[PATCH v3 0/5] ARM64: Add kernel probes(Kprobes) support
David Long
dave.long at linaro.org
Sun Dec 14 22:29:20 PST 2014
On 12/12/14 18:10, Steve Capper wrote:
> On 12 December 2014 at 22:42, David Long <dave.long at linaro.org> wrote:
>> On 12/10/14 11:38, Steve Capper wrote:
>>>
>>> On Tue, Dec 09, 2014 at 09:27:18AM -0500, David Long wrote:
>>>>
>>>> On 12/09/14 08:33, Steve Capper wrote:
>>>>>
>>>>> On Thu, Dec 04, 2014 at 08:53:03PM +0900, Masami Hiramatsu wrote:
>>>
>>>
>>> [...]
>>>
>>>>>
>>>>> Not sure if this is helpful, but the following also caused a crash for
>>>>> me:
>>>>>
>>>>> echo "p:trace_event_buffer_lock_reserve trace_event_buffer_lock_reserve"
>>>>>> /sys/kernel/debug/tracing/kprobe_events
>>>>> echo "p:memcpy memcpy" >> /sys/kernel/debug/tracing/kprobe_events
>>>>> echo 1 > /sys/kernel/debug/tracing/events/kprobes/enable
>>>>>
>>>>> [immediate crash]
>>>>>
>>>>> The crash point for me is in the arm64 ASID allocator, it again looks
>>>>> like the interrupts are in an unexpected state.
>>>>> (check_and_switch_context goes down the irqs disabled code path, I
>>>>> think incorrectly).
>>>>>
>>>>> This occurred for me both with and without the proposed irq saving fix.
>>>>>
>>>>> I will do some more digging.
>>>>>
>>>>
>>>> Thanks, more information is good.
>>>>
>>>
>>> Hi,
>>>
>>> Some good news, I think I've fixed the problem I've been experiencing.
>>>
>>> Basically, I've torn out all the interrupt save/restore and have
>>> narrowed the scope to just sandwich the instruction single-step. This
>>> simplifies a lot of logic, and I've now been able to perf record a
>>> kprobe on memcpy (and the trace_event_buffer_lock_reserve + memcpy
>>> test) without any issues on a Juno platform.
>>>
>>> I may have been somewhat over-zealous with the chainsaw, so please do
>>> put this fix through its paces.
>>>
>>> Cheers,
>>> --
>>> Steve
>>>
>>>
>>> From d3f4d80ce19bec71bd03209beb2fbfd8084d6543 Mon Sep 17 00:00:00 2001
>>> From: Steve Capper <steve.capper at linaro.org>
>>> Date: Mon, 1 Dec 2014 11:30:10 +0000
>>> Subject: [PATCH] Fix the interrupt handling for kprobes
>>>
>>> ---
>>> arch/arm64/kernel/kprobes.c | 16 ++--------------
>>> 1 file changed, 2 insertions(+), 14 deletions(-)
>>>
>>> diff --git a/arch/arm64/kernel/kprobes.c b/arch/arm64/kernel/kprobes.c
>>> index be7c330..d39d826 100644
>>> --- a/arch/arm64/kernel/kprobes.c
>>> +++ b/arch/arm64/kernel/kprobes.c
>>> @@ -229,10 +229,6 @@ skip_singlestep_missed(struct kprobe_ctlblk *kcb,
>>> struct pt_regs *regs)
>>> {
>>> /* set return addr to next pc to continue */
>>> instruction_pointer(regs) += sizeof(kprobe_opcode_t);
>>> -
>>> - if (kcb->kprobe_status != KPROBE_REENTER)
>>> - kprobes_restore_local_irqflag(regs);
>>> -
>>> }
>>>
>>> static void __kprobes setup_singlestep(struct kprobe *p,
>>> @@ -259,7 +255,7 @@ static void __kprobes setup_singlestep(struct kprobe
>>> *p,
>>> spsr_set_debug_flag(regs, 0);
>>>
>>> /* IRQs and single stepping do not mix well. */
>>> - local_irq_disable();
>>> + kprobes_save_local_irqflag(regs);
>>> kernel_enable_single_step(regs);
>>> instruction_pointer(regs) = slot;
>>> } else {
>>> @@ -326,7 +322,6 @@ post_kprobe_handler(struct kprobe_ctlblk *kcb, struct
>>> pt_regs *regs)
>>> }
>>>
>>> reset_current_kprobe();
>>> - kprobes_restore_local_irqflag(regs);
>>> }
>>>
>>> int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int
>>> fsr)
>>> @@ -380,8 +375,6 @@ int __kprobes kprobe_fault_handler(struct pt_regs
>>> *regs, unsigned int fsr)
>>> return 1;
>>>
>>> break;
>>> - default:
>>> - break;
>>> }
>>> return 0;
>>> }
>>> @@ -446,7 +439,6 @@ void __kprobes kprobe_handler(struct pt_regs *regs)
>>> * handling of this interrupt is appropriate.
