libbpf: Preliminary support for fd_idx

Prep libbpf to use FD_IDX kernel feature when generating loader program.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210514003623.28033-14-alexei.starovoitov@gmail.com

tools/lib/bpf/libbpf.c

@@ -412,6 +412,8 @@ struct module_btf {
 	int fd;
 };
 
+struct bpf_gen;
+
 struct bpf_object {
 	char name[BPF_OBJ_NAME_LEN];
 	char license[64];
@@ -432,6 +434,8 @@ struct bpf_object {
 	bool loaded;
 	bool has_subcalls;
 
+	struct bpf_gen *gen_loader;
+
 	/*
 	 * Information when doing elf related work. Only valid if fd
 	 * is valid.
@@ -6388,19 +6392,34 @@ bpf_object__relocate_data(struct bpf_object *obj, struct bpf_program *prog)
 
 		switch (relo->type) {
 		case RELO_LD64:
+			if (obj->gen_loader) {
+				insn[0].src_reg = BPF_PSEUDO_MAP_IDX;
+				insn[0].imm = relo->map_idx;
+			} else {
 				insn[0].src_reg = BPF_PSEUDO_MAP_FD;
 				insn[0].imm = obj->maps[relo->map_idx].fd;
+			}
 			break;
 		case RELO_DATA:
-			insn[0].src_reg = BPF_PSEUDO_MAP_VALUE;
 			insn[1].imm = insn[0].imm + relo->sym_off;
+			if (obj->gen_loader) {
+				insn[0].src_reg = BPF_PSEUDO_MAP_IDX_VALUE;
+				insn[0].imm = relo->map_idx;
+			} else {
+				insn[0].src_reg = BPF_PSEUDO_MAP_VALUE;
 				insn[0].imm = obj->maps[relo->map_idx].fd;
+			}
 			break;
 		case RELO_EXTERN_VAR:
 			ext = &obj->externs[relo->sym_off];
 			if (ext->type == EXT_KCFG) {
+				if (obj->gen_loader) {
+					insn[0].src_reg = BPF_PSEUDO_MAP_IDX_VALUE;
+					insn[0].imm = obj->kconfig_map_idx;
+				} else {
 					insn[0].src_reg = BPF_PSEUDO_MAP_VALUE;
 					insn[0].imm = obj->maps[obj->kconfig_map_idx].fd;
+				}
 				insn[1].imm = ext->kcfg.data_off;
 			} else /* EXT_KSYM */ {
 				if (ext->ksym.type_id) { /* typed ksyms */