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

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Date: Mon, 17 Jun 2024 11:12:44 -0700
From: Eduard Zingerman <eddyz87@...il.com>
To: Donglin Peng <dolinux.peng@...il.com>, ast@...nel.org
Cc: 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 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>

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