Re: [PATCH] kdb/mdb hardware breakpoints broken 2.4.17/18

[Suparna Bhattacharya <suparna@in.ibm.com>]

Keith Owens wrote:
> 
> On Wed, 23 Jan 2002 14:00:45 -0700,
> "Jeff V. Merkey" <jmerkey@vger.timpanogas.org> wrote:
> >Please find a patch that corrects the problem with hardware
> >breakpoints not working with kdb.  I have noticed that gdb uses
> >inserted int3 (0xCC) breakpoints (as does kdb) for soft breakpoint
> >support, so this fix may not affect these programs.  It is not
> >clear why every signal handled is writing a 0 t the DR7 register.
> 
> Not a 0, it is %0, gcc asm parameter 0.  There used to be a bug in
> ptrace handling where db7 would get cleared in taps.c and would not get
> reinstated until one timeslice after the initial trap.  Fixed by James
> Cownie in June 2000 by reinstating db7 before passing the signal to
> user space.
> 
> The conflict with kdb is inherent in the lack of a kernel interface for
> assigning debug registers.  

As Bharata mentioned this sort of a thing is exactly what we use today 
in dprobes. Guess we could separate out that piece as a patch so you can
take a look. We had posted one long back on this list, but have now
cleaned
up and reorganized it a bit. In fact we've used this for a while for
preparing 
consolidated patches containing both dprobes and kdb, in a way that also
interoperates with gdb (we don't want our watchpoint settings to get
overwritten
on a context switch for example).


>gdb/ptrace always uses i386 db4-7 no matter
> what kdb is doing.  The kernel always uses db7, even if no tracing is
> being done.  If you want to use gdb/ptrace then restrict your kdb usage
> to db0-3, without gdb/ptrace kdb can use db0-6.
> 
> It would be nice to have a proper debug register function to
> automatically detect conflicts and tell one of the debuggers to go
> away.   However Linus "I don't want kernel debuggers" Torvalds does not
> care about this problem and I don't want the footprint of kdb to be any
> bigger than it has to be.  So kdb relies on the user to know which
> debug registers then can use.
> 
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