fs/squashfs: sqfs_read: Prevent arbitrary code execution

Following Jincheng's report, an out-of-band write leading to arbitrary
code execution is possible because on one side the squashfs logic
accepts directory names up to 65535 bytes (u16), while U-Boot fs logic
accepts directory names up to 255 bytes long.

Prevent such an exploit from happening by capping directory name sizes
to 255. Use a define for this purpose so that developers can link the
limitation to its source and eventually kill it some day by dynamically
allocating this array (if ever desired).

Link: https://lore.kernel.org/all/CALO=DHFB+yBoXxVr5KcsK0iFdg+e7ywko4-e+72kjbcS8JBfPw@mail.gmail.com
Reported-by: Jincheng Wang <jc.w4ng@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Tested-by: Jincheng Wang <jc.w4ng@gmail.com>

include/fs.h

@@ -174,6 +174,8 @@ int fs_write(const char *filename, ulong addr, loff_t offset, loff_t len,
 #define FS_DT_REG  8         /* regular file */
 #define FS_DT_LNK  10        /* symbolic link */
 
+#define FS_DIRENT_NAME_LEN 256
+
 /**
  * struct fs_dirent - directory entry
  *
@@ -194,7 +196,7 @@ struct fs_dirent {
 	/** change_time:	time of last modification */
 	struct rtc_time change_time;
 	/** name:		file name */
-	char name[256];
+	char name[FS_DIRENT_NAME_LEN];
 };
 
 /* Note: fs_dir_stream should be treated as opaque to the user of fs layer */