fgraph: Keep track of when fgraph_ops are registered or not

Add a warning if unregister_ftrace_graph() is called without ever
registering it, or if register_ftrace_graph() is called twice. This can
detect errors when they happen and not later when there's a side effect:

Link: https://lore.kernel.org/all/20250617120830.24fbdd62@gandalf.local.home/
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/20250701194451.22e34724@gandalf.local.home
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index c5b207992fb49e369ae7f6777952067899256729..f4d200f0c610e3c096ec14db55800b4e99b8a202 100644 (file)
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c
@@ -1325,6 +1325,10 @@ int register_ftrace_graph(struct fgraph_ops *gops)
        int ret = 0;
        int i = -1;
 
+       if (WARN_ONCE(gops->ops.flags & FTRACE_OPS_FL_GRAPH,
+                     "function graph ops registered again"))
+               return -EBUSY;
+
        guard(mutex)(&ftrace_lock);
 
        if (!fgraph_stack_cachep) {
@@ -1401,17 +1405,21 @@ void unregister_ftrace_graph(struct fgraph_ops *gops)
 {
        int command = 0;
 
+       if (WARN_ONCE(!(gops->ops.flags & FTRACE_OPS_FL_GRAPH),
+                     "function graph ops unregistered without registering"))
+               return;
+
        guard(mutex)(&ftrace_lock);
 
        if (unlikely(!ftrace_graph_active))
-               return;
+               goto out;
 
        if (unlikely(gops->idx < 0 || gops->idx >= FGRAPH_ARRAY_SIZE ||
                     fgraph_array[gops->idx] != gops))
-               return;
+               goto out;
 
        if (fgraph_lru_release_index(gops->idx) < 0)
-               return;
+               goto out;
 
        fgraph_array[gops->idx] = &fgraph_stub;
 
@@ -1434,4 +1442,6 @@ void unregister_ftrace_graph(struct fgraph_ops *gops)
                unregister_trace_sched_switch(ftrace_graph_probe_sched_switch, NULL);
        }
        gops->saved_func = NULL;
+ out:
+       gops->ops.flags &= ~FTRACE_OPS_FL_GRAPH;
 }