selftests/bpf: Add test for truncated dmabuf_iter reads

If many dmabufs are present, reads of the dmabuf iterator can be
truncated at PAGE_SIZE or user buffer size boundaries before the fix in
"bpf: Fix truncated dmabuf iterator reads". Add a test to
confirm truncation does not occur.

Signed-off-by: T.J. Mercier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

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

···
 	return -1;
 }
 
-static int create_sys_heap_dmabuf(void)
+static int create_sys_heap_dmabuf(size_t bytes)
 {
-	sysheap_test_buffer_size = 20 * getpagesize();
-
 	struct dma_heap_allocation_data data = {
-		.len = sysheap_test_buffer_size,
+		.len = bytes,
 		.fd = 0,
 		.fd_flags = O_RDWR | O_CLOEXEC,
 		.heap_flags = 0,
···
 static int create_test_buffers(void)
 {
 	udmabuf = create_udmabuf();
-	sysheap_dmabuf = create_sys_heap_dmabuf();
+
+	sysheap_test_buffer_size = 20 * getpagesize();
+	sysheap_dmabuf = create_sys_heap_dmabuf(sysheap_test_buffer_size);
 
 	if (udmabuf < 0 || sysheap_dmabuf < 0)
 		return -1;
···
 	close(iter_fd);
 }
 
+static void subtest_dmabuf_iter_check_lots_of_buffers(struct dmabuf_iter *skel)
+{
+	int iter_fd;
+	char buf[1024];
+	size_t total_bytes_read = 0;
+	ssize_t bytes_read;
+
+	iter_fd = bpf_iter_create(bpf_link__fd(skel->links.dmabuf_collector));
+	if (!ASSERT_OK_FD(iter_fd, "iter_create"))
+		return;
+
+	while ((bytes_read = read(iter_fd, buf, sizeof(buf))) > 0)
+		total_bytes_read += bytes_read;
+
+	ASSERT_GT(total_bytes_read, getpagesize(), "total_bytes_read");
+
+	close(iter_fd);
+}
+
+
 static void subtest_dmabuf_iter_check_open_coded(struct dmabuf_iter *skel, int map_fd)
 {
 	LIBBPF_OPTS(bpf_test_run_opts, topts);
···
 		subtest_dmabuf_iter_check_no_infinite_reads(skel);
 	if (test__start_subtest("default_iter"))
 		subtest_dmabuf_iter_check_default_iter(skel);
+	if (test__start_subtest("lots_of_buffers")) {
+		size_t NUM_BUFS = 100;
+		int buffers[NUM_BUFS];
+		int i;
+
+		for (i = 0; i < NUM_BUFS; ++i) {
+			buffers[i] = create_sys_heap_dmabuf(getpagesize());
+			if (!ASSERT_OK_FD(buffers[i], "dmabuf_fd"))
+				goto cleanup_bufs;
+		}
+
+		subtest_dmabuf_iter_check_lots_of_buffers(skel);
+
+cleanup_bufs:
+		for (--i; i >= 0; --i)
+			close(buffers[i]);
+	}
 	if (test__start_subtest("open_coded"))
 		subtest_dmabuf_iter_check_open_coded(skel, map_fd);