selftests/bpf: Test for checking return code for the extended prog

This adds test to enforce same check for the return code for the extended prog
as it is enforced for the target program. It asserts failure for a
return code, which is permitted without the patch in this series, while
it is restricted after the application of this patch.
Signed-off-by: Udip Pant <udippant@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200825232003.2877030-4-udippant@fb.com

tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c

@@ -142,10 +142,50 @@ static void test_func_replace_verify(void)
 				  prog_name, false);
 }
 
+static void test_func_replace_return_code(void)
+{
+	/*
+	 * standalone test that asserts failure to load freplace prog
+	 * because of invalid return code.
+	 */
+	struct bpf_object *obj = NULL, *pkt_obj;
+	int err, pkt_fd;
+	__u32 duration = 0;
+	const char *target_obj_file = "./connect4_prog.o";
+	const char *obj_file = "./freplace_connect_v4_prog.o";
+
+	err = bpf_prog_load(target_obj_file, BPF_PROG_TYPE_UNSPEC,
+			    &pkt_obj, &pkt_fd);
+	/* the target prog should load fine */
+	if (CHECK(err, "tgt_prog_load", "file %s err %d errno %d\n",
+		  target_obj_file, err, errno))
+		return;
+	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts,
+			    .attach_prog_fd = pkt_fd,
+			   );
+
+	obj = bpf_object__open_file(obj_file, &opts);
+	if (CHECK(IS_ERR_OR_NULL(obj), "obj_open",
+		  "failed to open %s: %ld\n", obj_file,
+		  PTR_ERR(obj)))
+		goto close_prog;
+
+	/* It should fail to load the program */
+	err = bpf_object__load(obj);
+	if (CHECK(!err, "bpf_obj_load should fail", "err %d\n", err))
+		goto close_prog;
+
+close_prog:
+	if (!IS_ERR_OR_NULL(obj))
+		bpf_object__close(obj);
+	bpf_object__close(pkt_obj);
+}
+
 void test_fexit_bpf2bpf(void)
 {
 	test_target_no_callees();
 	test_target_yes_callees();
 	test_func_replace();
 	test_func_replace_verify();
+	test_func_replace_return_code();
 }

tools/testing/selftests/bpf/progs/freplace_connect_v4_prog.c

+// SPDX-License-Identifier: GPL-2.0
+// Copyright (c) 2020 Facebook
+
+#include <linux/stddef.h>
+#include <linux/ipv6.h>
+#include <linux/bpf.h>
+#include <linux/in.h>
+#include <sys/socket.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_endian.h>
+
+SEC("freplace/connect_v4_prog")
+int new_connect_v4_prog(struct bpf_sock_addr *ctx)
+{
+	// return value thats in invalid range
+	return 255;
+}
+
+char _license[] SEC("license") = "GPL";