>>> * Return back to original instruction, and continue.
>>> */
>>> - kprobes_restore_local_irqflag(regs);
>>> return;
>>> } else if (cur) {
>>> /* We probably hit a jprobe. Call its break handler. */
>>> @@ -459,7 +451,6 @@ void __kprobes kprobe_handler(struct pt_regs *regs)
>>> /* breakpoint is removed, now in a race
>>> * Return back to original instruction & continue.
>>> */
>>> - kprobes_restore_local_irqflag(regs);
>>> }
>>> }
>>>
>>> @@ -485,6 +476,7 @@ kprobe_single_step_handler(struct pt_regs *regs,
>>> unsigned int esr)
>>> retval = kprobe_ss_hit(kcb, instruction_pointer(regs));
>>>
>>> if (retval == DBG_HOOK_HANDLED) {
>>> + kprobes_restore_local_irqflag(regs);
>>> kernel_disable_single_step();
>>>
>>> if (kcb->kprobe_status == KPROBE_REENTER)
>>> @@ -499,7 +491,6 @@ kprobe_single_step_handler(struct pt_regs *regs,
>>> unsigned int esr)
>>> static int __kprobes
>>> kprobe_breakpoint_handler(struct pt_regs *regs, unsigned int esr)
>>> {
>>> - kprobes_save_local_irqflag(regs);
>>> kprobe_handler(regs);
>>> return DBG_HOOK_HANDLED;
>>> }
>>> @@ -563,7 +554,6 @@ int __kprobes longjmp_break_handler(struct kprobe *p,
>>> struct pt_regs *regs)
>>> memcpy((void *)stack_addr, kcb->jprobes_stack,
>>> MIN_STACK_SIZE(stack_addr));
>>> preempt_enable_no_resched();
>>> - kprobes_restore_local_irqflag(regs);
>>> return 1;
>>> }
>>> return 0;
>>> @@ -655,8 +645,6 @@ trampoline_probe_handler(struct kprobe *p, struct
>>> pt_regs *regs)
>>> kfree(ri);
>>> }
>>>
>>> - kprobes_restore_local_irqflag(regs);
>>> -
>>> /* return 1 so that post handlers not called */
>>> return 1;
>>> }
>>>
>>
>> Sorry for the delay in responding. The assumption with the existing code is
>> that you can't enable single-stepping in MDSCR until you disable interrupts.
>> But since we get to that point (in this case) with debug exceptions masked
>> in daif I suppose this is an unnecessary constraint, as you have
>> demonstrated. I do actually wonder if we need to set the bit in MDSCR_EL1
>> at all since we're also setting it in the spsr in the regs structure that
>> will be restored when the eret is done, but it looks like
>> kernel_enable_single_step() sets both.
>>
>
> I do think we need to disable interrupts, but only when we're about to
> single-step.
> (My explanation before was imprecise, apologies).
> The change I posted disables interrupts in __kprobes_single_step, and
> restores them in kprobe_single_step_handler.
> I did try removing the interrupt logic completely, but this then
> resulted in a system hang after a few seconds.
>
I worded that badly. You still have to disable interrupts but I had
thought we needed to do that before setting SS in MDSCR. You've shown
we can do it anytime before we unmask debug exceptions.
> I will think about MDSCR_EL1, when I'm awake :-).
>
> Cheers,
> --
> Steve
>
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