[PATCH 1/2] perf/probe: Report possible permission error for map__load failure

[Masami Hiramatsu <mhiramat@kernel.org>]

Report possible permission error including kptr_restrict setting
for map__load() failure. This can happen when non-superuser runs
perf probe.

With this patch, perf probe shows the following message.

 $ perf probe vfs_read
 Failed to load symbols from /proc/kallsyms
 Please ensure you can read the /proc/kallsyms symbol addresses.
 If the /proc/sys/kernel/kptr_restrict is '2', you can not read
 kernel symbol address even if you are a superuser. Please change
 it to '1'. If kptr_restrict is '1', the superuser can read the
 symbol addresses.
 In that case, please run this command again with sudo.
   Error: Failed to add events.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
 tools/perf/util/probe-event.c |   25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
index 3a7649835ec9..8fe179d671c3 100644
--- a/tools/perf/util/probe-event.c
+++ b/tools/perf/util/probe-event.c
@@ -2936,7 +2936,7 @@ static int find_probe_functions(struct map *map, char *name,
 	bool cut_version = true;
 
 	if (map__load(map) < 0)
-		return 0;
+		return -EACCES;	/* Possible permission error to load symbols */
 
 	/* If user gives a version, don't cut off the version from symbols */
 	if (strchr(name, '@'))
@@ -2975,6 +2975,17 @@ void __weak arch__fix_tev_from_maps(struct perf_probe_event *pev __maybe_unused,
 				struct map *map __maybe_unused,
 				struct symbol *sym __maybe_unused) { }
 
+
+static void pr_kallsyms_access_error(void)
+{
+	pr_err("Please ensure you can read the /proc/kallsyms symbol addresses.\n"
+	       "If the /proc/sys/kernel/kptr_restrict is '2', you can not read\n"
+	       "kernel symbol address even if you are a superuser. Please change\n"
+	       "it to '1'. If kptr_restrict is '1', the superuser can read the\n"
+	       "symbol addresses.\n"
+	       "In that case, please run this command again with sudo.\n");
+}
+
 /*
  * Find probe function addresses from map.
  * Return an error or the number of found probe_trace_event
@@ -3011,8 +3022,16 @@ static int find_probe_trace_events_from_map(struct perf_probe_event *pev,
 	 */
 	num_matched_functions = find_probe_functions(map, pp->function, syms);
 	if (num_matched_functions <= 0) {
-		pr_err("Failed to find symbol %s in %s\n", pp->function,
-			pev->target ? : "kernel");
+		if (num_matched_functions == -EACCES) {
+			pr_err("Failed to load symbols from %s\n",
+			       pev->target ?: "/proc/kallsyms");
+			if (pev->target)
+				pr_err("Please ensure the file is not stripped.\n");
+			else
+				pr_kallsyms_access_error();
+		} else
+			pr_err("Failed to find symbol %s in %s\n", pp->function,
+				pev->target ? : "kernel");
 		ret = -ENOENT;
 		goto out;
 	} else if (num_matched_functions > probe_conf.max_probes) {