Re: Call graph dwarf unwinding fails with lld

["Fāng-ruì Sòng" <maskray@google.com>]

On Wed, May 25, 2022 at 12:50 AM Ian Rogers <irogers@google.com> wrote:
>
> On Tue, May 24, 2022 at 1:50 PM Travis Downs <travis.downs@gmail.com> wrote:
> >
> > I have been tracking down an issue locally where `perf record
> > --call-graph=dwarf` stopped working in the sense that unwinding always
> > failed.
> >
> > Turns out it was related to `lld` - binaries linked with `lld` don't
> > seem to unwind correctly with `--call-graph=dwarf`. I've confirmed
> > that the `.eh_frame` sections are there, but I guess they are
> > different after using this linker.
> >
> > Has anyone run into this?
>
> I was unaware but thanks for filing the bug!
>
> > There's an open LLVM issue at https://github.com/llvm/llvm-project/issues/53156
> >
> > ... but no activity since it was filed, though it is not clear if the
> > problem is really with lld or the unwinders.
> >
> > In a related question: perf supports more than one DWARF unwinding
> > implementation, depending on what libraries are available at build
> > time, right? Is there a way to select which unwinder is used at
> > runtime?
>
> I don't think so. There are two build options NO_LIBUNWIND=1 and
> NO_LIBDW_DWARF_UNWIND=1. I played around with the example from the
> bug, confirmed no difference with the eh_frames and hand verified
> them, I also added -fno-omit-frame-pointer -O0 and -static. I've not
> got a good handle on the issue but my suspicion is that the
> eh_frame_hdr is broken - it is at least something that lld generates.
> If I change the eh_frame_hdr finding to some findable but unexpected
> offset like eh_frame here:
> https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/perf/util/unwind-libunwind-local.c?h=perf/core#n286
> and then test with a working linker the unwinding is broken similarly
> to with lld. I was looking for a useful tool (objdump, llvm-objdump,
> readelf, dwarfdump) to look at the eh_frame_hdr but I didn't see one.
> Perhaps someone has a suggestion or maybe pahole can do it. Hopefully
> this gives a lead for you to dig into for the bug.
>
> Thanks,
> Ian

I left a comment on
https://github.com/llvm/llvm-project/issues/53156#issuecomment-1136777149
that the perf issue is related to unsupported `ld.lld --rosegments`
and `ld.bfd -z separate-code` layouts.

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