SUMMARY: 4.0B and GDB

From: Eric Z. Ayers <eric_at_compgen.com>
Date: Wed, 29 Jan 1997 08:51:20 -0500

Well, this pretty much sums up the problem and the fix:

-----------------------------------------------------------------------------

> Hello,
>
> We've been having a kernel panic using Digital Unix 4.0B and gdb-4.16.
> All a user has to do is run gdb and try to step through a program. At
> first we were using a version compiled on Digital Unix 3.2D, but then we
> re-compiled the source on version 4.0 and still have the same problem.
> The system will hang with a message something like 'kernel memory
> refernece error... Syncing Disks' You have to press the halt button in
> order to force a panic.
>
> Anyone heard of a patch/workaround for this?
>
> Thanks,
> -Eric.

There is a problem with V4.0B when you have options KDEBUG (kernel
debugging) built into the kernel; any attempt to single-step a user
program using a user-mode debugger will panic the system.

There *should* be a "service blitz notification" coming out on this
in the near future.

  Here are the suggested work-arounds (from our dbx developer):

  This is a known kernel problem. I imagine people are working on it as
  I've filed a show stopper on it. But, in the meantime...

  This will only happen on a system that is built with KDEBUG set up
  in the kernel config file, /sys/conf/XXXX where XXXX is the
  uppercase system name.

  The problem has to do with the kernel variable db_enable. This is a
  relatively new kernel variable, and gets set to 1 when KDEBUG is configured.
  You can resolve this by going into ladebug or dbx as superuser and
  doing the following

  dbx -k /vmunix /* or whatever you are running as the OS
  assign db_enable = 0

  and then quit. This will take care of the current session, but db_enable will
  be reset if you have to reboot. To take care of that:

  dbx /vmunix /* no -k this time, we want the disk version...
  patch db_enable = 0

  and you'll be set for the life of this particular kernel. Note that you'll
  have to do this all over again if you rebuild the kernel and leave KDEBUG
  set. As I found out this morning. Sigh....

A lot of work was done on the kernel to implement enhanced debugging
support for V4.0B, and clearly something slipped through the cracks.

Tom
 
 Dr. Thomas P. Blinn, UNIX Software Group, Digital Equipment Corporation
 
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Received on Wed Jan 29 1997 - 15:09:02 NZDT

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