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From: Donglin Peng <dolinux.peng@gmail.com>
To: ast@kernel.org
Cc: daniel@iogearbox.net,
	song@kernel.org,
	andrii@kernel.org,
	eddyz87@gmail.com,
	haoluo@google.com,
	yonghong.song@linux.dev,
	bpf@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Donglin Peng <dolinux.peng@gmail.com>
Subject: [PATCH] libbpf: checking the btf_type kind when fixing variable offsets
Date: Sat, 15 Jun 2024 17:29:58 -0700
Message-Id: <20240616002958.2095829-1-dolinux.peng@gmail.com>
Precedence: bulk
MIME-Version: 1.0
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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@gmail.com>
---
 tools/lib/bpf/linker.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
index 0d4be829551b..7f5fc9ac4ad6 100644
--- a/tools/lib/bpf/linker.c
+++ b/tools/lib/bpf/linker.c
@@ -2213,10 +2213,17 @@ static int linker_fixup_btf(struct src_obj *obj)
 		vi = btf_var_secinfos(t);
 		for (j = 0, m = btf_vlen(t); j < m; j++, vi++) {
 			const struct btf_type *vt = btf__type_by_id(obj->btf, vi->type);
-			const char *var_name = btf__str_by_offset(obj->btf, vt->name_off);
-			int var_linkage = btf_var(vt)->linkage;
+			const char *var_name;
+			int var_linkage;
 			Elf64_Sym *sym;
 
+			/* should be a variable */
+			if (btf_kind(vt) != BTF_KIND_VAR)
+				continue;
+
+			var_name = btf__str_by_offset(obj->btf, vt->name_off);
+			var_linkage = btf_var(vt)->linkage;
+
 			/* no need to patch up static or extern vars */
 			if (var_linkage != BTF_VAR_GLOBAL_ALLOCATED)
 				continue;
-- 
2.25.1