[PATCH] perf probe: Adjust dso->long_name for offline module

[Wang Nan]

If libelf unable to open debuginfo for an offline module but the ko has
symtab, something unexpected may happen.

 # rm -rf ~/.debug/
 # mv /usr/lib64/elfutils/libebl_x86_64.so{,.bak}
 # ./perf probe -m /home/wangnan/kmodule/mymodule.ko my_func
 [mymodule] with build id 326ab42550ef3d24944f53c817533728367effeb not found, continuing without symbols
 Failed to find symbol my_func in /home/wangnan/kmodule/mymodule.ko
   Error: Failed to add events
 # ./perf buildid-cache -a ./mymodule.ko
 # ./perf probe -m /home/wangnan/kmodule/mymodule.ko my_func
 Added new event:
   probe:my_func        (on my_func in /home/wangnan/kmodule/mymodule.ko)

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

	 perf record -e probe:my_func -aR sleep 1

In the above example, probe fails if it isn't in buildid-cache. However,
user would expect it success in both case because perf is able to find
probe points actually.

The problem is because perf won't utilize module's full path if it
failed to open debuginfo. In
 convert_to_probe_trace_events ->
 find_probe_trace_events_from_map ->
 get_target_map ->
 kernel_get_module_map ->
 machine__findnew_module_map ->
 map_groups__find_by_name

map_groups__find_by_name() is able to find the map of that module, but
this information is found from /proc/modules before it knows the real
path of the offline module. Therefore, the map->dso->long_name is
set to something like '[mymodule]', which prevents dso__load() find
the real path of the module file.

In another aspect, if dso__load() can get the offline module through
buildid cache, it can read symble table from that ko. Even if debuginfo
is not available, 'perf probe' can success if the '.symtab' can be
found.

This patch fixes long_name so dso__load() is able to find module's path
and read symbol table in this case.

After this patch:

 # rm -rf ~/.debug/
 # mv /usr/lib64/elfutils/libebl_x86_64.so{,.bak}
 # ./perf probe -m /home/wangnan/kmodule/mymodule.ko my_func
 Added new event:
   probe:my_func        (on my_func in /home/wangnan/kmodule/mymodule.ko)

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

 	perf record -e probe:my_func -aR sleep 1

 # mv /usr/lib64/elfutils/libebl_x86_64.so{.bak,}

Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
Cc: Zefan Li <lizefan@xxxxxxxxxx>
Cc: pi3orama@xxxxxxx
---
 tools/perf/util/probe-event.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
index 93996ec..ea4f79f 100644
--- a/tools/perf/util/probe-event.c
+++ b/tools/perf/util/probe-event.c
@@ -2516,6 +2516,7 @@ static int find_probe_trace_events_from_map(struct perf_probe_event *pev,
 	struct probe_trace_point *tp;
 	int num_matched_functions;
 	int ret, i, j, skipped = 0;
+	const char *dup_filename;
 
 	map = get_target_map(pev->target, pev->uprobes);
 	if (!map) {
@@ -2523,6 +2524,21 @@ static int find_probe_trace_events_from_map(struct perf_probe_event *pev,
 		goto out;
 	}
 
+	/*
+	 * If the map's dso is an offline module, give dso__load() a chance
+	 * to find the file path of that module by fixing long_name.
+	 */
+	if (map->dso && strchr(pev->target, '/')) {
+		if (!map->dso->long_name || map->dso->long_name[0] == '[') {
+			dup_filename = strdup(pev->target);
+			if (!dup_filename) {
+				ret = -ENOMEM;
+				goto out;
+			}
+			dso__set_long_name(map->dso, dup_filename, true);
+		}
+	}
+
 	syms = malloc(sizeof(struct symbol *) * probe_conf.max_probes);
 	if (!syms) {
 		ret = -ENOMEM;
-- 
1.8.3.4

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