[PATCH bpf] selftests/bpf: Test that a drained dmabuf iterator keeps returning NULL

["T.J. Mercier" <tjmercier@google.com>]

The BPF iterator docs in bpf_iterators.rst specify that the next
function of an open-coded iterator should keep returning NULL after it
has been returned once.

Add a subtest that checks this for the dmabuf iterator. It drains an
iterator, calls bpf_iter_dmabuf_next() once more and checks that the
result is NULL. The dma-bufs the test creates keep the list from being
empty, so an iterator that starts over would return one of them.

The existing open_coded subtest uses bpf_for_each(), which stops at the
first NULL and never calls next() on a drained iterator.

Signed-off-by: T.J. Mercier <tjmercier@google.com>
---
This depends on Kang Chen's fix and fails without it:
https://lore.kernel.org/bpf/20261110125106.2935677-1-k.chen@smail.nju.edu.cn/
Kang, feel free to carry it as 2/2 in your v2.

 .../selftests/bpf/prog_tests/dmabuf_iter.c    | 19 +++++++++++++++++++
 .../testing/selftests/bpf/progs/dmabuf_iter.c | 19 +++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/dmabuf_iter.c b/tools/testing/selftests/bpf/prog_tests/dmabuf_iter.c
index 1a5dc1adae73..c1f81b5dccdc 100644
--- a/tools/testing/selftests/bpf/prog_tests/dmabuf_iter.c
+++ b/tools/testing/selftests/bpf/prog_tests/dmabuf_iter.c
@@ -265,6 +265,23 @@ static void subtest_dmabuf_iter_check_open_coded(struct dmabuf_iter *skel, int m
 	} while (bpf_map_get_next_key(map_fd, key, key));
 }
 
+static void subtest_dmabuf_iter_check_next_after_drain(struct dmabuf_iter *skel)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, topts);
+	int err, fd;
+
+	fd = bpf_program__fd(skel->progs.iter_dmabuf_next_after_drain);
+
+	err = bpf_prog_test_run_opts(fd, &topts);
+	if (!ASSERT_OK(err, "test_run_opts err"))
+		return;
+	if (!ASSERT_OK(topts.retval, "test_run_opts retval"))
+		return;
+
+	/* The test buffers exist, so a restarted iteration would find one */
+	ASSERT_EQ(skel->data->next_after_drain, 0, "next_after_drain");
+}
+
 void test_dmabuf_iter(void)
 {
 	struct dmabuf_iter *skel = NULL;
@@ -315,6 +332,8 @@ void test_dmabuf_iter(void)
 	}
 	if (test__start_subtest("open_coded"))
 		subtest_dmabuf_iter_check_open_coded(skel, map_fd);
+	if (test__start_subtest("next_after_drain"))
+		subtest_dmabuf_iter_check_next_after_drain(skel);
 
 destroy:
 	destroy_test_buffers();
diff --git a/tools/testing/selftests/bpf/progs/dmabuf_iter.c b/tools/testing/selftests/bpf/progs/dmabuf_iter.c
index 9cbb7442646e..d5421b75c39c 100644
--- a/tools/testing/selftests/bpf/progs/dmabuf_iter.c
+++ b/tools/testing/selftests/bpf/progs/dmabuf_iter.c
@@ -99,3 +99,22 @@ int iter_dmabuf_for_each(const void *ctx)
 
 	return 0;
 }
+
+/* Result of a bpf_iter_dmabuf_next() call after the iterator was drained */
+__u64 next_after_drain = -1;
+
+SEC("syscall")
+int iter_dmabuf_next_after_drain(const void *ctx)
+{
+	struct bpf_iter_dmabuf it;
+
+	bpf_iter_dmabuf_new(&it);
+	while (bpf_iter_dmabuf_next(&it))
+		;
+
+	/* Store the raw value; the verifier would remove a branch on it */
+	next_after_drain = (__u64)bpf_iter_dmabuf_next(&it);
+	bpf_iter_dmabuf_destroy(&it);
+
+	return 0;
+}
-- 
2.56.0.567.gdb80ae8cd5-goog