debugfs: check for NULL pointer in debugfs_create_str()

Passing a NULL pointer to debugfs_create_str() leads to a NULL pointer
dereference when the debugfs file is read. Following upstream
discussions, forbid the creation of debugfs string files with NULL
pointers. Add a WARN_ON() to expose offending callers and return early.

Fixes: 9af0440 ("debugfs: Implement debugfs_create_str()")
Reported-by: yangshiguang <yangshiguang@xiaomi.com>
Closes: https://lore.kernel.org/lkml/2025122221-gag-malt-75ba@gregkh/
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Gui-Dong Han <hanguidong02@gmail.com>
Link: https://patch.msgid.link/20260323085930.88894-2-hanguidong02@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: 2045gemini

fs/debugfs/file.c

@@ -1127,7 +1127,7 @@ static const struct file_operations fops_str_wo = {
  *          directory dentry if set.  If this parameter is %NULL, then the
  *          file will be created in the root of the debugfs filesystem.
  * @value: a pointer to the variable that the file should read to and write
- *         from.
+ *         from. This pointer and the string it points to must not be %NULL.
  *
  * This function creates a file in debugfs with the given name that
  * contains the value of the variable @value.  If the @mode variable is so
@@ -1136,6 +1136,9 @@ static const struct file_operations fops_str_wo = {
 void debugfs_create_str(const char *name, umode_t mode,
 			struct dentry *parent, char **value)
 {
+	if (WARN_ON(!value || !*value))
+		return;
+
 	debugfs_create_mode_unsafe(name, mode, parent, value, &fops_str,
 				   &fops_str_ro, &fops_str_wo);
 }