libbpf: Split CO-RE logic into relo_core.c.

Move CO-RE logic into separate file.
The internal interface between libbpf and CO-RE is through
bpf_core_apply_relo_insn() function and few structs defined in relo_core.h.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210721000822.40958-5-alexei.starovoitov@gmail.com

tools/lib/bpf/Build

 libbpf-y := libbpf.o bpf.o nlattr.o btf.o libbpf_errno.o str_error.o \
 	    netlink.o bpf_prog_linfo.o libbpf_probes.o xsk.o hashmap.o \
-	    btf_dump.o ringbuf.o strset.o linker.o gen_loader.o
+	    btf_dump.o ringbuf.o strset.o linker.o gen_loader.o relo_core.o

tools/lib/bpf/libbpf_internal.h

@@ -425,4 +425,14 @@ static inline void *libbpf_ptr(void *ret)
 	return ret;
 }
 
+static inline bool str_is_empty(const char *s)
+{
+	return !s || !s[0];
+}
+
+static inline bool is_ldimm64_insn(struct bpf_insn *insn)
+{
+	return insn->code == (BPF_LD | BPF_IMM | BPF_DW);
+}
+
 #endif /* __LIBBPF_LIBBPF_INTERNAL_H */

tools/lib/bpf/relo_core.h

@@ -75,8 +75,7 @@ struct bpf_core_relo {
 	enum bpf_core_relo_kind kind;
 };
 
-struct bpf_core_cand
-{
+struct bpf_core_cand {
 	const struct btf *btf;
 	const struct btf_type *t;
 	const char *name;
@@ -89,4 +88,13 @@ struct bpf_core_cand_list {
 	int len;
 };
 
+int bpf_core_apply_relo_insn(const char *prog_name,
+			     struct bpf_insn *insn, int insn_idx,
+			     const struct bpf_core_relo *relo, int relo_idx,
+			     const struct btf *local_btf,
+			     struct bpf_core_cand_list *cands);
+int bpf_core_types_are_compat(const struct btf *local_btf, __u32 local_id,
+			      const struct btf *targ_btf, __u32 targ_id);
+
+size_t bpf_core_essential_name_len(const char *name);
 #endif