libbpf: Support .text sub-calls relocations

The LLVM patch https://reviews.llvm.org/D72197 makes LLVM emit function call
relocations within the same section. This includes a default .text section,
which contains any BPF sub-programs. This wasn't the case before and so libbpf
was able to get a way with slightly simpler handling of subprogram call
relocations.

This patch adds support for .text section relocations. It needs to ensure
correct order of relocations, so does two passes:
- first, relocate .text instructions, if there are any relocations in it;
- then process all the other programs and copy over patched .text instructions
for all sub-program calls.

v1->v2:
- break early once .text program is processed.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200115190856.2391325-1-andriin@fb.com

tools/lib/bpf/libbpf.c

@@ -4692,13 +4692,7 @@ bpf_program__reloc_text(struct bpf_program *prog, struct bpf_object *obj,
 	size_t new_cnt;
 	int err;
 
-	if (prog->idx == obj->efile.text_shndx) {
-		pr_warn("relo in .text insn %d into off %d (insn #%d)\n",
-			relo->insn_idx, relo->sym_off, relo->sym_off / 8);
-		return -LIBBPF_ERRNO__RELOC;
-	}
-
-	if (prog->main_prog_cnt == 0) {
+	if (prog->idx != obj->efile.text_shndx && prog->main_prog_cnt == 0) {
 		text = bpf_object__find_prog_by_idx(obj, obj->efile.text_shndx);
 		if (!text) {
 			pr_warn("no .text section found yet relo into text exist\n");
@@ -4728,6 +4722,7 @@ bpf_program__reloc_text(struct bpf_program *prog, struct bpf_object *obj,
 			 text->insns_cnt, text->section_name,
 			 prog->section_name);
 	}
+
 	insn = &prog->insns[relo->insn_idx];
 	insn->imm += relo->sym_off / 8 + prog->main_prog_cnt - relo->insn_idx;
 	return 0;
@@ -4807,8 +4802,28 @@ bpf_object__relocate(struct bpf_object *obj, const char *targ_btf_path)
 			return err;
 		}
 	}
+	/* ensure .text is relocated first, as it's going to be copied as-is
+	 * later for sub-program calls
+	 */
+	for (i = 0; i < obj->nr_programs; i++) {
+		prog = &obj->programs[i];
+		if (prog->idx != obj->efile.text_shndx)
+			continue;
+
+		err = bpf_program__relocate(prog, obj);
+		if (err) {
+			pr_warn("failed to relocate '%s'\n", prog->section_name);
+			return err;
+		}
+		break;
+	}
+	/* now relocate everything but .text, which by now is relocated
+	 * properly, so we can copy raw sub-program instructions as is safely
+	 */
 	for (i = 0; i < obj->nr_programs; i++) {
 		prog = &obj->programs[i];
+		if (prog->idx == obj->efile.text_shndx)
+			continue;
 
 		err = bpf_program__relocate(prog, obj);
 		if (err) {