Re: [PATCH net-next 10/15] selftests/bpf: Test passing metadata to TCP egress

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] bpf: test_skb_trace__tcp_retrans flakiness due to premature map assertion
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commit 2cb521395baafb017de2859f1a86b8fa86c112a8
Author: Jakub Sitnicki <jakub@cloudflare.com>

selftests/bpf: Test passing metadata to TCP egress
    
Tag the skb at cgroup/skb egress and read the stashed metadata back at TC
egress: for a regular TCP transmission, and for a retransmission, where
the first transmission is dropped at TC and the write-queue original,
tagged via bpf_skb_fclone_orig, passes the metadata to the retransmitted
fast clone via the copy tracepoint.

> diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_context_test_run.c b/tools/testing/selftests/bpf/prog_tests/xdp_context_test_run.c
> index 890a881a6a1f8..ac5c0f067b3f1 100644
> --- a/tools/testing/selftests/bpf/prog_tests/xdp_context_test_run.c
> +++ b/tools/testing/selftests/bpf/prog_tests/xdp_context_test_run.c
[ ... ]
> +static void test_skb_trace__tcp_retrans(struct test_xdp_meta *skel)
> +{
[ ... ]
> +	/* First transmission is dropped at TC; recv unblocks on retransmit */
> +	ret = recv(conn_fd, buf, sizeof(buf), 0);
> +	ASSERT_EQ(ret, TEST_PAYLOAD_LEN, "recv");
> +
> +	/* Poll every 10 msec up to 1 sec */
> +	for (i = 0; i < 100 && !skel->bss->test_pass; i++)
> +		usleep(10000);
> +	ASSERT_TRUE(skel->bss->test_pass, "test_pass");
> +
> +	/* Skb is freed after RX delivery; wait for free tp */
> +	for (i = 0; i < 100 && !skel->bss->free_count; i++)
> +		usleep(10000);
> +	ASSERT_GE(skel->bss->free_count, 1, "free_count");
> +
> +	assert_meta_stash_empty(skel->maps.meta_map);

[Severity: Medium]
Will this polling loop terminate prematurely?

Since the first transmission is dropped at TC egress, it gets freed and
increments free_count to 1 early. This causes the loop to exit immediately
because !skel->bss->free_count is already false.

Could this cause assert_meta_stash_empty() to fail intermittently because the
retransmitted clone and original write-queue SKB haven't been asynchronously
freed yet?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260928-bpf-meta-gated-tracepoints-v1-0-844dbf3e1edf@cloudflare.com?part=10