bpftool: Add gen min_core_btf command

This command is implemented under the "gen" command in bpftool and the
syntax is the following:

$ bpftool gen min_core_btf INPUT OUTPUT OBJECT [OBJECT...]

INPUT is the file that contains all the BTF types for a kernel and
OUTPUT is the path of the minimize BTF file that will be created with
only the types needed by the objects.
Signed-off-by: Mauricio Vásquez <mauricio@kinvolk.io>
Signed-off-by: Rafael David Tinoco <rafael.tinoco@aquasec.com>
Signed-off-by: Lorenzo Fontana <lorenzo.fontana@elastic.co>
Signed-off-by: Leonardo Di Donato <leonardo.didonato@elastic.co>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220215225856.671072-4-mauricio@kinvolk.io

tools/bpf/bpftool/bash-completion/bpftool

@@ -1003,9 +1003,13 @@ _bpftool()
                             ;;
                     esac
                     ;;
+                min_core_btf)
+                    _filedir
+                    return 0
+                    ;;
                 *)
                     [[ $prev == $object ]] && \
-                        COMPREPLY=( $( compgen -W 'object skeleton help' -- "$cur" ) )
+                        COMPREPLY=( $( compgen -W 'object skeleton help min_core_btf' -- "$cur" ) )
                     ;;
             esac
             ;;

tools/bpf/bpftool/gen.c

@@ -1108,6 +1108,7 @@ static int do_help(int argc, char **argv)
 	fprintf(stderr,
 		"Usage: %1$s %2$s object OUTPUT_FILE INPUT_FILE [INPUT_FILE...]\n"
 		"       %1$s %2$s skeleton FILE [name OBJECT_NAME]\n"
+		"       %1$s %2$s min_core_btf INPUT OUTPUT OBJECT [OBJECT...]\n"
 		"       %1$s %2$s help\n"
 		"\n"
 		"       " HELP_SPEC_OPTIONS " |\n"
@@ -1118,10 +1119,45 @@ static int do_help(int argc, char **argv)
 	return 0;
 }
 
+/* Create minimized BTF file for a set of BPF objects */
+static int minimize_btf(const char *src_btf, const char *dst_btf, const char *objspaths[])
+{
+	return -EOPNOTSUPP;
+}
+
+static int do_min_core_btf(int argc, char **argv)
+{
+	const char *input, *output, **objs;
+	int i, err;
+
+	if (!REQ_ARGS(3)) {
+		usage();
+		return -1;
+	}
+
+	input = GET_ARG();
+	output = GET_ARG();
+
+	objs = (const char **) calloc(argc + 1, sizeof(*objs));
+	if (!objs) {
+		p_err("failed to allocate array for object names");
+		return -ENOMEM;
+	}
+
+	i = 0;
+	while (argc)
+		objs[i++] = GET_ARG();
+
+	err = minimize_btf(input, output, objs);
+	free(objs);
+	return err;
+}
+
 static const struct cmd cmds[] = {
-	{ "object",	do_object },
-	{ "skeleton",	do_skeleton },
-	{ "help",	do_help },
+	{ "object",		do_object },
+	{ "skeleton",		do_skeleton },
+	{ "min_core_btf",	do_min_core_btf},
+	{ "help",		do_help },
 	{ 0 }
 };