5 Oct 2012 21:37:58 +0200, Daniel Borkmann wrote:

quoted

On Fri, Oct 5, 2012 at 9:21 PM, pchavent [off-list ref] wrote:

quoted

On Fri, 5 Oct 2012 16:17:12 +0200, Daniel Borkmann wrote:

quoted

On Fri, Oct 5, 2012 at 3:10 PM, Paul Chavent [off-list ref]
wrote:

quoted

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 <redacted>

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

Yes, i also expect to be able to include the link level header when i use
SOCK_RAW.

My intention is to send a frame with this payload (for example) :
typedef struct
{
  double   ts;
  uint64_t foo;
} test_t;

So i get a pointer to the raw packet :
void * raw_packet = frame_base + (TPACKET_HDRLEN - sizeof(struct
sockaddr_ll));

I cook the header :
memcpy(raw_packet +  0, dst_addr, sizeof(dst_addr));
memcpy(raw_packet +  6, src_addr, sizeof(src_addr));
memcpy(raw_packet + 12, type    , sizeof(type));

Then i get a pointer to the beginning of payload :
test_t * payload = raw_packet + 14;

Here payload is at 58 bytes from the beginning of the frame.

Then i fill the payload :
payload->ts = 1.0;
payload->foo = 2;
...

These are misaligned accesses.

I don't care to fill the cooked header if it's misaligned, but i would like
to be able to fill the frame directly in the ring buffer being on aligned
boundaries.

Okay.

Maybe what you could do in a new version of your patch is to introduce
a TP_STATUS flag, e.g. TP_STATUS_SEND_HAS_OFF that you pass along
binary or'ed with the commonly used flags, and then you can fill
tp_mac resp. tp_net with offsets. By that, you won't break legacy
stuff.