On 9/2/21 12:32 PM, Alexei Starovoitov wrote:

On Thu, Sep 2, 2021 at 10:08 AM Toke Høiland-Jørgensen [off-list ref] wrote:

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Yonghong Song [off-list ref] writes:

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On 8/31/21 3:28 AM, Toke Høiland-Jørgensen wrote:

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Andrii Nakryiko [off-list ref] writes:

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On Thu, Aug 26, 2021 at 5:10 AM Toke Høiland-Jørgensen [off-list ref] wrote:

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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")

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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?

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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).

I cannot reproduce it by self with self built llvm (11, 12, 13, 14).
But I can reproduce it with an upstream built llvm12.

/bin/clang \
         -I. \
         -I/home/yhs/work/bpf-next/tools/include/uapi/ \
         -I/home/yhs/work/bpf-next/tools/lib/bpf/ \
         -I/home/yhs/work/bpf-next/tools/lib \
         -g -O2 -Wall -target bpf -c skeleton/pid_iter.bpf.c -o 
pid_iter.bpf.o && llvm-strip -g pid_iter.bpf.o
   GEN     pid_iter.skel.h
libbpf: elf: skipping unrecognized data section(11) .eh_frame
libbpf: elf: skipping relo section(12) .rel.eh_frame for section(11) 
.eh_frame

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 ?

Sure. I will try to get upstream build flags, reproduce and fix it
in llvm.