libbpf: provide more helpful message on uninitialized global var

When BPF program defines uninitialized global variable, it's put into
a special COMMON section. Libbpf will reject such programs, but will
provide very unhelpful message with garbage-looking section index.

This patch detects special section cases and gives more explicit error
message.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/libbpf.c

@@ -1760,15 +1760,22 @@ bpf_program__collect_reloc(struct bpf_program *prog, GElf_Shdr *shdr,
 			 (long long) sym.st_value, sym.st_name, name);
 
 		shdr_idx = sym.st_shndx;
+		insn_idx = rel.r_offset / sizeof(struct bpf_insn);
+		pr_debug("relocation: insn_idx=%u, shdr_idx=%u\n",
+			 insn_idx, shdr_idx);
+
+		if (shdr_idx >= SHN_LORESERVE) {
+			pr_warning("relocation: not yet supported relo for non-static global \'%s\' variable in special section (0x%x) found in insns[%d].code 0x%x\n",
+				   name, shdr_idx, insn_idx,
+				   insns[insn_idx].code);
+			return -LIBBPF_ERRNO__RELOC;
+		}
 		if (!bpf_object__relo_in_known_section(obj, shdr_idx)) {
 			pr_warning("Program '%s' contains unrecognized relo data pointing to section %u\n",
 				   prog->section_name, shdr_idx);
 			return -LIBBPF_ERRNO__RELOC;
 		}
 
-		insn_idx = rel.r_offset / sizeof(struct bpf_insn);
-		pr_debug("relocation: insn_idx=%u\n", insn_idx);
-
 		if (insns[insn_idx].code == (BPF_JMP | BPF_CALL)) {
 			if (insns[insn_idx].src_reg != BPF_PSEUDO_CALL) {
 				pr_warning("incorrect bpf_call opcode\n");