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

["Wangnan (F)" <wangnan0@huawei.com>]

On 2015/11/26 9:10, 平松雅巳 / HIRAMATU，MASAMI wrote:
> From: Wang Nan [mailto:wangnan0@huawei.com]
>> 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}
> Please do give more possible usecase. removing libebl is crazy,
> and broken environment.

It is a real problem we met, where perf itself is statically linked
and copied to target platform.

> If you'd like to use perf probe without debuginfo, make NO_DWARF=1 or
> strip the target binary.

Can't simply stripping the target binary because the symtab would
also be stipped out.

>> # ./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.
> Hmm, if so, it should be fixed in map or machine, not in probe-event.c.

Will do in next version.

Thank you.