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

[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@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,

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

-dl

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