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

[Masami Hiramatsu]

(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@xxxxxxxxxx>
>>
>> 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" > ...

> 
> 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).

Thank you,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx


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