How to debug kernels (was: cvs commit: src/sys/conf kern.post.mk)

[Kris Kennaway]

 On Mon, Sep 12, 2005 at 10:21:34AM +0930, Greg 'groggy' Lehey wrote:
> On Saturday, 10 September 2005 at 22:29:02 -0400, Kris Kennaway wrote:
> > On Sat, Sep 10, 2005 at 10:01:15PM -0400, Garance A Drosihn wrote:
> >> At 6:05 PM -0700 9/10/05, Nate Lawson wrote:
> >>> David E. O'Brien wrote:
> >>>> obrien      2005-09-11 00:22:21 UTC
> >>>>
> >>>>  FreeBSD src repository
> >>>>
> >>>>  Modified files:
> >>>>    sys/conf             kern.post.mk   Log:
> >>>>  For HEAD, install a kernel with debug information if DEBUG is a kernel
> >>>>  config option.  It is too easy to loose the build directory and not have
> >>>>  symbols for kgdb to read.
> >>>>    Revision  Changes    Path
> >>>>  1.84      +4 -17     src/sys/conf/kern.post.mk
> >>>
> >>> I disagree with this change.  We do not need to waste the space
> >>> in /. If I'm running a debug kernel, it is based on the latest
> >>> version of kernel.debug in my kernel compile dir and I know to
> >>> find it there.
> >>
> >> Fwiw, I've been burned by building a debug kernel, only to have
> >> removed the original compile-directory for that kernel by the time I
> >> actually *needed* the debug symbols.  It's one thing if you're building
> >> a debug kernel because you know you're going to spend the next hour
> >> debugging some change.  It's another if you're building a debug kernel
> >> because your machine might panic sometime in the next two or three
> >> weeks.
> >
> > Likewise, I also find this change very useful.  When I'm juggling a
> > few dozen panics on a few dozen machines with a few dozen different
> > customized source trees, it's hard to keep track of all the
> > kernel.debugs.  Now I don't have to.
> 
> A lot must depend on how you use your debug kernel.  One of the
> biggest problems I've found is keeping the kernel and the sources in
> sync.  This change makes it more difficult.  Also, the kernel build
> can install debugging macros in the build directory; if you blow away
> that directory, you've lost the macros too.
> 
> The method I use is described in
> http://wwww.lemis.com/grog/Papers/Debug-tutorial/tutorial.pdf.
> Basically, you debug from the kernel build directory and just pull in
> the dump from /var/crash.  I'm planning to present it again in Basel
> in November, so if anybody disagrees with the approach, now is the
> time to tell me.  With this method, having symbols in the booted
> kernel is a waste of time and space.

Please reread my earlier posts for why this is a hindrance in my
environment.

Kris
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