Re: [RFC PATCH bpf-next v2 2/2] selftests/livepatch: update test-ftrace.sh for deprecated ftrace_enabled

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: Miroslav Benes <mbenes@suse.cz>
Assisted-by: Claude:claude-sonnet-5
Signed-off-by: Andrey Grodzovsky <redacted>
---
 .../testing/selftests/livepatch/functions.sh  | 13 ++++++
 .../selftests/livepatch/test-ftrace.sh        | 45 ++++++++++++-------
 2 files changed, 41 insertions(+), 17 deletions(-)

diff --git a/tools/testing/selftests/livepatch/functions.sh b/tools/testing/selftests/livepatch/functions.sh
index 30dc677b2f45..7fa4bda70221 100644
--- a/tools/testing/selftests/livepatch/functions.sh
+++ b/tools/testing/selftests/livepatch/functions.sh

@@ -126,6 +126,19 @@ 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 result
+
+	sysctl -q kernel.ftrace_enabled=0 &> /dev/null
+	result=$(sysctl --values kernel.ftrace_enabled)
+	sysctl -q kernel.ftrace_enabled=1 &> /dev/null
+
+	[[ "$result" == "0" ]]
+}