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/[e-mail]/

Fixes: 8fd27bf69b86 ("libbpf: Add BPF static linker BTF and BTF.ext support")
Signed-off-by: Donglin Peng <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Reviewed-by: Alan Maguire <[e-mail]>
Acked-by: Eduard Zingerman <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/lib/bpf/linker.c

···
 		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)