Re: [PATCH bpf 3/3] selftests/bpf: Add test for cgroup iterator on a dead cgroup

[Yonghong Song <yhs@meta.com>]

On 11/6/22 11:42 PM, Hou Tao wrote:
> From: Hou Tao <houtao1@huawei.com>
> 
> The test closes both iterator link fd and cgroup fd, and removes the
> cgroup file to make a dead cgroup before reading cgroup iterator fd. It
> also uses kern_sync_rcu() and usleep() to wait for the release of
> start cgroup. If the start cgroup is not pinned by cgroup iterator,
> reading iterator fd will trigger use-after-free.
> 
> Signed-off-by: Hou Tao <houtao1@huawei.com>

LGTM with a few nits below.

Acked-by: Yonghong Song <yhs@fb.com>

> ---
>   .../selftests/bpf/prog_tests/cgroup_iter.c    | 78 +++++++++++++++++++
>   1 file changed, 78 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/cgroup_iter.c b/tools/testing/selftests/bpf/prog_tests/cgroup_iter.c
> index c4a2adb38da1..d64ed1cf1554 100644
> --- a/tools/testing/selftests/bpf/prog_tests/cgroup_iter.c
> +++ b/tools/testing/selftests/bpf/prog_tests/cgroup_iter.c
> @@ -189,6 +189,82 @@ static void test_walk_self_only(struct cgroup_iter *skel)
>   			      BPF_CGROUP_ITER_SELF_ONLY, "self_only");
>   }
>   
> +static void test_walk_dead_self_only(struct cgroup_iter *skel)
> +{
> +	DECLARE_LIBBPF_OPTS(bpf_iter_attach_opts, opts);
> +	char expected_output[128], buf[128];
> +	const char *cgrp_name = "/dead";
> +	union bpf_iter_link_info linfo;
> +	int len, cgrp_fd, iter_fd;
> +	struct bpf_link *link;
> +	size_t left;
> +	char *p;
> +
> +	cgrp_fd = create_and_get_cgroup(cgrp_name);
> +	if (!ASSERT_GE(cgrp_fd, 0, "create cgrp"))
> +		return;
> +
> +	/* The cgroup is already dead during iteration, so it only has epilogue
> +	 * in the output.
> +	 */

Let us reword the comment like
	The cgroup will be dead during read() iteration, and it only has
	epilogue in the output.

> +	snprintf(expected_output, sizeof(expected_output), EPILOGUE);
> +
> +	memset(&linfo, 0, sizeof(linfo));
> +	linfo.cgroup.cgroup_fd = cgrp_fd;
> +	linfo.cgroup.order = BPF_CGROUP_ITER_SELF_ONLY;
> +	opts.link_info = &linfo;
> +	opts.link_info_len = sizeof(linfo);
> +
> +	link = bpf_program__attach_iter(skel->progs.cgroup_id_printer, &opts);
> +	if (!ASSERT_OK_PTR(link, "attach_iter"))
> +		goto close_cg;
> +
> +	iter_fd = bpf_iter_create(bpf_link__fd(link));
> +	if (!ASSERT_GE(iter_fd, 0, "iter_create"))
> +		goto free_link;
> +
> +	/* Close link fd and cgroup fd */
> +	bpf_link__destroy(link);
> +	link = NULL;
> +	close(cgrp_fd);
> +	cgrp_fd = -1;

We can remove 'link = NULL' and 'cgroup_fd = -1' and
add 'return' after 'close(iter_fd)', which seems better
to understand the code.

> +
> +	/* Remove cgroup to mark it as dead */
> +	remove_cgroup(cgrp_name);
> +
> +	/* Two kern_sync_rcu() and usleep() pair are used to wait for the
> +	 * releases of cgroup css, and the last kern_sync_rcu() and usleep()
> +	 * pair is used to wait for the free of cgroup itself.
> +	 */
> +	kern_sync_rcu();
> +	usleep(8000);
> +	kern_sync_rcu();
> +	usleep(8000);
> +	kern_sync_rcu();
> +	usleep(1000);
> +
> +	memset(buf, 0, sizeof(buf));
> +	left = ARRAY_SIZE(buf);
> +	p = buf;
> +	while ((len = read(iter_fd, p, left)) > 0) {
> +		p += len;
> +		left -= len;
> +	}
> +
> +	ASSERT_STREQ(buf, expected_output, "dead cgroup output");
> +
> +	/* read() after iter finishes should be ok. */
> +	if (len == 0)
> +		ASSERT_OK(read(iter_fd, buf, sizeof(buf)), "second_read");
> +
> +	close(iter_fd);
> +free_link:
> +	bpf_link__destroy(link);
> +close_cg:
> +	if (cgrp_fd >= 0)
> +		close(cgrp_fd);
> +}
> +
>   void test_cgroup_iter(void)
>   {
>   	struct cgroup_iter *skel = NULL;
> @@ -217,6 +293,8 @@ void test_cgroup_iter(void)
>   		test_early_termination(skel);
>   	if (test__start_subtest("cgroup_iter__self_only"))
>   		test_walk_self_only(skel);
> +	if (test__start_subtest("cgroup_iter_dead_self_only"))

Let us follow the convention in this file with
	cgroup_iter__dead_self_only

> +		test_walk_dead_self_only(skel);
>   out:
>   	cgroup_iter__destroy(skel);
>   	cleanup_cgroups();