selftests: Add selftest for disallowing modify_return attachment to freplace

This adds a selftest that ensures that modify_return tracing programs
cannot be attached to freplace programs. The security_ prefix is added to
the freplace program because that would otherwise let it pass the check for
modify_return.

Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>

Author: tohojo

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

@@ -232,6 +232,60 @@ static void test_func_replace_multi(void)
 				  prog_name, true, test_second_attach);
 }
 
+static void test_fmod_ret_freplace(void)
+{
+	struct bpf_object *freplace_obj = NULL, *pkt_obj, *fmod_obj = NULL;
+	const char *freplace_name = "./freplace_get_constant.o";
+	const char *fmod_ret_name = "./fmod_ret_freplace.o";
+	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts);
+	const char *tgt_name = "./test_pkt_access.o";
+	struct bpf_link *freplace_link = NULL;
+	struct bpf_program *prog;
+	__u32 duration = 0;
+	int err, pkt_fd;
+
+	err = bpf_prog_load(tgt_name, 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",
+		  tgt_name, err, errno))
+		return;
+	opts.attach_prog_fd = pkt_fd;
+
+	freplace_obj = bpf_object__open_file(freplace_name, &opts);
+	if (CHECK(IS_ERR_OR_NULL(freplace_obj), "freplace_obj_open",
+		  "failed to open %s: %ld\n", freplace_name,
+		  PTR_ERR(freplace_obj)))
+		goto out;
+
+	err = bpf_object__load(freplace_obj);
+	if (CHECK(err, "freplace_obj_load", "err %d\n", err))
+		goto out;
+
+	prog = bpf_program__next(NULL, freplace_obj);
+	freplace_link = bpf_program__attach_trace(prog);
+	if (CHECK(IS_ERR(freplace_link), "freplace_attach_trace", "failed to link\n"))
+		goto out;
+
+	opts.attach_prog_fd = bpf_program__fd(prog);
+	fmod_obj = bpf_object__open_file(fmod_ret_name, &opts);
+	if (CHECK(IS_ERR_OR_NULL(fmod_obj), "fmod_obj_open",
+		  "failed to open %s: %ld\n", fmod_ret_name,
+		  PTR_ERR(fmod_obj)))
+		goto out;
+
+	err = bpf_object__load(fmod_obj);
+	if (CHECK(!err, "fmod_obj_load", "loading fmod_ret should fail\n"))
+		goto out;
+
+out:
+	bpf_link__destroy(freplace_link);
+	bpf_object__close(freplace_obj);
+	bpf_object__close(fmod_obj);
+	bpf_object__close(pkt_obj);
+}
+
+
 static void test_func_sockmap_update(void)
 {
 	const char *prog_name[] = {
@@ -314,4 +368,6 @@ void test_fexit_bpf2bpf(void)
 		test_func_map_prog_compatibility();
 	if (test__start_subtest("func_replace_multi"))
 		test_func_replace_multi();
+	if (test__start_subtest("fmod_ret_freplace"))
+		test_fmod_ret_freplace();
 }

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

@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+volatile __u64 test_fmod_ret = 0;
+SEC("fmod_ret/security_new_get_constant")
+int BPF_PROG(fmod_ret_test, long val, int ret)
+{
+	test_fmod_ret = 1;
+	return 120;
+}
+
+char _license[] SEC("license") = "GPL";

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

@@ -5,7 +5,7 @@
 
 volatile __u64 test_get_constant = 0;
 SEC("freplace/get_constant")
-int new_get_constant(long val)
+int security_new_get_constant(long val)
 {
 	if (val != 123)
 		return 0;