Re: [PATCH v3 0/5] ARM64: Add kernel probes(Kprobes) support

[Steve Capper <steve.capper@linaro.org>]

On 12 December 2014 at 22:42, David Long <dave.long@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@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 will think about MDSCR_EL1, when I'm awake :-).

Cheers,
--
Steve