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SubjectRe: [PATCH v3 1/2] kretprobe: produce sane stack traces
On 2018-11-01, Steven Rostedt <rostedt@goodmis.org> wrote:
> On Thu, 1 Nov 2018 19:35:50 +1100
> Aleksa Sarai <cyphar@cyphar.com> wrote:
> > @@ -1834,6 +1853,11 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
> > ri->rp = rp;
> > ri->task = current;
> >
> > + trace.entries = &ri->entry.entries[0];
> > + trace.max_entries = KRETPROBE_TRACE_SIZE;
> > + save_stack_trace_regs(regs, &trace);
> > + ri->entry.nr_entries = trace.nr_entries;
> > +
>
> So basically your saving a copy of the stack trace for each probe, for
> something that may not ever be used?
>
> This adds quite a lot of overhead for something not used often.
>
> I think the real answer is to fix kretprobes (and I just checked, the
> return call of function graph tracer stack trace doesn't go past the
> return_to_handler function either. It's just that a stack trace was
> never done on the return side).
>
> The more I look at this patch, the less I like it.

That's more than fair.

There's also an issue that Masami noted -- nested kretprobes don't give
you the full stack trace with this solution since the stack was already
overwritten. I think that the only real option is to fix the unwinder to
work in a kretprobe context -- which is what I'm looking at now.

Unfortunately, I'm having a lot of trouble understanding how the current
ftrace hooking works -- ORC has a couple of ftrace hooks that seem
reasonable on the surface but I don't understand (for instance) how
HAVE_FUNCTION_GRAPH_RET_ADDR_PTR *actually* works. Though your comment
appears to indicate that it doesn't work for stack traces?

For kretprobes I think it would be fairly easy to reconstruct what
landed you into a kretprobe_trampoline by walking the set of
kretprobe_instances (since all new ones are added to the head, you can
get the real return address in-order).

But I still have to figure out what is actually stopping the
save_stack_trace() unwinder that isn't stopping the show_stacks()
unwinder (though the show_stacks() code is more ... liberal with the
degree of certainty it has about the unwind).

Am I barking up the wrong tree?

--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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