Re: [PATCH bpf-next v2] libbpf: ignore .eh_frame sections when parsing elf files

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:

> On Thu, Aug 26, 2021 at 5:10 AM Toke Høiland-Jørgensen <toke@redhat.com> 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-asynchronous-unwind-tables 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...

> So instead of hiding the problem, let's use this as an opportunity to
> fix those user's compilation flags instead.

This really doesn't seem like something that's helping anyone, it's just
annoying and confusing users...

-Toke