Missing .gnu_debugdata section on ARM platform when libdw is used by systemd-coredump

[Mark Wielaard]

Hi Tino,

On Tue, 2021-05-04 at 14:15 +0200, Tino Mettler via Elfutils-devel
wrote:
> I have a system running on 2 different architectures: AMD64 and ARM.
> When a coredump happens, I want systemd-coredump to generate a stack
> trace of the crashing application. Systemd depends on elfutils for
> this feature. I use binaries with minidebuginfo (the LZMA compressed
> symbol table in the .gnu_debugdata section) on both architectures.
> 
> I get a stack trace on AMD64, but not on ARM. While debugging this, I
> saw that find_aux_sym() from dwfl_module_getdwarf.c does not find a
> .gnu_debugdata section when iterating through the ELF using
> elf_nextscn().
> 
> However, it finds a .gnu_debuglink section. I inspected the
> executable and verified that it contains a .gnu_debugdata section and
> it looks fine. Interestingly, there is no .gnu_debuglink section in
> the executable despite elf_nextscn() seems to find one.
> 
> It looks like libdw does not process the actual executable, but a
> modified variant, and the .gnu_debugdata section gets lost at some
> point on my ARM device. Can you give me a hint where I need to look
> at? Both devices run a different kernel with a different
> configuration. Could that be related?
> 
> I also tried gdb using the coredump file from systemd-coredump and
> the same executable, and a stack trace worked as expected.

If possible you might want to post the core file and/or executables
somewhere so others can inspect them. I am not completely sure how the
.gnu_debugdata (aux symbols) is related. Are you getting a backtrace,
but not function symbols for the addresses?

For ARM it might depend on whether the executable contains an .eh_frame
sections. If it has (or the .debug file has an .debug_frame section)
then libdw should be able to produce a backtrace.

But there are also ARM binaries which only come with IDX data, which
libdw doesn't handle.

Cheers,

Mark