On Thu, Sep 2, 2021 at 10:08 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
>
> Yonghong Song <yhs@xxxxxx> writes:
>
> > On 8/31/21 3:28 AM, Toke Høiland-Jørgensen wrote:
> >> Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes:
> >>
> >>> On Thu, Aug 26, 2021 at 5:10 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
> >>>>
> >>>> When .eh_frame and .rel.eh_frame sections are present in BPF object files,
> >>>> libbpf produces errors like this when loading the file:
> >>>>
> >>>> libbpf: elf: skipping unrecognized data section(32) .eh_frame
> >>>> libbpf: elf: skipping relo section(33) .rel.eh_frame for section(32) .eh_frame
> >>>>
> >>>> It is possible to get rid of the .eh_frame section by adding
> >>>> -fno-asynchronous-unwind-tables to the compilation, but we have seen
> >>>> multiple examples of these sections appearing in BPF files in the wild,
> >>>> most recently in samples/bpf, fixed by:
> >>>> 5a0ae9872d5c ("bpf, samples: Add -fno-
/to BPF Clang invocation")
> >>>>
> >>>> While the errors are technically harmless, they look odd and confuse users.
> >>>
> >>> These warnings point out invalid set of compiler flags used for
> >>> compiling BPF object files, though. Which is a good thing and should
> >>> incentivize anyone getting those warnings to check and fix how they do
> >>> BPF compilation. Those .eh_frame sections shouldn't be present in BPF
> >>> object files at all, and that's what libbpf is trying to say.
> >>
> >> Apart from triggering that warning, what effect does this have, though?
> >> The programs seem to work just fine (as evidenced by the fact that
> >> samples/bpf has been built this way for years, for instance)...
> >>
> >> Also, how is a user supposed to go from that cryptic error message to
> >> figuring out that it has something to do with compiler flags?
> >>
> >>> I don't know exactly in which situations that .eh_frame section is
> >>> added, but looking at our selftests (and now samples/bpf as well),
> >>> where we use -target bpf, we don't need
> >>> -fno-asynchronous-unwind-tables at all.
> >>
> >> This seems to at least be compiler-dependent. We ran into this with
> >> bpftool as well (for the internal BPF programs it loads whenever it
> >> runs), which already had '-target bpf' in the Makefile. We're carrying
> >> an internal RHEL patch adding -fno-asynchronous-unwind-tables to the
> >> bpftool build to fix this...
> >
> > I haven't seen an instance of .eh_frame as well with -target bpf.
> > Do you have a reproducible test case? I would like to investigate
> > what is the possible cause and whether we could do something in llvm
> > to prevent its generatin. Thanks!
>
> We found this in the RHEL builds of bpftool. I don't think we're doing
> anything special, other than maybe building with a clang version that's
> a few versions behind:
>
> # clang --version
> clang version 11.0.0 (Red Hat 11.0.0-1.module+el8.4.0+8598+a071fcd5)
> Target: x86_64-unknown-linux-gnu
> Thread model: posix
> InstalledDir: /usr/bin
>
> So I suppose it may resolve itself once we upgrade LLVM?
That's odd. I don't think I've seen this issue even with clang 11
(but I built it myself).
If there is a fix indeed let's backport it to llvm 11. The user
experience matters.
It could be llvm configuration too.
I'm guessing some build flags might influence default settings
for unwind tables.
Yonghong, can we make bpf backend to ignore needsUnwindTableEntry ?
