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

[Wangnan (F)]

On 2015/11/26 9:10, åæéå / HIRAMATUïMASAMI wrote:
> From: Wang Nan [mailto:wangnan0@xxxxxxxxxx]
> > 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.

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