CONFIG_FILTER error on arm

[Bjørn Mork]

Mukund Deshpande <sai.mukund.sagar at gmail.com> writes:

> I added extra message for each of the cases where this error was occuring.
> It has failed at bind call..

That's interesting.

> Any suggestions how to find if there is any problem in ifindex detection?

I see now that I was a bit too quick as usual when thinking about what
could fail, looking only briefly at the lines before the bind() call and
assuming that the code must be correct on most systems. 

Looking at it more in detail, I now believe this code is utterly wrong
on *any* system....

Quoting from packet(7):

</quote>
NAME
       packet, AF_PACKET - packet interface on device level.

SYNOPSIS
       #include <sys/socket.h>
       #include <netpacket/packet.h>
       #include <net/ethernet.h> /* the L2 protocols */

       packet_socket = socket(AF_PACKET, int socket_type, int protocol);

DESCRIPTION
       Packet sockets are used to receive or send raw packets at the device driver (OSI Layer 2) level.
       They allow the user to implement protocol modules in user space on top of the physical layer.

       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.  The link level header information is
       available in a common format in a sockaddr_ll.  protocol is the IEEE 802.3  protocol  number  in
       network  order.   See the <linux/if_ether.h> include file for a list of allowed protocols.  When
       protocol is set to htons(ETH_P_ALL) then all protocols are received.  All  incoming  packets  of
       that  protocol  type will be passed to the packet socket before they are passed to the protocols
       implemented in the kernel.

       Only processes with effective UID 0 or the CAP_NET_RAW capability may open packet sockets.

       SOCK_RAW packets are passed to and from the device driver without  any  changes  in  the  packet
       data.  When receiving a packet, the address is still parsed and passed in a standard sockaddr_ll
       address structure.  When transmitting a packet, the user  supplied  buffer  should  contain  the
       physical  layer  header.   That  packet  is  then queued unmodified to the network driver of the
       interface defined by the destination address.  Some device drivers  always  add  other  headers.
       SOCK_RAW is similar to but not compatible with the obsolete AF_INET/SOCK_PACKET of Linux 2.0.

       SOCK_DGRAM  operates  on  a  slightly  higher  level.  The physical header is removed before the
       packet is passed to the user.  Packets sent through a SOCK_DGRAM packet socket  get  a  suitable
       physical  layer  header  based  on the information in the sockaddr_ll destination address before
       they are queued.

       By default all packets of the specified protocol type are passed to a packet  socket.   To  only
       get  packets from a specific interface use bind(2) specifying an address in a struct sockaddr_ll
       to bind the packet socket to an interface.  Only the sll_protocol and  the  sll_ifindex  address
       fields are used for purposes of binding.
</quote>


So you can't really do socket(PF_PACKET, SOCK_PACKET, ...) and expect it
to work.  And the bind() may be correct for the obsolete SOCK_PACKET
sockets, but I don't think it will work correctly with AF_PACKET
sockets. They need to have sll_protocol and sll_ifindex set, and not the
ifreq structure in info->ifp.

I have a feeling I must be missing something here, as this code
obviously does work for most of us.  Maybe the bind() call does in fact
work, but the interface name in info->ifp is wrong somehow?

Maybe someone else has a comment?



Bjørn