Re: [PATCH] packet: Deliver VLAN TPID to userspace

[Atzm Watanabe <atzm@stratosphere.co.jp>]

At Fri, 18 Oct 2013 10:56:55 -0700,
Stephen Hemminger wrote:
> 
> On Sat, 19 Oct 2013 02:08:11 +0900
> Atzm Watanabe <atzm@stratosphere.co.jp> wrote:
> 
> > diff --git a/include/uapi/linux/if_packet.h b/include/uapi/linux/if_packet.h
> > index dbf0666..6e36e0a 100644
> > --- a/include/uapi/linux/if_packet.h
> > +++ b/include/uapi/linux/if_packet.h
> > @@ -83,7 +83,7 @@ struct tpacket_auxdata {
> >  	__u16		tp_mac;
> >  	__u16		tp_net;
> >  	__u16		tp_vlan_tci;
> > -	__u16		tp_padding;
> > +	__u16		tp_vlan_tpid;
> >  };
> >  
> >  /* Rx ring - header status */
> > @@ -132,12 +132,13 @@ struct tpacket2_hdr {
> >  	__u32		tp_sec;
> >  	__u32		tp_nsec;
> >  	__u16		tp_vlan_tci;
> > -	__u16		tp_padding;
> > +	__u16		tp_vlan_tpid;
> >  };
> >  
> >  struct tpacket_hdr_variant1 {
> >  	__u32	tp_rxhash;
> >  	__u32	tp_vlan_tci;
> > +	__u32	tp_vlan_tpid;
> >  };
> 
> The last change will break ABI to userspace applications.
> You can reuse padding elements; but you can't increase (or shrink)
> an existing structure.

Thank you for pointing.
But I have two questions:

  - The patch that increases existing structures was posted and
    accepted in the past (e.g 393e52e33c6c26ec7db290dab803bac1bed962d4
    "packet: deliver VLAN TCI to userspace").
    What is the difference between them and my patch?

  - I tested using tools/testing/selftests/net/psock_tpacket.c built
    before applying my patch, and all test cases were passed.
    Also I tested by the code that was listed in
    Documentation/networking/packet_mmap.txt "AF_PACKET TPACKET_V3
    example".  It seems that problem was not caused.
    What situation causes the problem that you assumed?

Thank you.