linux-kernel - Re: [RFC Patch 2/2][Bugfix][x86][hw-breakpoint] Fix return-code to notifier chain in hw_breakpoint

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Message-ID: <20100110031857.GB15195@nowhere>
Date:	Sun, 10 Jan 2010 04:18:58 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [RFC Patch 2/2][Bugfix][x86][hw-breakpoint] Fix return-code to
	notifier chain in hw_breakpoint_handler

On Fri, Jan 01, 2010 at 12:32:17AM +0530, K.Prasad wrote:
> On Thu, Dec 31, 2009 at 01:38:09AM +0100, Frederic Weisbecker wrote:
> > On Sat, Dec 26, 2009 at 11:58:33PM +0530, K.Prasad wrote:
> > > The hw-breakpoint handler will return NOTIFY_DONE for user-space breakpoints
> > > to generate SIGTRAP signal (and not for kernel-space addresses). 
> > > 
> > > Signed-off-by: K.Prasad <prasad@...ux.vnet.ibm.com>
> > > ---
> > >  arch/x86/kernel/hw_breakpoint.c |    9 +++++++--
> > >  1 file changed, 7 insertions(+), 2 deletions(-)
> > > 
> > > Index: linux-2.6-tip/arch/x86/kernel/hw_breakpoint.c
> > > ===================================================================
> > > --- linux-2.6-tip.orig/arch/x86/kernel/hw_breakpoint.c
> > > +++ linux-2.6-tip/arch/x86/kernel/hw_breakpoint.c
> > > @@ -502,8 +502,6 @@ static int __kprobes hw_breakpoint_handl
> > >  		rcu_read_lock();
> > >  
> > >  		bp = per_cpu(bp_per_reg[i], cpu);
> > > -		if (bp)
> > > -			rc = NOTIFY_DONE;
> > >  		/*
> > >  		 * Reset the 'i'th TRAP bit in dr6 to denote completion of
> > >  		 * exception handling
> > > @@ -517,6 +515,13 @@ static int __kprobes hw_breakpoint_handl
> > >  			rcu_read_unlock();
> > >  			break;
> > >  		}
> > > +		/*
> > > +		 * Further processing in do_debug() is needed for a) user-space
> > > +		 * breakpoints (to generate signals) and b) when the system has
> > > +		 * taken exception due to multiple causes
> > > +		 */
> > > +		if (bp->attr.bp_addr < TASK_SIZE)
> > > +			rc = NOTIFY_DONE;
> > >  
> > >  		perf_bp_event(bp, args->regs);
> > >  
> > > 
> > 
> > 
> > Oh and now that I see this patch, the previous one indeed makes sense
> > with this check:
> > 
> > if (dr6 & (~DR_TRAP_BITS))
> > 	rc = NOTIFY_DONE;
> > 
> > That said, it means thread.debugreg6 won't get the reserved bits anymore.
> > I see some use of them from kvm (it restores the reserved bits on guest<->host
> > switch). Not sure if this inconsistency could affect kvm...
> > 
> 
> Can you point me to the relevant code?


I see various uses of DR6_VOLATILE and DR6_FIXED_1 in arch/x86/kvm/,
DR6_FIXED_1 being the fixed unused bits in dr6. Not sure how
this patch would affect what's set there.

I'll wait for Jan's answer.

Thanks.

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