On Fri, 16 Aug 2002, Maxim Sobolev wrote:
> >
> > On Thu, 15 Aug 2002, Maxim Sobolev wrote:
> >
> > > When implementing ability to switch interface into promisc mode using
> > > ifconfig(8) I've stumbled into the problem with already exhausted
> > > space in the `short if_flags' field in the ifnet structure. I need to
> > > allocate one new flag, while we already have 16 IFF_* flags, and even
> > > one additional flag which is implemented using currently free
> > > if_ipending field of the said structure. Attached patch is aimed at
> > > increasing size of if_flags to 32 bits, as well as to clean-up
> > > if_ipending abuse. Granted, it will break backward ABI compatibility,
> > > but IMO it is not a big problem.
> >
> > Why isn't it a bug problem? It affects an application ABI (most socket
> > ioctls). We have whole syscalls whose purpose is to avoid breaking
> > application ABIs back to about 4.3BSD. Some of them may even work.
> >
> > > Index: src/share/man/man4/netintro.4
> > > ===================================================================
> > > RCS file: /home/ncvs/src/share/man/man4/netintro.4,v
> > > retrieving revision 1.20
> > > diff -d -u -r1.20 netintro.4
> > > --- src/share/man/man4/netintro.4 18 Mar 2002 12:39:32 -0000 1.20
> > > +++ src/share/man/man4/netintro.4 15 Aug 2002 18:33:42 -0000
> > > @@ -197,7 +197,7 @@
> > > struct sockaddr ifru_addr;
> > > struct sockaddr ifru_dstaddr;
> > > struct sockaddr ifru_broadaddr;
> > > - short ifru_flags;
> > > + int ifru_flags;
> > > int ifru_metric;
> > > int ifru_mtu;
> > > int ifru_phys;
> >
> > This particular ABI seems to have been broken before (in if.h 1.50 on
> > 1999/02/09), since the actual struct has "short ifru_flags[2];" followed
> > by "short if_index;" instead of "short ifru_flags;", and it has 2 new
> > struct members at the end too. If the struct were actually as above,
> > then changing the short to an int would almost be binary compatible
> > since it would just expand ifru_flags to use the 2 bytes of unnamed
> > padding caused by the poor layout, so the struct wouldn't expand and
> > the other members wouldn't move. Enlarging ifru_flags itself might
> > only break big-endian machines (little-endian ones wouldn't notice
> > providing the padding is zeroed).
> >
> > > Index: src/share/man/man9/ifnet.9
> >
> > Breaking kernel ABIs isn't so important. They should only be compatible
> > within major releases.
>
> BTW, I've just realised that we can easily avoid breaking application
> ABI by using currently unused ifr_ifru.ifru_flags[2] (aka. ifr_prevflags)
> for storing another 16 flags. What do people think?
If something uses ifr_prevflags this could break compatibility, but if
not it is the best solution I think :).
>
> -Maxim
>
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