linux-kernel - [PATCH v2] libbpf: checking the btf_type kind when fixing variable offsets

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Date: Wed, 19 Jun 2024 05:23:55 -0700
From: Donglin Peng <dolinux.peng@...il.com>
To: alan.maguire@...cle.com,
	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,
	Donglin Peng <dolinux.peng@...il.com>
Subject: [PATCH v2] libbpf: checking the btf_type kind when fixing variable offsets

I encountered an issue when building the test_progs from the repository[1]:

$ pwd
/work/Qemu/x86_64/linux-6.10-rc2/tools/testing/selftests/bpf/

$ make test_progs V=1
...
./tools/sbin/bpftool gen object ./ip_check_defrag.bpf.linked2.o ./ip_check_defrag.bpf.linked1.o
libbpf: failed to find symbol for variable 'bpf_dynptr_slice' in section '.ksyms'
Error: failed to link './ip_check_defrag.bpf.linked1.o': No such file or directory (2)
...

Upon investigation, I discovered that the btf_types referenced in the '.ksyms'
section had 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
...

For a detailed analysis, please refer to [2]. We can add a kind checking to
fix the issue.

[1] https://github.com/eddyz87/bpf/tree/binsort-btf-dedup
[2] https://lore.kernel.org/all/0c0ef20c-c05e-4db9-bad7-2cbc0d6dfae7@oracle.com/

Fixes: 8fd27bf69b86 ("libbpf: Add BPF static linker BTF and BTF.ext support")
Acked-by: Eduard Zingerman <eddyz87@...il.com>
Reviewed-by: Alan Maguire <alan.maguire@...cle.com>
Signed-off-by: Donglin Peng <dolinux.peng@...il.com>
---
v2:
 - Refactored the code using btf_is_var
 - Improved the comment to be more reasonable
---
 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..5a583053e311 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;
 
+			/* could be a variable or function */
+			if (!btf_is_var(vt))
+				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