scripts: add CAMEO tag generator

This script inserts CAMEO tags into an uImage to make U-Boot
of DGS-1210 switches happy.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Suggested-by: Sander Vanheule <sander@svanheule.net> # Mutual checksum algorithm
[commit title prefix, trailing whitespace, OpenWrt capitalisation, move
CRC calculation comment, use UIMAGE_NAME_*, remove parentheses for
return, use f-string instead of str()]
Signed-off-by: Sander Vanheule <sander@svanheule.net>

diff --git a/scripts/cameo-tag.py b/scripts/cameo-tag.py
new file mode 100755 (executable)
index 0000000..643dd38
--- /dev/null
+++ b/scripts/cameo-tag.py
@@ -0,0 +1,117 @@
+#!/usr/bin/python3
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# Copyright (C) 2022 OpenWrt.org
+#
+# ./cameo-tag.py <uImageFileName> <OffsetOfRootFS>
+#
+# CAMEO tag generator used for the D-Link DGS-1210 switches. Their U-Boot
+# loader checks for the string CAMEOTAG and a checksum in the kernel and
+# rootfs partitions. If not found it complains about the boot image.
+# Nevertheless it will boot if the tags are available in the secondary
+# boot partitions. If some day we want to overwrite the original vendor
+# partition we must have the tags in place. To solve this we insert the
+# tag two times into the kernel image.
+#
+# To understand what we do here it is helpful to explain how the original
+# CAMEO tag generation/checking works. The firmware consists of two parts.
+# A kernel uImage (<1.5MB) and a rootfs image (<12MB) that are written to
+# their respective mtd partitions. The default generator simply checksums
+# both parts and appends 16 bytes [<CAMEOTAG><0001><checksum>] to each part.
+# The checksum is only an addition of all preceding bytes (b0+b1+b2+...).
+# A tag does not interfere with any data in the images itself. During boot
+# the loader will scan all primary/secondary partitions (2*kernel, 2*rootfs)
+# until it finds the CAMEO tag. If checksums match everything is fine.
+# If all 4 fail we are lost. Luckily the loader does not care about where
+# the tags are located and ignores any data beyond a tag.
+#
+# The OpenWrt image consists of a kernel (>1.5MB) and a rootfs. There is
+# no chance to add CAMEO tags at the default locations, since the kernel spans
+# both the original kernel partition and the start of the rootfs partition.
+# This would leave the kernel partition without a tag. So we must find suitable
+# space.
+#
+# Location for original kernel partition is at the end of the uImage header.
+# We will reuse the last bytes of the IH_NAME field. This is the tricky part
+# because we have the header CRC and the CAMEO checksum that must match the
+# whole header. uImage header CRC checksums all data except the CRC itself. The
+# for CAMEO checksum in turn, checksums all preceding data except itself.
+# Changing one of both results in a change of the other, but data trailing the
+# CAMEO checksum only influences the CRC.
+#
+# Location for original rootfs partition is very simple. It is behind the
+# OpenWrt compressed kernel image file that spans into the rootfs. So
+# the tag will be written somewhere to the following rootfs partition and
+# can be found by U-Boot. The CAMEO checksum calculation must start at the
+# offset of the original rootfs partition and includes the "second" half of the
+# "split" kernel uImage.
+
+import argparse
+import os
+import zlib
+
+READ_UNTIL_EOF = -1
+UIMAGE_HEADER_SIZE = 64
+UIMAGE_CRC_SLICE = 4
+UIMAGE_CRC_END = 8
+UIMAGE_NAME_OFF = 32
+UIMAGE_NAME_END = 56
+UIMAGE_SUM_OFF = 56
+UIMAGE_SUM_END = 60
+UIMAGE_INV_OFF = 60
+UIMAGE_INV_END = 64
+CAMEO_TAG = bytes([0x43, 0x41, 0x4d, 0x45, 0x4f, 0x54, 0x41, 0x47, 0x00, 0x00, 0x00, 0x01])
+IMAGE_NAME = bytes([0x4f, 0x70, 0x65, 0x6e, 0x57, 0x72, 0x74, 0x00, 0x00, 0x00, 0x00, 0x00])
+CRC_00 = bytes([0x00] * 4)
+CRC_FF = bytes([0xff] * 4)
+
+def read_buffer(offset, count):
+    args.uimage_file.seek(offset)
+    return bytearray(args.uimage_file.read(count))
+
+def write_buffer(whence, buf):
+    args.uimage_file.seek(0, whence)
+    args.uimage_file.write(buf)
+
+def cameosum(buf):
+    return (sum(buf) & 0xffffffff).to_bytes(4, 'big')
+
+def invertcrc(buf):
+    return (zlib.crc32(buf) ^ 0xffffffff).to_bytes(4, 'little')
+
+def checksum_header(buf):
+    # To efficently get a combination, we will make use of the following fact:
+    #     crc32(data + littleendian(crc32(data) ^ 0xffffffff)) = 0xffffffff
+    #
+    # After manipulation the uImage header looks like this:
+    #     [...<ffffffff>...<OpenWrt><000000><CAMEOTAG><0001><checksum><InvCRC>]
+    buf[UIMAGE_NAME_OFF:UIMAGE_NAME_END] = IMAGE_NAME + CAMEO_TAG
+    buf[UIMAGE_CRC_OFF:UIMAGE_CRC_END] = CRC_FF
+    buf[UIMAGE_SUM_OFF:UIMAGE_SUM_END] = cameosum(buf[0:UIMAGE_NAME_END])
+    buf[UIMAGE_CRC_OFF:UIMAGE_CRC_END] = CRC_00
+    buf[UIMAGE_INV_OFF:UIMAGE_INV_END] = invertcrc(buf[0:UIMAGE_SUM_END])
+    buf[UIMAGE_CRC_OFF:UIMAGE_CRC_END] = CRC_FF
+    return buf
+
+parser = argparse.ArgumentParser(description='Insert CAMEO firmware tags.')
+parser.add_argument('uimage_file', type=argparse.FileType('r+b'))
+parser.add_argument('rootfs_start', type=int)
+args = parser.parse_args()
+
+args.uimage_file.seek(0, os.SEEK_END)
+if args.uimage_file.tell() <= args.rootfs_start:
+    raise ValueError(f"uImage must be larger than {args.rootfs_start} bytes")
+
+# tag for the uImage Header of 64 bytes inside the kernel
+# partition. Read and mangle it so it contains a valid CAMEO tag
+# and checksum that matches perfectly to the uImage header CRC.
+
+buf = checksum_header(read_buffer(0, UIMAGE_HEADER_SIZE))
+write_buffer(os.SEEK_SET, buf)
+
+# tag for the second part of the kernel that resides in the
+# vendor rootfs partition. For this we will add the CAMEO tag
+# and the checksum to the end of the image.
+
+buf = read_buffer(args.rootfs_start, READ_UNTIL_EOF)
+write_buffer(os.SEEK_END, CAMEO_TAG + cameosum(buf + CAMEO_TAG))