kernel/trace: Add READ_ONCE() when accessing fgraph_array[]

In __ftrace_return_to_handler(), a loop iterates over the fgraph_array[]
elements, which are fgraph_ops. The loop checks if an element is a
fgraph_stub to prevent using a fgraph_stub afterward.

However, if the compiler reloads fgraph_array[] after this check, it might
race with an update to fgraph_array[] that introduces a fgraph_stub. This
could result in the stub being processed, but the stub contains a null
"func_hash" field, leading to a NULL pointer dereference.

To ensure that the gops compared against the fgraph_stub matches the gops
processed later, add a READ_ONCE(). A similar patch appears in commit
63a8dfb ("function_graph: Add READ_ONCE() when accessing fgraph_array[]").

Signed-off-by: Zilin Guan <redacted>
---
 kernel/trace/fgraph.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index ddedcb509..30e3ddc8a 100644
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c

@@ -833,7 +833,7 @@ static unsigned long __ftrace_return_to_handler(struct fgraph_ret_regs *ret_regs
 #endif
 	{
 		for_each_set_bit(i, &bitmap, sizeof(bitmap) * BITS_PER_BYTE) {
-			struct fgraph_ops *gops = fgraph_array[i];
+			struct fgraph_ops *gops = READ_ONCE(fgraph_array[i]);
 
 			if (gops == &fgraph_stub)
 				continue;

-- 
2.34.1