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SubjectRe: [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@linux.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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