powerpc/rtas: Warn if per-function lock isn't held

If the function descriptor has a populated lock member, then callers
are required to hold it across calls. Now that the firmware activation
sequence is appropriately guarded, we can warn when the requirement
isn't satisfied.

__do_enter_rtas_trace() gets reorganized a bit as a result of
performing the function descriptor lookup unconditionally now.
Reviewed-by: "Aneesh Kumar K.V (IBM)" <aneesh.kumar@kernel.org>
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231212-papr-sys_rtas-vs-lockdown-v6-8-e9eafd0c8c6c@linux.ibm.com

arch/powerpc/kernel/rtas.c

@@ -671,28 +671,25 @@ static void __do_enter_rtas(struct rtas_args *args)
 
 static void __do_enter_rtas_trace(struct rtas_args *args)
 {
-	const char *name = NULL;
+	const struct rtas_function *func = rtas_token_to_function(be32_to_cpu(args->token));
 
-	if (args == &rtas_args)
-		lockdep_assert_held(&rtas_lock);
 	/*
-	 * If the tracepoints that consume the function name aren't
-	 * active, avoid the lookup.
+	 * If there is a per-function lock, it must be held by the
+	 * caller.
 	 */
-	if ((trace_rtas_input_enabled() || trace_rtas_output_enabled())) {
-		const s32 token = be32_to_cpu(args->token);
-		const struct rtas_function *func = rtas_token_to_function(token);
+	if (func->lock)
+		lockdep_assert_held(func->lock);
 
-		name = func->name;
-	}
+	if (args == &rtas_args)
+		lockdep_assert_held(&rtas_lock);
 
-	trace_rtas_input(args, name);
+	trace_rtas_input(args, func->name);
 	trace_rtas_ll_entry(args);
 
 	__do_enter_rtas(args);
 
 	trace_rtas_ll_exit(args);
-	trace_rtas_output(args, name);
+	trace_rtas_output(args, func->name);
 }
 
 static void do_enter_rtas(struct rtas_args *args)