Re: [PATCH bpf-next v2 2/2] selftests/bpf: Verify rejection of sleepable tracing prog

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The test contains dead code after the expected failure of `fentry_sleepable__load()`. If this code executes on an unfixed kernel, it intentionally triggers a kernel BUG and crashes the test environment.
--

--- Patch [2]: [PATCH bpf-next v2 2/2] selftests/bpf: Verify rejection of sleepable tracing prog ---
commit 44292a230f0467503b72b0ecb8c6c25244de310b
Author: Leon Hwang <leon.hwang@linux.dev>

selftests/bpf: Verify rejection of sleepable tracing prog

This commit adds a selftest to verify that a sleepable tracing program
cannot attach to a '__x64_sys' prefix program target. The test attempts
to load the program and expects the load to fail, avoiding a kernel BUG
on unpatched kernels.

> diff --git a/tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c b/tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c
> --- a/tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c
> +++ b/tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c
[ ... ]
> @@ -576,6 +577,60 @@ static void test_func_replace_progmap(void)
[ ... ]
> +static void test_sleepable_fentry_to_xdp(void)
> +{
[ ... ]
> +	err = fentry_sleepable__load(skel);
> +	ASSERT_ERR(err, "fentry_sleepable__load");
> +	if (err)
> +		goto out;
> +
> +	skel->links.fentry_xdp = bpf_program__attach_trace(skel->progs.fentry_xdp);
> +	if (!ASSERT_OK_PTR(skel->links.fentry_xdp, "bpf_program__attach_trace"))
> +		goto out;
> +
> +	skel_xdp->links.__x64_sys_nop = bpf_program__attach_xdp(skel_xdp->progs.__x64_sys_nop,
> +								ifindex);
> +	if (!ASSERT_OK_PTR(skel_xdp->links.__x64_sys_nop, "bpf_program__attach_xdp"))
> +		goto out;
> +
> +	err = system("ping -q -c 1 -W 1 127.0.0.1 > /dev/null");
> +	ASSERT_OK(err, "ping");
> +	ASSERT_ERR(skel->bss->retval, "retval");
> +
> +out:
> +	fentry_sleepable__destroy(skel);
> +	xdp_dummy__destroy(skel_xdp);
> +}

[Severity: Medium]
Since the test expects fentry_sleepable__load() to fail (meaning err < 0),
won't the "if (err) goto out;" statement cause the test to immediately branch
to out?

If this code behaves as expected, it appears all the subsequent logic for
bpf_program__attach_trace(), bpf_program__attach_xdp(), and the ping command
becomes unreachable dead code.

Conversely, if the test runs on an unpatched kernel where the load
unexpectedly succeeds (err == 0), the test would fall through to the attach
and ping logic. Could executing this sequence on an unpatched kernel trigger
the exact kernel BUG mentioned in the commit message, potentially crashing
the test environment instead of gracefully failing the test?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260725132624.78373-1-leon.hwang@linux.dev?part=2