Re: Inconsistency between net/if.c and several ethernet drivers
I'm working on just such a driver. It has layers that communicate with assumptions about structure alignment. Moving struct arpcom to the beginning breaks this, and requires scattered changes throughout. :-( Even so I'd rather do it the right way. Unless, I hear someone else disagreeing, I'll assume that arpcom must be at the top of softc and write a problem report against the offending drivers (awi,lnc,pdq,ray). - Bill Baumann On Wed, 17 Jul 2002, Garrett Wollman wrote: On Wed, 17 Jul 2002 10:58:12 -0700 (PDT), Bill Baumann [EMAIL PROTECTED] said: 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... if_softc was added to pacify those who either didn't understand the C language, or thought they had a good reason why the softc structure had to begin with some other structure than ifnet. -GAWollman To Unsubscribe: send mail to [EMAIL PROTECTED] with unsubscribe freebsd-net in the body of the message
Re: Inconsistency between net/if.c and several ethernet drivers
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 [EMAIL PROTECTED]
with unsubscribe freebsd-net in the body of the message
To Unsubscribe: send mail to [EMAIL PROTECTED]
with unsubscribe freebsd-net in the body of the message
RE: Inconsistency between net/if.c and several ethernet drivers
Any driver that uses miibus_attach() is broken if struct arpcom is not at
the beginning of the softc structure. There was some discussion of this
more than a year ago when this bug showed up in the wx driver.
Jim
-Original Message-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]]On Behalf Of Bill Baumann
Sent: Wednesday, July 17, 2002 1:58 PM
To: Kelly Yancey
Cc: [EMAIL PROTECTED]
Subject: Re: Inconsistency between net/if.c and several ethernet drivers
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 [EMAIL PROTECTED]
with unsubscribe freebsd-net in the body of the message
To Unsubscribe: send mail to [EMAIL PROTECTED]
with unsubscribe freebsd-net in the body of the message
To Unsubscribe: send mail to [EMAIL PROTECTED]
with unsubscribe freebsd-net in the body of the message
Re: Inconsistency between net/if.c and several ethernet drivers
On Wed, 17 Jul 2002 10:58:12 -0700 (PDT), Bill Baumann [EMAIL PROTECTED] said: 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... if_softc was added to pacify those who either didn't understand the C language, or thought they had a good reason why the softc structure had to begin with some other structure than ifnet. -GAWollman To Unsubscribe: send mail to [EMAIL PROTECTED] with unsubscribe freebsd-net in the body of the message
RE: Inconsistency between net/if.c and several ethernet drivers
Fortunately, none of the offending drivers (awi,lnc,pdq,ray) use mii.
On Wed, 17 Jul 2002, Jim McGrath wrote:
Any driver that uses miibus_attach() is broken if struct arpcom is not at
the beginning of the softc structure. There was some discussion of this
more than a year ago when this bug showed up in the wx driver.
Jim
-Original Message-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]]On Behalf Of Bill Baumann
Sent: Wednesday, July 17, 2002 1:58 PM
To: Kelly Yancey
Cc: [EMAIL PROTECTED]
Subject: Re: Inconsistency between net/if.c and several ethernet drivers
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 [EMAIL PROTECTED]
with unsubscribe freebsd-net in the body of the message
To Unsubscribe: send mail to [EMAIL PROTECTED]
with unsubscribe freebsd-net in the body of the message
To Unsubscribe: send mail to [EMAIL PROTECTED]
with unsubscribe freebsd-net in the body of the message
Re: Inconsistency between net/if.c and several ethernet drivers
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 [EMAIL PROTECTED]
with unsubscribe freebsd-net in the body of the message
