perf kretprobes: Offset from reloc_sym if kernel supports it

We indicate support for accepting sym+offset with kretprobes through a
line in ftrace README. Parse the same to identify support and choose the
appropriate format for kprobe_events.

As an example, without this perf patch, but with the ftrace changes:

  naveen@ubuntu:~/linux/tools/perf$ sudo cat /sys/kernel/debug/tracing/README | grep kretprobe
  place (kretprobe): [<module>:]<symbol>[+<offset>]|<memaddr>
  naveen@ubuntu:~/linux/tools/perf$
  naveen@ubuntu:~/linux/tools/perf$ sudo ./perf probe -v do_open%return
  probe-definition(0): do_open%return
  symbol:do_open file:(null) line:0 offset:0 return:1 lazy:(null)
  0 arguments
  Looking at the vmlinux_path (8 entries long)
  Using /boot/vmlinux for symbols
  Open Debuginfo file: /boot/vmlinux
  Try to find probe point from debuginfo.
  Matched function: do_open [2d0c7d8]
  Probe point found: do_open+0
  Matched function: do_open [35d76b5]
  found inline addr: 0xc0000000004ba984
  Failed to find "do_open%return",
   because do_open is an inlined function and has no return point.
  An error occurred in debuginfo analysis (-22).
  Trying to use symbols.
  Opening /sys/kernel/debug/tracing//kprobe_events write=1
  Writing event: r:probe/do_open do_open+0
  Writing event: r:probe/do_open_1 do_open+0
  Added new events:
    probe:do_open        (on do_open%return)
    probe:do_open_1      (on do_open%return)

  You can now use it in all perf tools, such as:

	  perf record -e probe:do_open_1 -aR sleep 1

  naveen@ubuntu:~/linux/tools/perf$ sudo cat /sys/kernel/debug/kprobes/list
  c000000000041370  k  kretprobe_trampoline+0x0    [OPTIMIZED]
  c0000000004433d0  r  do_open+0x0    [DISABLED]
  c0000000004433d0  r  do_open+0x0    [DISABLED]

And after this patch (and the subsequent powerpc patch):

  naveen@ubuntu:~/linux/tools/perf$ sudo ./perf probe -v do_open%return
  probe-definition(0): do_open%return
  symbol:do_open file:(null) line:0 offset:0 return:1 lazy:(null)
  0 arguments
  Looking at the vmlinux_path (8 entries long)
  Using /boot/vmlinux for symbols
  Open Debuginfo file: /boot/vmlinux
  Try to find probe point from debuginfo.
  Matched function: do_open [2d0c7d8]
  Probe point found: do_open+0
  Matched function: do_open [35d76b5]
  found inline addr: 0xc0000000004ba984
  Failed to find "do_open%return",
   because do_open is an inlined function and has no return point.
  An error occurred in debuginfo analysis (-22).
  Trying to use symbols.
  Opening /sys/kernel/debug/tracing//README write=0
  Opening /sys/kernel/debug/tracing//kprobe_events write=1
  Writing event: r:probe/do_open _text+4469712
  Writing event: r:probe/do_open_1 _text+4956248
  Added new events:
    probe:do_open        (on do_open%return)
    probe:do_open_1      (on do_open%return)

  You can now use it in all perf tools, such as:

	  perf record -e probe:do_open_1 -aR sleep 1

  naveen@ubuntu:~/linux/tools/perf$ sudo cat /sys/kernel/debug/kprobes/list
  c000000000041370  k  kretprobe_trampoline+0x0    [OPTIMIZED]
  c0000000004433d0  r  do_open+0x0    [DISABLED]
  c0000000004ba058  r  do_open+0x8    [DISABLED]
Signed-off-by: NNaveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Acked-by: NMasami Hiramatsu <mhiramat@kernel.org>
Cc: Ananth N Mavinakayanahalli <ananth@linux.vnet.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: linuxppc-dev@lists.ozlabs.org
Link: http://lkml.kernel.org/r/496ef9f33c1ab16286ece9dd62aa672807aef91c.1488961018.git.naveen.n.rao@linux.vnet.ibm.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/probe-event.c

@@ -757,7 +757,9 @@ post_process_kernel_probe_trace_events(struct probe_trace_event *tevs,
 	}
 
 	for (i = 0; i < ntevs; i++) {
-		if (!tevs[i].point.address || tevs[i].point.retprobe)
+		if (!tevs[i].point.address)
+			continue;
+		if (tevs[i].point.retprobe && !kretprobe_offset_is_supported())
 			continue;
 		/* If we found a wrong one, mark it by NULL symbol */
 		if (kprobe_warn_out_range(tevs[i].point.symbol,
@@ -1528,11 +1530,6 @@ static int parse_perf_probe_point(char *arg, struct perf_probe_event *pev)
 		return -EINVAL;
 	}
 
-	if (pp->retprobe && !pp->function) {
-		semantic_error("Return probe requires an entry function.\n");
-		return -EINVAL;
-	}
-
 	if ((pp->offset || pp->line || pp->lazy_line) && pp->retprobe) {
 		semantic_error("Offset/Line/Lazy pattern can't be used with "
 			       "return probe.\n");
@@ -2841,7 +2838,8 @@ static int find_probe_trace_events_from_map(struct perf_probe_event *pev,
 	}
 
 	/* Note that the symbols in the kmodule are not relocated */
-	if (!pev->uprobes && !pp->retprobe && !pev->target) {
+	if (!pev->uprobes && !pev->target &&
+			(!pp->retprobe || kretprobe_offset_is_supported())) {
 		reloc_sym = kernel_get_ref_reloc_sym();
 		if (!reloc_sym) {
 			pr_warning("Relocated base symbol is not found!\n");

tools/perf/util/probe-file.c

@@ -879,6 +879,7 @@ int probe_cache__show_all_caches(struct strfilter *filter)
 
 enum ftrace_readme {
 	FTRACE_README_PROBE_TYPE_X = 0,
+	FTRACE_README_KRETPROBE_OFFSET,
 	FTRACE_README_END,
 };
 
@@ -889,6 +890,7 @@ static struct {
 #define DEFINE_TYPE(idx, pat)			\
 	[idx] = {.pattern = pat, .avail = false}
 	DEFINE_TYPE(FTRACE_README_PROBE_TYPE_X, "*type: * x8/16/32/64,*"),
+	DEFINE_TYPE(FTRACE_README_KRETPROBE_OFFSET, "*place (kretprobe): *"),
 };
 
 static bool scan_ftrace_readme(enum ftrace_readme type)
@@ -939,3 +941,8 @@ bool probe_type_is_available(enum probe_type type)
 
 	return true;
 }
+
+bool kretprobe_offset_is_supported(void)
+{
+	return scan_ftrace_readme(FTRACE_README_KRETPROBE_OFFSET);
+}

tools/perf/util/probe-file.h

@@ -65,6 +65,7 @@ struct probe_cache_entry *probe_cache__find_by_name(struct probe_cache *pcache,
 					const char *group, const char *event);
 int probe_cache__show_all_caches(struct strfilter *filter);
 bool probe_type_is_available(enum probe_type type);
+bool kretprobe_offset_is_supported(void);
 #else	/* ! HAVE_LIBELF_SUPPORT */
 static inline struct probe_cache *probe_cache__new(const char *tgt __maybe_unused)
 {