linux-kernel - Re: [RFC][PATCH] ppc: Implement save_stack_trace

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Date:	Mon, 11 Jan 2016 14:30:31 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, linuxppc-dev@...ts.ozlabs.org,
	chuhu@...hat.com
Subject: Re: [RFC][PATCH] ppc: Implement save_stack_trace_regs()

On Fri, 2016-01-08 at 17:50 -0500, Steven Rostedt wrote:
> On Wed, 16 Dec 2015 12:24:19 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
> > On Wed, 09 Dec 2015 12:03:05 +1100
> > Michael Ellerman <mpe@...erman.id.au> wrote:
> > > > > Should I take this via powerpc or do you want it to go in via tracing?
> > > >
> > > > You can take it. And you can replace the PT_R1 if you want. I just
> > > > noticed that it was defined, and I try to use macro names instead of
> > > > hard coded numbers. I was actually looking for a "PT_SP" :-)
> > >
> > > OK thanks.
> > >
> > > Looks like we actually have:
> > >
> > >   #define kernel_stack_pointer(regs) ((regs)->gpr[1])
> > >
> > > So that would be the most self documenting way to do it I guess, though I've
> > > never actually seen that macro used anywhere before :)
> >
> > Hi,
> >
> > Are you going to take this, or do you want me to?

Sorry, yep I'll take it.

I trimmed the change log a bit, final version below.

cheers


powerpc: Implement save_stack_trace_regs() to enable kprobe stack tracing

It has come to my attention that kprobe event stack tracing does not
work on powerpc. You can see with the following:

  # cd /sys/kernel/debug/tracing
  # echo stacktrace > trace_options
  # echo 'p kfree' > kprobe_events
  # echo 1 > events/kprobes/enable

Will print the following warning:
  save_stack_trace_regs() not implemented yet.

Although save_stack_trace() (which normal event stack traces use) is
implemented, save_stack_trace_regs() which kprobe events use is not.
This is a cheap attempt to implement that function.

Note, This may have issues if a task tries to get a stack trace from
another task with its regs, because it just passes in "current" to
save_context_stack(). But this does solve the issue with stack tracing
kprobe events.

Reported-by: Chunyu Hu <chuhu@...hat.com>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>

diff --git a/arch/powerpc/kernel/stacktrace.c b/arch/powerpc/kernel/stacktrace.c
index ea43a347a104..4f24606afc3f 100644
--- a/arch/powerpc/kernel/stacktrace.c
+++ b/arch/powerpc/kernel/stacktrace.c
@@ -61,3 +61,10 @@ void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
 	save_context_stack(trace, tsk->thread.ksp, tsk, 0);
 }
 EXPORT_SYMBOL_GPL(save_stack_trace_tsk);
+
+void
+save_stack_trace_regs(struct pt_regs *regs, struct stack_trace *trace)
+{
+	save_context_stack(trace, regs->gpr[1], current, 0);
+}
+EXPORT_SYMBOL_GPL(save_stack_trace_regs);