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

[dave.long@linaro.org (David Long)]

On 11/26/14 05:03, Steve Capper wrote:
> On Wed, Nov 26, 2014 at 05:33:05PM +0900, Masami Hiramatsu wrote:
>> (2014/11/21 0:02), Steve Capper wrote:
>>> On Tue, Nov 18, 2014 at 01:32:50AM -0500, David Long wrote:
>>>> From: "David A. Long" <dave.long@linaro.org>
>>>>
>>>> This patchset is heavily based on Sandeepa Prabhu's ARM v8 kprobes patches, first
>>>> seen in October 2013.  This version attempts to address concerns raised by
>>>> reviewers and also fixes problems discovered during testing, particularly during
>>>> SMP testing.
>>>>
>>>> This patchset adds support for kernel probes(kprobes), jump probes(jprobes)
>>>> and return probes(kretprobes) support for ARM64.
>>>>
>>>> Kprobes mechanism makes use of software breakpoint and single stepping
>>>> support available in the ARM v8 kernel.
>>>>
>>>> Changes since v2 include:
>>>>
>>>> 1) Removal of NOP padding in kprobe XOL slots.  Slots are now exactly one
>>>> instruction long.
>>>> 2) Disabling of interrupts during execution in single-step mode.
>>>> 3) Fixing of numerous problems in instruction simulation code.
>>>> 4) Support for the HAVE_REGS_AND_STACK_ACCESS_API feature is added, to allow
>>>> access to kprobes through debugfs.
>>>> 5) kprobes is *not* enabled in defconfig.
>>>> 6) Numerous complaints from checkpatch have been cleaned up, although a couple
>>>> remain as removing the function pointer typedefs results in ugly code.
>>>
>>> Hi David,
>>> I've been playing with this on a Juno board.
>>> I ran into one crash, which I'm not yet sure is an issue, but thought I
>>> would flag it.
>>>
>>> I opted to put a kprobe on memcpy, this is an assembler function so I
>>> located it via:
>>> $ nm ./vmlinux | grep \ memcpy$
>>> fffffe0000408a00 T memcpy
>>>
>>> Then placed a probe as follows:
>>> echo "p:memcpy 0xfffffe0000408a00 %x2" > /sys/kernel/debug/tracing/kprobe_events
>>
>> You can also do "p:memcpy memcpy %x2" > ...
>
> Thanks, that is easier :-).
>
>>
>>>
>>> I was able to cat out the /sys/kernel/debug/tracing/trace_pipe file and
>>> activate the probe via:
>>> echo 1 > /sys/kernel/debug/tracing/events/kprobes/enable
>>>
>>> Everything worked well, and I got the expected output.
>>>
>>> I then tried to record events with perf via:
>>> perf record -e kprobes:memcpy -a sleep 5
>>>
>>> Then I got an, easily reproducible, panic (pasted below).
>>
>> On x86, I didn't get a panic.
>>
>>>
>>> The point of failure in the panic was:
>>> fs/buffer.c:1257
>>>
>>> static inline void check_irqs_on(void)
>>> {
>>> #ifdef irqs_disabled
>>>          BUG_ON(irqs_disabled());
>>> #endif
>>> }
>>>
>>> I will do some more digging; but I have managed to code up an ftrace
>>> static probe on memcpy and record that using perf on arm64 without
>>> issue.
>>
>> Yeah, this can be a bug related to kprobes recursive call.
>> Could you do "cat /sys/kernel/debug/tracing/kprobe_profile" (before
>> run perf)?
>> The first digit is # of hit, and the second is # of missed (since
>> recursively called).
>>
>> On x86, right after tracing by ftrace, we have no missed probe.
>>
>>   # cat /sys/kernel/debug/tracing/kprobe_profile
>>    memcpy                                                  4547               0
>>
>> But after tracing by perf, many missed events I could see.
>>
>>   # cat /sys/kernel/debug/tracing/kprobe_profile
>>    memcpy                                                413983            7632
>>
>> So I guess this can be related to the recursive call (which
>> is correctly handled on x86).
>>
>
> Before running perf, I got the following:
>
>   # cat /sys/kernel/debug/tracing/kprobe_profile
>     memcpy                                                   838               0
>
> Unfortunately, after the crash, I was then unable to take any other
> measurements.
>
> I rebooted, set up the kprobe, then ran `./hackbench 100 process 1000',
> to try and exacerbate things, and got the following:
>   # cat /sys/kernel/debug/tracing/kprobe_profile
>     memcpy                                                100677               0
>
> So no missed events thusfar.
>
> Cheers,
>

So I take it from this we can conclude the problem is not reliably 
reproducible?

-dl