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: <20100120060159.GA4859@in.ibm.com>
Date:	Wed, 20 Jan 2010 11:31:59 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	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 Sun, Jan 17, 2010 at 01:10:58AM +0530, K.Prasad wrote:
> On Mon, Jan 11, 2010 at 08:15:29PM +0100, Jan Kiszka wrote:
> > Frederic Weisbecker wrote:
> > > 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.
> > > 
> > 
> > You may need to synchronize me: What does the patch change, the shadow
> > register KVM will restore into DR6 on return to the host? Or the
> > register content KVM finds on guest entry?
> > 
> 
> Sorry, this mail got buried deeply in my mailbox (hence the delay).
> 
> Basically, this patch tries to remove DR6 from its reserved bits to help
> easy checks for certain status bits (such as DR_STEP). For instance, in
> order to verify if DR_STEP (Bit 14) is set we must now do
> if ((DR6 & ~DR6_RESERVED) & DR_STEP) {}
> or
> if (DR6 & (DR_STEP | DR6_RESERVED)) {}
> which is redundant.
> 
> Instead this patch would expunge all reserved bits in DR6 before checks
> for various status bits (to detect the cause of exception) are made in
> do_debug().
> 
> At the outset, I don't think changes in the way the value of DR6 is used
> for comparison in do_debug() would affect exception handling for either
> KVM's guest or host OS (given that there are no hooks for the same in
> do_debug()).
> 
> > The rules are simple: On entry, KVM assumes nothing about the register
> > state, just overwrites it (on demand) with the guest state. On exit, it
> > calls into hw_breakpoint_restore to ensure the host sees a proper state
> > (if required). But there is at no time an architecturally invalid state
> > loaded into the real register (that's basically what DR6_VOLATILE and
> > DR6_FIXED_1 are used for while in guest mode).
> > 
> 
> Such a behaviour shouldn't be affected by the above change...your
> confirmation would help!
> 

Hi Jan,
	I presume that the above explanation makes the role of this
patch/bugfix clear.

Kindly let me know if you have any further queries.

Thanks,
K.Prasad

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