PPC KGDB changes and some help?

Amit S. Kale amitkale at emsyssoft.com
Thu Jan 22 04:01:45 EST 2004

On Wednesday 21 Jan 2004 9:00 pm, Tom Rini wrote:
> On Wed, Jan 21, 2004 at 07:46:17PM +0530, Amit S. Kale wrote:
> > Hi Tom,
> >
> > Yes. Software breakpoints have been tested in the TimeSys ppc kernel
> > source. They work quite well!! I'll be releasing that code soon.
> Any chance you can give me what they gave you?  I can try and merge
> and test things.


> > The breakpoint 0xc0000000 placed by gdb is _evil_ It may clobber data.
> > The gdb at kgdb.sourceforge.net places it correctly at module_event.
> I'm not quite sure what you're getting at.   The gdb binary I'm using is
> a good one (It's happy w/ the current kgdb stub, working in tandem w/ a
> BDI2000, etc).  If the breakpoints being set aren't right, I suspect
> that it's related to the other problems I'm seeing.

Stock gdb places a breakpoint to detect loading of shared libraries. Since
kernel doesn't have the symbols that ld-linux-* has, it places that at
begining of the kernel (or elsewhere I haven't been able to figure out
exactly where it places it). This breakpoint corrupts kernel data many a

The gdb I maintain at kgdb.sourceforge.net places a breakpoint correctly at
module_event and detects loading of modules.

> > Where is the other breakpoint placed? While you would have certainly done
> > that, please confirm that kgdb actually inserts a breakpoint where you
> > have asked it to: a simple printk at the address where the breakpoint is
> > placed should be sufficient. printing from gdb will not work as gdb
> > removes all breakpoints before giving control to a user.
> The thing is the kernel gets into an infinite loop of stopping, as far
> as gdb can tell, at the initial breakpoint

I thought you could place a breakpoint somewhere and the breakpoint was never

ok. Now I know where it went wrong: nip is instruction pointer, not
instruction contents. The change you had done compared nip to breakpoint
instruction contents.

> > +       if (linux_regs->nip == 0x7d821008 )
> > +               /* Skip over breakpoint trap insn */
> > +               linux_regs->nip += 4;

Checking for kgdb_setting_breakpoint is better. Following code from my patch
is correct.

> > +       extern atomic_t kgdb_setting_breakpoint;
> > +       if (atomic_read(&kgdb_setting_breakpoint))
> > +               regs->nip += 4;

Amit Kale
EmSysSoft (http://www.emsyssoft.com)
KGDB: Linux Kernel Source Level Debugger (http://kgdb.sourceforge.net)

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