selftests/bpf: Add a test for using a cpumap from an freplace-to-XDP program

This adds a simple test for inserting an XDP program into a cpumap that is
"owned" by an XDP program that was loaded as PROG_TYPE_EXT (as libxdp
does). Prior to the kernel fix this would fail because the map type
ownership would be set to PROG_TYPE_EXT instead of being resolved to
PROG_TYPE_XDP.

v5:
- Fix a few nits from Andrii, add his ACK
v4:
- Use skeletons for selftest
v3:
- Update comment to better explain the cause
- Add Yonghong's ACK

Acked-by: Yonghong Song <[e-mail]>
Acked-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Toke Høiland-Jørgensen <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Martin KaFai Lau <[e-mail]>

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

···
 #include <network_helpers.h>
 #include <bpf/btf.h>
 #include "bind4_prog.skel.h"
+#include "freplace_progmap.skel.h"
+#include "xdp_dummy.skel.h"
 
 typedef int (*test_cb)(struct bpf_object *obj);
 
···
 	bind4_prog__destroy(skel);
 }
 
+static void test_func_replace_progmap(void)
+{
+	struct bpf_cpumap_val value = { .qsize = 1 };
+	struct freplace_progmap *skel = NULL;
+	struct xdp_dummy *tgt_skel = NULL;
+	__u32 key = 0;
+	int err;
+
+	skel = freplace_progmap__open();
+	if (!ASSERT_OK_PTR(skel, "prog_open"))
+		return;
+
+	tgt_skel = xdp_dummy__open_and_load();
+	if (!ASSERT_OK_PTR(tgt_skel, "tgt_prog_load"))
+		goto out;
+
+	err = bpf_program__set_attach_target(skel->progs.xdp_cpumap_prog,
+					     bpf_program__fd(tgt_skel->progs.xdp_dummy_prog),
+					     "xdp_dummy_prog");
+	if (!ASSERT_OK(err, "set_attach_target"))
+		goto out;
+
+	err = freplace_progmap__load(skel);
+	if (!ASSERT_OK(err, "obj_load"))
+		goto out;
+
+	/* Prior to fixing the kernel, loading the PROG_TYPE_EXT 'redirect'
+	 * program above will cause the map owner type of 'cpumap' to be set to
+	 * PROG_TYPE_EXT. This in turn will cause the bpf_map_update_elem()
+	 * below to fail, because the program we are inserting into the map is
+	 * of PROG_TYPE_XDP. After fixing the kernel, the initial ownership will
+	 * be correctly resolved to the *target* of the PROG_TYPE_EXT program
+	 * (i.e., PROG_TYPE_XDP) and the map update will succeed.
+	 */
+	value.bpf_prog.fd = bpf_program__fd(skel->progs.xdp_drop_prog);
+	err = bpf_map_update_elem(bpf_map__fd(skel->maps.cpu_map),
+				  &key, &value, 0);
+	ASSERT_OK(err, "map_update");
+
+out:
+	xdp_dummy__destroy(tgt_skel);
+	freplace_progmap__destroy(skel);
+}
+
 /* NOTE: affect other tests, must run in serial mode */
 void serial_test_fexit_bpf2bpf(void)
 {
···
 		test_func_replace_global_func();
 	if (test__start_subtest("fentry_to_cgroup_bpf"))
 		test_fentry_to_cgroup_bpf();
+	if (test__start_subtest("func_replace_progmap"))
+		test_func_replace_progmap();
 }

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

···
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+struct {
+	__uint(type, BPF_MAP_TYPE_CPUMAP);
+	__type(key, __u32);
+	__type(value, struct bpf_cpumap_val);
+	__uint(max_entries, 1);
+} cpu_map SEC(".maps");
+
+SEC("xdp/cpumap")
+int xdp_drop_prog(struct xdp_md *ctx)
+{
+	return XDP_DROP;
+}
+
+SEC("freplace")
+int xdp_cpumap_prog(struct xdp_md *ctx)
+{
+	return bpf_redirect_map(&cpu_map, 0, XDP_PASS);
+}
+
+char _license[] SEC("license") = "GPL";