cvs commit: src/sys/dev/usb if_ural.c

[Robert Watson]

On Wed, 26 Jul 2006, Sam Leffler wrote:

> sam         2006-07-26 03:30:50 UTC
>
>  FreeBSD src repository
>
>  Modified files:
>    sys/dev/usb          if_ural.c
>  Log:
>  support for 802.11 packet injection via bpf
>
>  Reviewed by:    arch@
>  MFC after:      1 month

---------------------------------------
@@ -209,6 +211,9 @@
          */
         if (ic->ic_reset == NULL)
                 ic->ic_reset = ieee80211_default_reset;
+
+       KASSERT(ifp->if_spare2 == NULL, ("oops, hosed"));
+       ifp->if_spare2 = ic;                    /* XXX temp backpointer */
  }
---------------------------------------

Please don't use spare pointers without properly allocating them (i.e., 
renaming them).  I ran into this this morning when merging these changes into 
the rwatson_ifnet branch, where if_spare2 was renamed to if_startmbuf. 
However, if the above change is MFC'd as is, people may well find the problem 
through run-time corruption when the network stack tries to execute the ic 
contents thinking that it's a non-NULL if_startmbuf function pointer, which is 
a lot harder to debug.  The point of spare fields is that they be spare -- if 
they are no longer spare, they should not be left marked as spare, or they 
will get reused with unfortunate ABI consequences.  I don't mind if this spare 
is allocated (if_softc2, if_driver, or the like), as we have one more spare 
pointer I can use in 6.x, but the current use of the spare field is 
problematic and should not be MFC'd unless cleaned up.

Robert N M Watson
Computer Laboratory
University of Cambridge