[PATCH v3] perf probe: Adjust dso->long_name for offline module

[Wang Nan]

Something unexpected may happen if copy statically linked perf to a
production environment:

 # ./perf probe -m ./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 ./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

Where:

 # ldd ./perf
	not a dynamic executable
 # strace -e open ./perf probe -m ./mymodule.ko my_func
 ...
 open("/home/wangnan/kmodule/mymodule.ko", O_RDONLY) = 3
 open("/home/wangnan/kmodule/../lib64/elfutils/libebl_x86_64.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
 ...
 open("/lib64/tls/libebl_x86_64.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
 open("/lib64/libebl_x86_64.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
 open("/usr/lib64/tls/libebl_x86_64.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
 open("/usr/lib64/libebl_x86_64.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
 open("[mymodule]", O_RDONLY)            = -1 ENOENT (No such file or directory)
 open("/home/wangnan/.debug/.build-id/32/6ab42550ef3d24944f53c817533728367effeb", O_RDONLY) = -1 ENOENT (No such file or directory)
 open("[mymodule]", O_RDONLY)            = -1 ENOENT (No such file or directory)

In the above example, probe fails before we put the module into
buildid-cache. However, user would expect it success in both case
because perf is able to find probe points actually.

The reason 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/module before it knows the real
path of the offline module. Therefore, the map->dso->long_name is
set to something like '[mymodule]', which prevent 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 improves machine__findnew_module_map(): when dso->long_name
is leading with '[' (doesn't find path of module when parsing
/proc/modules), fixes it by dso__set_long_name(), so following
dso__load() is possible to find the symbol table.

This patch won't interfere with buildid matching. Here is the test
result:

 # ./perf probe -m ./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

 # ./perf probe -d '*'
 Removed event: probe:my_func
 # mv ./mymodule.{ko,.bak}
 # mv ./moduleb.ko mymodule.ko
 # ./perf probe -m ./mymodule.ko my_func
 /home/wangnan/kmodule/mymodule.ko 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 probe -v -m ./mymodule.ko my_func
 probe-definition(0): my_func
 symbol:my_func file:(null) line:0 offset:0 return:0 lazy:(null)
 0 arguments
 Could not open debuginfo. Try to use symbols.
 symsrc__init: build id mismatch for /home/wangnan/kmodule/mymodule.ko.
 /home/wangnan/kmodule/mymodule.ko 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. Reason: No such file or directory (Code: -2)

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
---

v2 -> v3: pass dso to adjust_dso_long_name() instead of map
          because adjust_dso_long_name() doesn't use other part of map.

---
 tools/perf/util/machine.c | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
index 7f5071a..5781992 100644
--- a/tools/perf/util/machine.c
+++ b/tools/perf/util/machine.c
@@ -561,6 +561,24 @@ int machine__process_switch_event(struct machine *machine __maybe_unused,
 	return 0;
 }
 
+static void adjust_dso_long_name(struct dso *dso, const char *filename)
+{
+	const char *dup_filename;
+
+	if (!filename || !dso || !dso->long_name)
+		return;
+	if (dso->long_name[0] != '[')
+		return;
+	if (!strchr(filename, '/'))
+		return;
+
+	dup_filename = strdup(filename);
+	if (!dup_filename)
+		return;
+
+	dso__set_long_name(dso, filename, true);
+}
+
 struct map *machine__findnew_module_map(struct machine *machine, u64 start,
 					const char *filename)
 {
@@ -573,8 +591,15 @@ struct map *machine__findnew_module_map(struct machine *machine, u64 start,
 
 	map = map_groups__find_by_name(&machine->kmaps, MAP__FUNCTION,
 				       m.name);
-	if (map)
+	if (map) {
+		/*
+		 * 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.
+		 */
+		adjust_dso_long_name(map->dso, filename);
 		goto out;
+	}
 
 	dso = machine__findnew_module_dso(machine, &m, filename);
 	if (dso == NULL)
-- 
1.8.3.4

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