libbpf: Allow WEAK and GLOBAL bindings during BTF fixup

During BTF fix up for global variables, global variable can be global
weak and will have STB_WEAK binding in ELF. Support such global
variables in addition to non-weak ones.

This is not the problem when using BPF static linking, as BPF static
linker "fixes up" BTF during generation so that libbpf doesn't have to
do it anymore during bpf_object__open(), which led to this not being
noticed for a while, along with a pretty rare (currently) use of __weak
variables and maps.

Reported-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220407230446.3980075-2-andrii@kernel.org

Author: anakryiko

tools/lib/bpf/libbpf.c

@@ -1401,8 +1401,11 @@ static int find_elf_var_offset(const struct bpf_object *obj, const char *name, _
 	for (si = 0; si < symbols->d_size / sizeof(Elf64_Sym); si++) {
 		Elf64_Sym *sym = elf_sym_by_idx(obj, si);
 
-		if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL ||
-		    ELF64_ST_TYPE(sym->st_info) != STT_OBJECT)
+		if (ELF64_ST_TYPE(sym->st_info) != STT_OBJECT)
+			continue;
+
+		if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL &&
+		    ELF64_ST_BIND(sym->st_info) != STB_WEAK)
 			continue;
 
 		sname = elf_sym_str(obj, sym->st_name);