resolve_btfids: Handle presence of .BTF.base section

Now that btf_parse_elf() handles .BTF.base section presence,
we need to ensure that resolve_btfids uses .BTF.base when present
rather than the vmlinux base BTF passed in via the -B option.
Detect .BTF.base section presence and unset the base BTF path
to ensure that BTF ELF parsing will do the right thing.
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20240613095014.357981-7-alan.maguire@oracle.com

tools/bpf/resolve_btfids/main.c

@@ -409,6 +409,14 @@ static int elf_collect(struct object *obj)
 			obj->efile.idlist       = data;
 			obj->efile.idlist_shndx = idx;
 			obj->efile.idlist_addr  = sh.sh_addr;
+		} else if (!strcmp(name, BTF_BASE_ELF_SEC)) {
+			/* If a .BTF.base section is found, do not resolve
+			 * BTF ids relative to vmlinux; resolve relative
+			 * to the .BTF.base section instead.  btf__parse_split()
+			 * will take care of this once the base BTF it is
+			 * passed is NULL.
+			 */
+			obj->base_btf_path = NULL;
 		}
 
 		if (compressed_section_fix(elf, scn, &sh))