linux-kernel - Re: [PATCH 4/4] perf-probe: Warn if the target function is GNU Indirect function

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Date:   Fri, 10 Jul 2020 08:55:40 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Andi Kleen <andi@...stfloor.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 4/4] perf-probe: Warn if the target function is GNU
 Indirect function

Em Fri, Jul 10, 2020 at 12:30:08PM +0900, Masami Hiramatsu escreveu:
> On Thu, 9 Jul 2020 07:36:54 -0700
> Andi Kleen <andi@...stfloor.org> wrote:
> 
> > > diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
> > > index 1e95a336862c..671176d39569 100644
> > > --- a/tools/perf/util/probe-event.c
> > > +++ b/tools/perf/util/probe-event.c
> > > @@ -379,6 +379,11 @@ static int find_alternative_probe_point(struct debuginfo *dinfo,
> > >  			address = sym->start;
> > >  		else
> > >  			address = map->unmap_ip(map, sym->start) - map->reloc;
> > > +		if (sym->type == STT_GNU_IFUNC) {
> > > +			pr_warning("Warning: The probe address (0x%lx) is in a GNU indirect function.\n"
> > > +				"This may not work as you expected unless you intend to probe the indirect function.\n",
> > 
> > I would say something like this.
> > 
> > Consider identifying the final function used at run time and set the
> > probe directly on that.
> > 
> > I think that's more useful to the user.
> 
> Hmm, would you mean the default function which may be used for the symbol?

Humm, I think he means that the user must somehow, knowing details
involved in picking the final function, probe that one instead of the
IFUNC one, right Andi?

- Arnaldo

> Let me check how we can find it.