mkbootimg can not rebuild mi-4c boot.img

[cong2014]

on xiaomi 4c, android version is 5.1.1, miui is v7.1

1. use tool to unpack boot.img
2. do nothing, but use mkbootimg to pack kernel and ramdisk again
3. the new boot.img is much smaller than before
4. push to device, can not boot.

[yunlai]

I have a similar issue

1. use unpackbootimg to unpack boot.img
2. do nothing, but use mkbootfs and mkbootimg to pack kernel and ramdisk again
3. push to the new boot img to device, boot failure

I compared the new boot.img with the original one ,fount out that even through the secondary partition size is 0, there is still 2048 bytes data seams like secondary partition data at the end of the
original boot.img, but the new one don't have this part.
I append this 2048 bytes to the new one ,still can not boot

[yunlai]

I hava the same issue with mi4s,miui 8.0.2