selftests/bpf: Test the length and the address of an arena mmap()

libbpf maps all max_entries pages of an arena and never maps it at
address 0. No test maps an arena differently. Thus the checks from
"bpf: Refuse an arena mmap() longer than max_entries pages" and
"bpf: Refuse to map an arena at address 0" have no test. Test that a longer
mmap() fails with E2BIG, and that an mmap() at address 0 fails with EINVAL.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261101190535.1454146-8-alexei.starovoitov@gmail.com

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

@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <test_progs.h>
+#include <sys/mman.h>
+
+#define ARENA_PAGES 8
+
+static void test_refused(int pages, int flags, int err)
+{
+	LIBBPF_OPTS(bpf_map_create_opts, opts, .map_flags = BPF_F_MMAPABLE);
+	void *area;
+	int fd;
+
+	fd = bpf_map_create(BPF_MAP_TYPE_ARENA, "arena", 0, 0, ARENA_PAGES, &opts);
+	if (fd == -EOPNOTSUPP) {
+		test__skip();
+		return;
+	}
+	if (!ASSERT_GE(fd, 0, "bpf_map_create"))
+		return;
+
+	area = mmap(NULL, pages * getpagesize(), PROT_READ | PROT_WRITE,
+		    MAP_SHARED | flags, fd, 0);
+	if (ASSERT_EQ(area, MAP_FAILED, "mmap"))
+		ASSERT_EQ(errno, err, "errno");
+	else
+		munmap(area, pages * getpagesize());
+	close(fd);
+}
+
+void test_arena_mmap(void)
+{
+	if (test__start_subtest("long"))
+		test_refused(ARENA_PAGES + 1, 0, E2BIG);
+	if (test__start_subtest("address_0"))
+		test_refused(ARENA_PAGES, MAP_FIXED, EINVAL);
+}