drivers: kernelsu: throne_tracker: avoid cross-fs traversal using s_magic check

Skip directories that does NOT have the same magic as /data/app.
This is to avoid scanning incfs and any other stacked filesystems.

While this is way dumber, it's way cheaper.
no kern_path(), no missable path_put(), no ref handling.

This supercedes:
- "throne_tracker: avoid cross fs access (tiann/KernelSU#2626)"
- upstream tiann/KernelSU@0b6998b

Signed-off-by: backslashxx <[email protected]>
Signed-off-by: Tashfin Shakeer Rhythm <[email protected]>

Author: backslashxx

drivers/kernelsu/throne_tracker.c

@@ -258,6 +258,24 @@ void search_manager(const char *path, int depth, struct list_head *uid_data)
 					goto skip_iterate;
 				}
 
+				// grab magic on first folder, which is /data/app
+				if (!data_app_magic) {
+					if (file->f_inode->i_sb->s_magic) {
+						data_app_magic = file->f_inode->i_sb->s_magic;
+						pr_info("%s: dir: %s got magic! 0x%lx\n", __func__, pos->dirpath, data_app_magic);
+					} else {
+						filp_close(file, NULL);
+						goto skip_iterate;
+					}
+				}
+
+				if (file->f_inode->i_sb->s_magic != data_app_magic) {
+					pr_info("%s: skip: %s magic: 0x%lx expected: 0x%lx\n", __func__, pos->dirpath,
+						file->f_inode->i_sb->s_magic, data_app_magic);
+					filp_close(file, NULL);
+					goto skip_iterate;
+				}
+
 				iterate_dir(file, &ctx.ctx);
 				filp_close(file, NULL);
 			}