Date: Wed, 17 Jul 2002 10:58:12 -0700 (PDT) From: Bill Baumann <bbaumann@isilon.com> To: Kelly Yancey <kbyanc@posi.net> Cc: freebsd-net@FreeBSD.ORG Subject: Re: Inconsistency between net/if.c and several ethernet drivers Message-ID: <Pine.BSF.4.21.0207170942280.35007-100000@isilon.com> In-Reply-To: <20020716180110.E4709-100000@gateway.posi.net>
next in thread | previous in thread | raw e-mail | index | archive | help
Pointers to interface and arpcom are clearly equivalent as arpcom contains
the interface structure. The one definition for the combined structure
makes it very safe. However, there are many definitions of the softc
structure. Requiring arpcom to be at the beginning of all softc structs
requires every device driver writer to either track this obscure detail,
or risk obscure bugs.
Why bother with a if_softc field when the interface and softc pointer are
supposed to be the same? Also, the very old Lance driver (lnc) has this
problem. It makes me wonder how true we are to TCP/IP Illustrated...
So while this wouldn't really be a bug fix, perhaps we should do my
suggested change anyway to avoid modifying the drivers.
Regards,
Bill Baumann
On Tue, 16 Jul 2002, Kelly Yancey wrote:
> On Tue, 16 Jul 2002, Bill Baumann wrote:
>
> >
> > In net/if.c in a couple of places, the ethernet address is needed. This
> > is stored in the arpcom structure. A couple lines of code in if.c require
> > struct arpcom be at the very begining of device softc structures. Nearly
> > all drivers observe this. However, several do not. Sadly, this includes
> > the one I'm working on.
> >
> > net/if.c routines if_findindex() and if_setlladdr() gain access to the
> > ethernet address via the following expression:
> >
> > ((struct arpcom *)ifp->if_softc)->ac_enaddr
> >
> > The above code assumes that the if_softc pointer is equivalent to an
> > struct arpcom pointer. The awi, ray, lnc and pdq drivers have other
> > fields at the beginning of their softc structures. Attempts to set the
> > ethernet address of these devices may cause corruption.
> >
> >
> > Shouldn't access of arpcom be via ifp instead?
> >
> > ((struct arpcom *)ifp)->ac_enaddr
> >
> >
> > For example, if_ethersubr.c uses the following macro:
> > #define IFP2AC(IFP) ((struct arpcom *)IFP)
> >
> > It looked to me like the other code in net, like if_ethersubr.c use ifp
> > rather than if_softc to find struct arpcom.
> >
> > Bug?
> >
>
> Design. :) See page 77 of Stevens' TCP/IP Illustrated Volume 2. By putting
> the structures at the beginning of the softc, the networking code can access
> them without any explicit knowledge of the driver's softc itself (i.e. it can
> use the softc as an opaque encapsulated version of either the arpcom or ifnet
> structures). The bug, then, would seem to be in the network drivers that
> don't follow this convention. But I'm not familiar with those particular
> drivers, so I cannot comment on them; perhaps they employ some cleverness to
> circumvent the requirement (by why?). Anyway, it should be obvious that
> accessing the arpcom structure via casting from the ifnet structure or the
> softc structure are supposed to have the same results, so the code your quoted
> above is fine.
>
> Kelly
>
> --
> Kelly Yancey -- kbyanc@{posi.net,FreeBSD.org}
> "The worst sin towards our fellow creatures is not to hate them, but to be
> indifferent to them; that's the essence of inhumanity."
> -- George Bernard Shaw
>
>
>
> To Unsubscribe: send mail to majordomo@FreeBSD.org
> with "unsubscribe freebsd-net" in the body of the message
>
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-net" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?Pine.BSF.4.21.0207170942280.35007-100000>