Re: A scenario where Perf cannot unwind the user stack by dwarf with libdw.so

[Mark Wielaard <mark@klomp.org>]

Hi,

On Mon, Feb 13, 2023 at 01:44:39AM +0000, liuxu (AJ) via Elfutils-devel wrote:
> I am failing to unwind the user stack with libdw on Ubuntu 18.04
> arm64 environment, the toolchain used is clang 12.0.1.  form the
> unwinding process, I found that the pc value passing to handle_cfi
> is 0x760, which is 0x10000 offset from the corresponding cfi address
> 0x10760.
>
> Futher analysis show that the bias calculated in
> __libdwfl_elf_address_range for ET_DYN type pick the first PT_LOAD
> segment, not the segment corresponds to code segment. So the bias
> result is wrong in current situation.

I have some problem replicating this because I don't have the same
toolchain and don't know enough about perf to know how to replicate.

So if I understand correctly you are using clang and lld to link
against a musl based libc/ld.so on aarch64? Does this generate PIE
executables?

Have you tried a different setup? I expect the issue is the PT_LOAD
segments generated by lld are different from ld (bfd). But it would be
good to know if this is the difference that makes things fail.

Are you able to show the issue using eu-stack instead of perf?

If not, could you show how to see the issue with perf?

> The actual parameters of handle_cfi when I report:
> Breakpoint 1, handle_cfi (state=state@entry=0xaaaaaadc45b0, pc=1888, cfi=0xaaaaaadc4900, bias=757661696) at frame_unwind.c:540
> warning: Source file is more recent than executable.

Could you show the backtrace at this point?  (Note that gdb believes
you have changed the sources, but haven't rebuild libdw.so)

Thanks,

Mark