Re: [RFC Patch 2/2][Bugfix][x86][hw-breakpoint] Fix return-code tonotifier chain in hw_breakpoint_handler

[K.Prasad]

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@xxxxxxxxxxxxxxxxxx>
> >>>> ---
> >>>>  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!

Thanks,
K.Prasad

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