[PATCH] perf probe: Improve error message when function not found

[David Ahern <dsahern@gmail.com>]

When requesting a function from a userspace library the error message to
the user is less than helpful. e.g.,

    perf probe -x /lib64/libpthread-2.14.90.so -a 'lock_full=__pthread_mutex_lock_full'
    no symbols found in /lib64/libpthread-2.14.90.so, maybe install a debug package?
    Failed to load map.
       Error: Failed to add events. (-22)

yet the symbol really does exist but is a local symbol which is filtered:

nm /lib64/libpthread-2.14.90.so | grep __pthread_mutex_lock_full
0000000000005700 t __pthread_mutex_lock_full

With this patch:
    perf probe -x /lib64/libpthread-2.14.90.so -a 'lock_full=__pthread_mutex_lock_full'

    no symbols found in /lib64/libpthread-2.14.90.so, maybe install a debug package?
    Failed to find function in /lib64/libpthread-2.14.90.so. Perhaps it is a local variable?
      Error: Failed to add events. (-22)

Signed-off-by: David Ahern <dsahern@gmail.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
index 9c6989ca2bea..2eac49096840 100644
--- a/tools/perf/util/probe-event.c
+++ b/tools/perf/util/probe-event.c
@@ -2309,7 +2309,8 @@ static int convert_name_to_addr(struct perf_probe_event *pev, const char *exec)
 	}
 	available_func_filter = strfilter__new(function, NULL);
 	if (map__load(map, filter_available_functions)) {
-		pr_err("Failed to load map.\n");
+		pr_err("Failed to find requested symbol in %s. Is it a global variable?\n",
+		       name);
 		goto out;
 	}
 
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1.8.3.4 (Apple Git-47)