On Mon, Jun 15, 2009 at 04:40:45PM +1000, David Gibson wrote:
> On Wed, Jun 10, 2009 at 12:13:49PM +0530, K.Prasad wrote:
> > On Fri, Jun 05, 2009 at 03:11:58PM +1000, David Gibson wrote:
> > > On Wed, Jun 03, 2009 at 10:05:11PM +0530, K.Prasad wrote:
> >
> > > > + else {
> > > > + /*
> > > > + * This exception is triggered not because of a memory
> > > > access on
> > > > + * the monitored variable but in the double-word
> > > > address range
> > > > + * in which it is contained. We will consume this
> > > > exception,
> > > > + * considering it as 'noise'.
> > > > + */
> > > > + rc = NOTIFY_STOP;
> > > > + goto out;
> > > > + }
> > > > + is_one_shot = (bp->triggered == ptrace_triggered) ? 1 : 0;
> > >
> > > Ouch, explicitly special-casing ptrace_triggered is pretty nasty.
> > > Since the bp_info is already arch specific, maybe it should include a
> > > flag to indicate whether the breakpoint is one-shot or not.
> > >
> >
> > The reason to check for ptrace_triggered is to contain the one-shot
> > behaviour only to ptrace (thus retaining the semantics) and not to extend
> > them to all user-space requests through
> > register_user_hw_breakpoint().
>
> Right, but couldn't you implement that withing ptrace_triggered
> itself, without a special test here, by having it cancel the
> breakpoint.
>
A special check (either using the callback routine as above, or using a
special flag) will be required in hw_breakpoint_handler() to enable
early return (without single-stepping). I'm not sure if I got your
suggestion right, and let me know if you think so.
> > A one-shot behaviour for all user-space requests would create more work
> > for the user-space programs (such as re-registration) and will leave open
> > a small window of opportunity for debug register grabbing by kernel-space
> > requests.
> >
> > So, in effect a request through register_user_hw_breakpoint() interface
> > will behave as under:
> > - Single-step over the causative instruction that triggered the
> > breakpoint exception handler.
> > - Deliver the SIGTRAP signal to user-space after executing the causative
> > instruction.
> >
> > This behaviour is in consonance with that of kernel-space requests and
> > those on x86 processors, and helps define a consistent behaviour across
> > architectures for user-space.
> >
> > Let me know what you think on the same.
>
> I certainly see the value in consistent semantics across archs.
> However, I can also see uses for the powerpc trap-before-execute
> behaviour. That's why I'm suggesting it might be worth having an
> arch-specific flag.
>
> [snip]
So, you suggest that the 'one-shot' behaviour should be driven by
user-request and not just confined to ptrace? (The default behaviour for
all breakpoints-minus-ptrace will remain 'continuous' though).
It can be implemented through an additional flag in 'struct
arch_hw_breakpoint'. I can send a new version 7 of the patchset with this
change (with the hope that the version 6 of the patchset looks fine in
its present form!). Meanwhile, we'd like to know what uses you see in
addition to the present one if the one-shot behaviour is made
user-defined. Are those uses beyond what can be achieved through the
present ptrace interface?
> > > > +int __kprobes single_step_dabr_instruction(struct die_args *args)
> > > > +{
> > > > + struct pt_regs *regs = args->regs;
> > > > + int cpu = get_cpu();
> > > > + int ret = NOTIFY_DONE;
> > > > + siginfo_t info;
> > > > + unsigned long this_dabr_data = per_cpu(dabr_data, cpu);
> > > > +
> > > > + /*
> > > > + * Check if we are single-stepping as a result of a
> > > > + * previous HW Breakpoint exception
> > > > + */
> > > > + if (this_dabr_data == 0)
> > > > + goto out;
> > > > +
> > > > + regs->msr &= ~MSR_SE;
> > > > + /* Deliver signal to user-space */
> > > > + if (this_dabr_data < TASK_SIZE) {
> > > > + info.si_signo = SIGTRAP;
> > > > + info.si_errno = 0;
> > > > + info.si_code = TRAP_HWBKPT;
> > > > + info.si_addr = (void __user *)(per_cpu(dabr_data, cpu));
> > > > + force_sig_info(SIGTRAP, &info, current);
> > >
> > > Uh.. I recall mentioning in my previous review that in order to match
> > > previous behaviour we need to deliver the userspace signal *before*
> > > stepping over the breakpointed instruction, rather than after (which
> > > I guess is why breakpoints are one-shot in the old scheme).
> >
> > This code would implement the behaviour as stated in the comment for
> > user-space requests above.
>
> And you're relying on the old trap-sending code in do_dabr for ptrace
> requests?
>
Yes.
> --
> David Gibson | I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_
> _other_
> | _way_ _around_!
> http://www.ozlabs.org/~dgibson
Thanks,
K.Prasad
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