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

["Christian König" <christian.koenig@amd.com>]

On 11/11/26 20:59, T.J. Mercier wrote:
> 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>

Acked-by: Christian König <christian.koenig@amd.com>

Let me know if you already have drm-misc commit rights.

Regards,
Christian.

> ---
> 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;
> +}