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: 8fd27bf6 ("libbpf: Add BPF static linker BTF and BTF.ext support")
Signed-off-by: Donglin Peng <dolinux.peng@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20240619122355.426405-1-dolinux.peng@gmail.com

tools/lib/bpf/linker.c

@@ -2227,10 +2227,17 @@ 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;