[RFC PATCH bpf-next 2/3] kprobes: mark ftrace-based kprobe ops permanent

kernel.ftrace_enabled=0 silently kills ftrace-based kprobes/kretprobes
the same way it does BPF trampolines: arm succeeds, the probe stops
firing with no error, and re-enabling silently restores it. Unlike
trampolines this is not a regression -- ftrace-based kprobes have
never carried this protection, so the bug is long-standing.

kprobe_ftrace_ops/kprobe_ipmodify_ops are shared only among
ftrace-based kprobe attachers, not with any other tracer, so the same
unconditional FTRACE_OPS_FL_PERMANENT livepatch already uses applies
cleanly here too, with no kernel/trace/ftrace.c changes needed.

Known limitation: kprobe.multi/kretprobe.multi/kprobe.session
(fprobe-backed, kernel/trace/fprobe.c) are not covered. Entry-only
fprobes could take the same fix, but return-capturing fprobes route
through the function-graph tracer's shared subops manager, which is
also used by unrelated tracers (function_graph, irqsoff, wakeup
latency, function profiler) -- marking it permanent would block
ftrace_enabled=0 for those too. Left for a follow-up.

Cc: <redacted>
Assisted-by: Claude:claude-sonnet-5
Signed-off-by: Andrey Grodzovsky <redacted>
---
 kernel/kprobes.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index bfc89083daa9..5a54511eee79 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c

@@ -1122,14 +1122,15 @@ static struct kprobe *alloc_aggr_kprobe(struct kprobe *p)
 #endif /* CONFIG_OPTPROBES */
 
 #ifdef CONFIG_KPROBES_ON_FTRACE
+/* Same protection livepatch gives its own ftrace_ops. */
 static struct ftrace_ops kprobe_ftrace_ops __read_mostly = {
 	.func = kprobe_ftrace_handler,
-	.flags = FTRACE_OPS_FL_SAVE_REGS,
+	.flags = FTRACE_OPS_FL_SAVE_REGS | FTRACE_OPS_FL_PERMANENT,
 };
 
 static struct ftrace_ops kprobe_ipmodify_ops __read_mostly = {
 	.func = kprobe_ftrace_handler,
-	.flags = FTRACE_OPS_FL_SAVE_REGS | FTRACE_OPS_FL_IPMODIFY,
+	.flags = FTRACE_OPS_FL_SAVE_REGS | FTRACE_OPS_FL_IPMODIFY | FTRACE_OPS_FL_PERMANENT,
 };
 
 static int kprobe_ipmodify_enabled;

-- 
2.34.1