perf probe: Fix return probe support

Fix a bug to support %return probe syntax again. Previous commit 4235b045 has a
bug which disables the %return syntax on perf probe.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <20100827113852.22882.87447.stgit@ltc236.sdl.hitachi.co.jp>
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/probe-event.c

@@ -1539,6 +1539,7 @@ static int convert_to_probe_trace_events(struct perf_probe_event *pev,
 		goto error;
 	}
 	tev->point.offset = pev->point.offset;
+	tev->point.retprobe = pev->point.retprobe;
 	tev->nargs = pev->nargs;
 	if (tev->nargs) {
 		tev->args = zalloc(sizeof(struct probe_trace_arg)

tools/perf/util/probe-finder.c

@@ -783,6 +783,16 @@ static int convert_probe_point(Dwarf_Die *sp_die, struct probe_finder *pf)
 		/* This function has no name. */
 		tev->point.offset = (unsigned long)pf->addr;
 
+	/* Return probe must be on the head of a subprogram */
+	if (pf->pev->point.retprobe) {
+		if (tev->point.offset != 0) {
+			pr_warning("Return probe must be on the head of"
+				   " a real function\n");
+			return -EINVAL;
+		}
+		tev->point.retprobe = true;
+	}
+
 	pr_debug("Probe point found: %s+%lu\n", tev->point.symbol,
 		 tev->point.offset);