Re: libdw dwarf unwind issue

[Jiri Olsa <jolsa@redhat.com>]

On Sun, Nov 22, 2020 at 11:27:51PM +0100, Jan Kratochvil wrote:

SNIP

>              --1.15%--iothread_run
>                        aio_poll
>                        qemu_poll_ns
> 
> 
> > any idea why libdw is doing that? ;-)
> 
> Because libdw was reporting separate debuginfo file as the main file:
> 
> (lldb) p thread->process->dwfl->modulelist->main.name
> (char *) $1 = 0x0000000000b52490 "/home/jkratoch/.debug/.build-id/fc/d0111443729957fa8ef9511ff5dc3e9a5ccf9d/debug"
> (lldb) p thread->process->dwfl->modulelist->debug.name
> (char *) $2 = 0x0000000000000000
> 
> The correct way is to report the main ELF file as the main file:
> 
> (lldb) p thread->process->dwfl->modulelist->main.name
> (char *) $1 = 0x0000000000b52490 "/home/jkratoch/.debug/.build-id/fc/d0111443729957fa8ef9511ff5dc3e9a5ccf9d/elf"
> (lldb) p thread->process->dwfl->modulelist->debug.name
> (char *) $2 = 0x0000000000000000
> 
> In some cases it may later fill-in even the ->debug.name with "/debug" filename.

hi,
it fixes the issue for me, but it breaks the dwarf test:

72: DWARF unwind                                                    : FAILED!
[jolsa@krava perf]$ sudo ./perf test dwarf -v
72: DWARF unwind                                                    :
--- start ---
test child forked, pid 2084484
unwind: failed with 'No such file or directory'
got wrong number of stack entries 0 != 8
test child finished with -1
---- end ----
DWARF unwind: FAILED!

thanks,
jirka