linux-kernel - Re: [Patch 00/11] Hardware Breakpoint interfaces

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Date:	Sat, 28 Mar 2009 14:16:48 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@....ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	maneesh@...ux.vnet.ibm.com, Roland McGrath <roland@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [Patch 00/11] Hardware Breakpoint interfaces

On Wed, Mar 25, 2009 at 03:48:31PM -0400, Alan Stern wrote:
> > +
> > +/*
> > + * Handle debug exception notifications.
> > + */
> > +int __kprobes hw_breakpoint_handler(struct die_args *args)
> > +{
> > +	int i, rc = NOTIFY_DONE;
> > +	struct hw_breakpoint *bp;
> > +	/* The DR6 value is stored in args->err */
> > +	unsigned long dr7, dr6 = args->err;
> 
> Please change this.  (I should have changed it long ago, but I never
> got around to it.)  Instead of passing the DR6 value in args->err,
> pass a pointer to the dr6 variable in do_debug().  That way the
> handler routines can turn off bits in that variable and do_debug() can
> see which bits remain set at the end.
> 
> Of course, this will require a corresponding change to the
> post_kprobe_handler() routine.
>

As I looked at the code with an intention of changing it, I don't find a
place - in hw_breakpoint_handler() and in functions called by
kprobe_exceptions_notify() where bits in dr6 are written into.
The thread-specific thread->debugreg6 is updated with causative bits in
ptrace_triggered() to help send signals to the user-space. I don't see a
user for the change you propose.

I should send out the revised patchset sometime tomorrow. Kindly let me
know your comments about them.

Thanks,
K.Prasad

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