Re: [PATCH 3/6] uprobes/tracing: Make uprobe_{trace,perf}_print() uretprobe-friendly

[Oleg Nesterov <oleg@redhat.com>]

On 04/07, Srikar Dronamraju wrote:
>
> * Oleg Nesterov <oleg@redhat.com> [2013-04-01 18:08:51]:
>
> > diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
> > index e91a354..db2718a 100644
> > --- a/kernel/trace/trace_uprobe.c
> > +++ b/kernel/trace/trace_uprobe.c
> > @@ -515,15 +515,26 @@ static void uprobe_trace_print(struct trace_uprobe *tu,
> >  	int size, i;
> >  	struct ftrace_event_call *call = &tu->call;
> >
> > -	size = SIZEOF_TRACE_ENTRY(1) + tu->size;
> > +	if (is_ret_probe(tu))
>
> One nit:
> Here and couple of places below .. we could check for func instead of
> is_ret_probe() right?

Yes we could. And note that we do not really need both uprobe_trace_func()
and uretprobe_perf_func(), we could use a single function and check "func".

But:

> Or is there an advantage of checking is_ret_probe() over func?

I believe yes. Firstly, we can't use 0ul as "invalid func address" to detect
the !is_ret_probe() case, we need, say, -1ul which probably needs a symbolic
name. In fact, I'd prefer to add another "is_return" argument if we want to
avoid is_ret_probe() and unify 2 functions.

But more importantly, I think that is_ret_probe() is much more grep-friendly
and thus more understandable and consistent with other checks which can not
rely on "func".

> >  static int uprobe_trace_func(struct trace_uprobe *tu, struct pt_regs *regs)
> >  {
> > -	uprobe_trace_print(tu, 0, regs);
> > +	if (!is_ret_probe(tu))
> > +		uprobe_trace_print(tu, 0, regs);
>
> Should this hunk be in the previous patch?

Well, I dunno. Even if this hunk goes into the previous patch it won't
make the "print" logic correct until we change uprobe_trace_print(), iow
to me this logically connects to uprobe_trace_print() changed by this patch.

And correctness-wise this doesn't matter, until 6/6 make is_ret_probe() == T
possible we should not worry about the "missed" is_ret_probe() checks.

> Also something for the future:
> Most times a user uses a return probe, the user probably wants to probe
> the function entry too. So should we extend the abi from p+r to
> p+r+..<something else> to mean it traces both function entry and return.
> Esp given that uretprobe has been elegantly been designed to make this a
> possibility.

Oh, perhaps, but this is really for the future. In particular, it is not
clear how we can specify normal-fetchargs + ret-fetchargs.

Oleg.