selftests/bpf: Add test for DEVMAP reuse

The test covers basic re-use of a pinned DEVMAP map,
with both matching and mismatching parameters.

Signed-off-by: Yureka Lilian <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Eduard Zingerman <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

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

···
+// SPDX-License-Identifier: GPL-2.0
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <test_progs.h>
+
+
+#include "test_pinning_devmap.skel.h"
+
+void test_pinning_devmap_reuse(void)
+{
+	const char *pinpath1 = "/sys/fs/bpf/pinmap1";
+	const char *pinpath2 = "/sys/fs/bpf/pinmap2";
+	struct test_pinning_devmap *skel1 = NULL, *skel2 = NULL;
+	int err;
+	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts);
+
+	/* load the object a first time */
+	skel1 = test_pinning_devmap__open_and_load();
+	if (!ASSERT_OK_PTR(skel1, "skel_load1"))
+		goto out;
+
+	/* load the object a second time, re-using the pinned map */
+	skel2 = test_pinning_devmap__open_and_load();
+	if (!ASSERT_OK_PTR(skel2, "skel_load2"))
+		goto out;
+
+	/* we can close the reference safely without
+	 * the map's refcount falling to 0
+	 */
+	test_pinning_devmap__destroy(skel1);
+	skel1 = NULL;
+
+	/* now, swap the pins */
+	err = renameat2(0, pinpath1, 0, pinpath2, RENAME_EXCHANGE);
+	if (!ASSERT_OK(err, "swap pins"))
+		goto out;
+
+	/* load the object again, this time the re-use should fail */
+	skel1 = test_pinning_devmap__open_and_load();
+	if (!ASSERT_ERR_PTR(skel1, "skel_load3"))
+		goto out;
+
+out:
+	unlink(pinpath1);
+	unlink(pinpath2);
+	test_pinning_devmap__destroy(skel1);
+	test_pinning_devmap__destroy(skel2);
+}

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

···
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+struct {
+	__uint(type, BPF_MAP_TYPE_DEVMAP);
+	__uint(max_entries, 1);
+	__type(key, __u32);
+	__type(value, __u32);
+	__uint(pinning, LIBBPF_PIN_BY_NAME);
+} pinmap1 SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_DEVMAP);
+	__uint(max_entries, 2);
+	__type(key, __u32);
+	__type(value, __u32);
+	__uint(pinning, LIBBPF_PIN_BY_NAME);
+} pinmap2 SEC(".maps");