linux-kernel - Re: [PATCH] libbpf: checking the btf_type kind when fixing variable offsets

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Message-ID: <CAErzpmvLeSkL3yTPoZyXiFVGRVFPoAa_4k=0Vtp6eu_Zu3c5Aw@mail.gmail.com>
Date: Wed, 19 Jun 2024 13:03:56 +0800
From: Donglin Peng <dolinux.peng@...il.com>
To: Eduard Zingerman <eddyz87@...il.com>
Cc: ast@...nel.org, daniel@...earbox.net, song@...nel.org, andrii@...nel.org, 
	haoluo@...gle.com, yonghong.song@...ux.dev, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] libbpf: checking the btf_type kind when fixing variable offsets

On Tue, Jun 18, 2024 at 2:12 AM Eduard Zingerman <eddyz87@...il.com> wrote:
>
> On Sat, 2024-06-15 at 17:29 -0700, Donglin Peng wrote:
> > I encountered an issue when building the test_progs using the repository[1]:
> >
> > $ clang --version
> > Ubuntu clang version 17.0.6 (++20231208085846+6009708b4367-1~exp1~20231208085949.74)
> > Target: x86_64-pc-linux-gnu
> > Thread model: posix
> > InstalledDir: /usr/bin
> >
> > $ pwd
> > /work/Qemu/x86_64/linux-6.10-rc2/tools/testing/selftests/bpf/
> >
> > $ make test_progs V=1
> > ...
> > /work/Qemu/x86_64/linux-6.10-rc2/tools/testing/selftests/bpf/tools/sbin/bpftool
> > gen object
> > /work/Qemu/x86_64/linux-6.10-rc2/tools/testing/selftests/bpf/ip_check_defrag.bpf.linked2.o
> > /work/Qemu/x86_64/linux-6.10-rc2/tools/testing/selftests/bpf/ip_check_defrag.bpf.linked1.o
> > libbpf: failed to find symbol for variable 'bpf_dynptr_slice' in section
> > '.ksyms'
> > Error: failed to link
> > '/work/Qemu/x86_64/linux-6.10-rc2/tools/testing/selftests/bpf/ip_check_defrag.bpf.linked1.o':
> > No such file or directory (2)
> > make: *** [Makefile:656:
> > /work/Qemu/x86_64/linux-6.10-rc2/tools/testing/selftests/bpf/ip_check_defrag.skel.h]
> > Error 254
> >
> > After investigation, I found that the btf_types in the '.ksyms' section have a kind of
> > BTF_KIND_FUNC instead of BTF_KIND_VAR:
> >
> > $ bpftool btf dump file ./ip_check_defrag.bpf.linked1.o
> > ...
> > [2] DATASEC '.ksyms' size=0 vlen=2
> >         type_id=16 offset=0 size=0 (FUNC 'bpf_dynptr_from_skb')
> >         type_id=17 offset=0 size=0 (FUNC 'bpf_dynptr_slice')
> > ...
> > [16] FUNC 'bpf_dynptr_from_skb' type_id=82 linkage=extern
> > [17] FUNC 'bpf_dynptr_slice' type_id=85 linkage=extern
> > ...
> >
> > To fix this, we can a add check for the kind.
> >
> > [1] https://github.com/eddyz87/bpf/tree/binsort-btf-dedup
> > Link: https://lore.kernel.org/all/4f551dc5fc792936ca364ce8324c0adea38162f1.camel@gmail.com/
> >
> > Fixes: 8fd27bf69b86 ("libbpf: Add BPF static linker BTF and BTF.ext support")
> > Signed-off-by: Donglin Peng <dolinux.peng@...il.com>
> > ---
>
> Good catch, thank you for narrowing this down.
>
> Acked-by: Eduard Zingerman <eddyz87@...il.com>

Thanks, I will update in v2.

>
> (Although, I agree with notes from Alan, having a comment would be good).