livepatch: Trigger WARNING if livepatch function fails due to recursion

If for some reason a function is called that triggers the recursion
detection of live patching, trigger a warning. By not executing the live
patch code, it is possible that the old unpatched function will be called
placing the system into an unknown state.

Link: https://lore.kernel.org/r/20201029145709.GD16774@alley
Link: https://lkml.kernel.org/r/20201106023547.312639435@goodmis.org

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Joe Lawrence <joe.lawrence@redhat.com>
Cc: live-patching@vger.kernel.org
Suggested-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/livepatch/patch.c

@@ -50,7 +50,7 @@ static void notrace klp_ftrace_handler(unsigned long ip,
 	ops = container_of(fops, struct klp_ops, fops);
 
 	bit = ftrace_test_recursion_trylock();
-	if (bit < 0)
+	if (WARN_ON_ONCE(bit < 0))
 		return;
 	/*
 	 * A variant of synchronize_rcu() is used to allow patching functions