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

[Joe Lawrence <joe.lawrence@redhat.com>]

On Fri, Jul 31, 2026 at 01:53:58PM -0400, Andrey Grodzovsky wrote:
> 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 <andrey.grodzovsky@crowdstrike.com>
> ---
>  .../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" ]]
> +}

Small nit: AFAICT, it doesn't really affect the test pattern, but it
would be a bit cleaner if this function restored the original value
instead of always turning it on, like (untested):

	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" ]]

--
Joe