Re: [PATCH bpf-next v4 6/8] bpftool: Implement btfgen()

btfgen() receives the path of a source and destination BTF files and a
list of BPF objects. This function records the relocations for all
objects and then generates the BTF file by calling btfgen_get_btf()
(implemented in the following commits).

btfgen_record_obj() loads the BTF and BTF.ext sections of the BPF
objects and loops through all CO-RE relocations. It uses
bpf_core_calc_relo_insn() from libbpf and passes the target spec to
btfgen_record_reloc() that saves the types involved in such relocation.

Signed-off-by: Mauricio Vásquez <redacted>
Signed-off-by: Rafael David Tinoco <redacted>
Signed-off-by: Lorenzo Fontana <redacted>
Signed-off-by: Leonardo Di Donato <redacted>
---
 tools/bpf/bpftool/gen.c | 221 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 219 insertions(+), 2 deletions(-)

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index 905ab0ee6542..cef0ea99d4d9 100644

+static int btfgen(const char *src_btf, const char *dst_btf, const char *objspaths[])
+{
+	struct btfgen_info *info;
+	struct btf *btf_new = NULL;
+	int err;
+
+	info = btfgen_new_info(src_btf);
+	if (!info) {
+		p_err("failed to allocate info structure: %s", strerror(errno));
+		err = -errno;
+		goto out;
+	}
+
+	for (int i = 0; objspaths[i] != NULL; i++) {
+		printf("OBJ : %s\n", objspaths[i]);

+
+		err = btfgen_record_obj(info, objspaths[i]);
+		if (err)
+			goto out;
+	}
+
+	btf_new = btfgen_get_btf(info);
+	if (!btf_new) {
+		err = -errno;
+		p_err("error generating btf: %s", strerror(errno));
+		goto out;
+	}
+
+	printf("DBTF: %s\n", dst_btf);