Re: [RFC PATCH bpf-next v3 2/2] selftests/livepatch: update test-ftrace.sh for deprecated ftrace_enabled

On Thu, Aug 6, 2026 at 5:18 PM Song Liu [off-list ref] wrote:

On Thu, Aug 6, 2026 at 8:30 AM Andrey Grodzovsky
[off-list ref] wrote:

quoted

kernel.ftrace_enabled=0 is now refused on kernels that deprecate the
knob, so the old disable/reload flow no longer applies there. Probe
for this with ftrace_disable_supported() and keep the full original
scenario (disable, fail to load a livepatch, re-enable, load, confirm
disable is refused while loaded) on kernels where it still works;
otherwise just confirm the write is refused.

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Suggested-by: Joe Lawrence <joe.lawrence@redhat.com>
Assisted-by: Claude:claude-sonnet-5
Signed-off-by: Andrey Grodzovsky <redacted>
---
 .../testing/selftests/livepatch/https://urldefense.com/v3/__http://functions.sh__;!!BmdzS3_lV9HdKG8!0iHqKyQrLfspbt4sQvekjrQjWLuiSTtsCXR9obFozOCSUhPg1KCQGTvfgZ9oTwgBkdB3nKZA1RCYtwwYj2qtEXgW$   | 14 ++++++
 .../selftests/livepatch/https://urldefense.com/v3/__http://test-ftrace.sh__;!!BmdzS3_lV9HdKG8!0iHqKyQrLfspbt4sQvekjrQjWLuiSTtsCXR9obFozOCSUhPg1KCQGTvfgZ9oTwgBkdB3nKZA1RCYtwwYj6oTNhr1$         | 45 ++++++++++++-------
 2 files changed, 42 insertions(+), 17 deletions(-)

diff --git a/tools/testing/selftests/livepatch/https://urldefense.com/v3/__http://functions.sh__;!!BmdzS3_lV9HdKG8!0iHqKyQrLfspbt4sQvekjrQjWLuiSTtsCXR9obFozOCSUhPg1KCQGTvfgZ9oTwgBkdB3nKZA1RCYtwwYj2qtEXgW$  b/tools/testing/selftests/livepatch/https://urldefense.com/v3/__http://functions.sh__;!!BmdzS3_lV9HdKG8!0iHqKyQrLfspbt4sQvekjrQjWLuiSTtsCXR9obFozOCSUhPg1KCQGTvfgZ9oTwgBkdB3nKZA1RCYtwwYj2qtEXgW$
index 30dc677b2f45..a65b7b1ac8ad 100644
--- a/tools/testing/selftests/livepatch/https://urldefense.com/v3/__http://functions.sh__;!!BmdzS3_lV9HdKG8!0iHqKyQrLfspbt4sQvekjrQjWLuiSTtsCXR9obFozOCSUhPg1KCQGTvfgZ9oTwgBkdB3nKZA1RCYtwwYj2qtEXgW$
+++ b/tools/testing/selftests/livepatch/https://urldefense.com/v3/__http://functions.sh__;!!BmdzS3_lV9HdKG8!0iHqKyQrLfspbt4sQvekjrQjWLuiSTtsCXR9obFozOCSUhPg1KCQGTvfgZ9oTwgBkdB3nKZA1RCYtwwYj2qtEXgW$

@@ -126,6 +126,20 @@ function set_ftrace_enabled() {
        echo "livepatch: kernel.ftrace_enabled = $result" > /dev/kmsg
 }

+# ftrace_disable_supported() - probe whether kernel.ftrace_enabled=0
+#      can still disable ftrace on this kernel. Newer kernels deprecate
+#      the knob and always refuse the write with -EOPNOTSUPP.
+function ftrace_disable_supported() {
+       local orig result
+
+       orig=$(sysctl --values kernel.ftrace_enabled)
+       sysctl -q kernel.ftrace_enabled=0 &> /dev/null
+       result=$(sysctl --values kernel.ftrace_enabled)
+       sysctl -q "kernel.ftrace_enabled=$orig" &> /dev/null
+
+       [[ "$result" == "0" ]]
+}

Since the selftests are shipped with the kernel code, I think
we don't need to handle backward compatibility here. Instead,
we can just assume disabling ftrace is no longer supported.

Thanks,
Song

I tend to agree but I think here[1] Steve and Miroslav asked for
backward compatability. V1 was actually the way you prefer.

Andrey

[1] - https://lore.kernel.org/bpf/alpine.LSU.2.21.2607311104390.25165@pobox.suse.cz/ (local)

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