soundwire: debugfs: initialize firmware_file to empty string

Passing NULL to debugfs_create_str() causes a NULL pointer dereference,
and creating debugfs nodes with NULL string pointers is no longer
permitted.

Additionally, firmware_file is a global pointer. Previously, adding every
new slave blindly overwrote it with NULL.

Fix these issues by initializing firmware_file to an allocated empty
string once in the subsystem init path (sdw_debugfs_init), and freeing
it in the exit path. Existing driver code handles empty strings
correctly.

Fixes: fe46d2a ("soundwire: debugfs: add interface to read/write commands")
Reported-by: yangshiguang <yangshiguang@xiaomi.com>
Closes: https://lore.kernel.org/lkml/17647e4c.d461.19b46144a4e.Coremail.yangshiguang1011@163.com/
Signed-off-by: Gui-Dong Han <hanguidong02@gmail.com>
Link: https://patch.msgid.link/20260323085930.88894-4-hanguidong02@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: 2045gemini

drivers/soundwire/debugfs.c

@@ -358,8 +358,8 @@ void sdw_slave_debugfs_init(struct sdw_slave *slave)
 	debugfs_create_file("go", 0200, d, slave, &cmd_go_fops);
 
 	debugfs_create_file("read_buffer", 0400, d, slave, &read_buffer_fops);
-	firmware_file = NULL;
-	debugfs_create_str("firmware_file", 0200, d, &firmware_file);
+	if (firmware_file)
+		debugfs_create_str("firmware_file", 0200, d, &firmware_file);
 
 	slave->debugfs = d;
 }
@@ -371,10 +371,15 @@ void sdw_slave_debugfs_exit(struct sdw_slave *slave)
 
 void sdw_debugfs_init(void)
 {
+	if (!firmware_file)
+		firmware_file = kstrdup("", GFP_KERNEL);
+
 	sdw_debugfs_root = debugfs_create_dir("soundwire", NULL);
 }
 
 void sdw_debugfs_exit(void)
 {
 	debugfs_remove_recursive(sdw_debugfs_root);
+	kfree(firmware_file);
+	firmware_file = NULL;
 }