function_graph: Add READ_ONCE() when accessing fgraph_array[]

In function_graph_enter() there's a loop that looks at fgraph_array[]
elements which are fgraph_ops. It first tests if it is a fgraph_stub op,
and if so skips it, as that's just there as a place holder. Then it checks
the fgraph_ops filters to see if the ops wants to trace the current
function.

But if the compiler reloads the fgraph_array[] after the check against
fgraph_stub, it could race with the fgraph_array[] being updated with the
fgraph_stub. That would cause the stub to be processed. But the stub has a
null "func_hash" field which will cause a NULL pointer dereference.

Add a READ_ONCE() so that the gops that is compared against the
fgraph_stub is also the gops that is processed later.

Link: https://lore.kernel.org/all/CA+G9fYsSVJQZH=nM=1cjTc94PgSnMF9y65BnOv6XSoCG_b6wmw@mail.gmail.com/
Link: https://lore.kernel.org/linux-trace-kernel/[email protected]

Cc: Mark Rutland <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Fixes: cc60ee813b ("function_graph: Use static_call and branch to optimize entry function")
Reported-by: Naresh Kamboju <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>
Acked-by: Masami Hiramatsu (Google) <[email protected]>

kernel/trace/fgraph.c

@@ -641,7 +641,7 @@ int function_graph_enter(unsigned long ret, unsigned long func,
 	{
 		for_each_set_bit(i, &fgraph_array_bitmask,
 					 sizeof(fgraph_array_bitmask) * BITS_PER_BYTE) {
-			struct fgraph_ops *gops = fgraph_array[i];
+			struct fgraph_ops *gops = READ_ONCE(fgraph_array[i]);
 			int save_curr_ret_stack;
 
 			if (gops == &fgraph_stub)