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

[Steve Capper]

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@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" > ...

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,
-- 
Steve
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