Re: [PATCH V2 4/4] arm64: disable irq between breakpoint and step exception

[Pratyush Anand <panand@redhat.com>]

Hi Will,

Thanks for your review.

On Tuesday 25 July 2017 06:55 PM, Will Deacon wrote:
> On Fri, Jul 07, 2017 at 05:34:00PM +0530, Pratyush Anand wrote:
>> If an interrupt is generated between breakpoint and step handler then
>> step handler can not get correct step address. This situation can easily
>> be invoked by samples/hw_breakpoint/data_breakpoint.c. It can also be
>> reproduced if we insert any printk() statement or dump_stack() in perf
>> overflow_handler. So, it seems that perf is working fine just luckily.
>> If the CPU which is handling perf breakpoint handler receives any
>> interrupt then, perf step handler will not execute sanely.
>>
>> This patch improves do_debug_exception() handling, which enforces now,
>> that exception handler function:
>> - should return 0 for any software breakpoint and hw
>> breakpoint/watchpoint handler if it does not expect a single step stage
>> - should return 1 if it expects single step.
>> - A single step handler should always return 0.
>> - All handler should return a -ve error in any other case.
>>
>> Now, we can know in do_debug_exception() that whether a step exception
>> will be followed or not. If there will a step exception then disable
>> irq. Re-enable it after single step handling.
> 
> AFAICT, this is only a problem for kernel-mode breakpoints where we end up
> stepping into the interrupt handler when trying to step over a breakpoint.

I think yes.

> 
> We'd probably be better off getting all users of kernel step (kprobes, kgdb
> and perf) to run the step with irqs disabled,


That should be doable. We can easily manage all of them in 
do_debug_exception() if individual brk handlers return correct value as per 
the rule mentioned in the commit log of this patch.

I think, I can take care of kprobes and kgdb as well in next version of patch.

> but I still have reservations
> about that:

So, IIUC, you have concern about faulting of a instruction being stepped. 
Since we will have a notion of *irq_en_needed*, so I think, if needed we can 
re-enable interrupt in fault handler do_mem_abort().

Whats your opinion here?

> 
>    http://lists.infradead.org/pipermail/linux-arm-kernel/2017-May/508066.html
>    http://lists.infradead.org/pipermail/linux-arm-kernel/2017-June/510814.html
>
> Wouldn't it be better to follow kprobes/kgdb and have perf run the step with
> irqs disabled?
-- 
Regards
Pratyush