modifying a cerbo swu file

mkdir tmp rootfs rootfs.user
(cd tmp && cpio -idv < ../venus-swu-einstein.swu)
gunzip tmp/venus-image-einstein.ext4.gz
e2fsck -f tmp/venus-image-einstein.ext4
resize2fs tmp/venus-image-einstein.ext4 1G
sudo mount tmp/venus-image-einstein.ext4 rootfs
sudo bindfs -u $(id -u) -g $(id -g) rootfs rootfs.user

Do what you want with rootfs.user...

sudo umount rootfs.user rootfs
pigz tmp/venus-image-einstein.ext4
(cd tmp && find -type f | sort | cpio -ov -H crc > ../venus-swu-einstein.swu)
rm -rf tmp rootfs rootfs.user
./fix-swu.py venus-swu-einstein.swu

fix-swu.py

#!/usr/bin/python3

import sys

# cpio seems to add zero padding to the swu file up to the block size. swupdate
# expects it to padding till it is 4 bytes aligned again. These trailing zeros
# can crash swupdate by causing a SIGPIPE. So make sure only the trailing zeros
# as expected by swupdate are in the swu file.

def swupdate_fix_cpio_padding(filename):
    file = open(filename, "r+b")
    file.seek(0, 2)
    addr = file.tell() - 1

    while addr:
        file.seek(addr, 0)

        if ord(file.read(1)) != 0:
            addr += 1
            addr += 4 - (addr % 4)
            print("truncating " + filename + " to " + str(addr))
            file.truncate(addr)
            break
        addr -= 1

if __name__ == "__main__":
   swupdate_fix_cpio_padding(sys.argv[1])