Edit system.img and repack then flash - Android Q&A, Help & Troubleshooting

Hello,
I've got all the ROM from sp flash tools, when I repack the folder where I unpacked everything gets a smaller size than the system.img file, when I repack, I also get a smaller size even that I use make_ext4fs with the correct size.
So when I flash the system.img the phone goes straight to bootloader and doesn't boot.
How can I fix this?
Also, I want to add system apps to the system.img, how can I do that?

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ProIEZRush said:
How can I fix this?
Also, I want to add system apps to the system.img, how can I do that?
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Hello and good afternoon, @ProIEZRush
Welcome to XDA! I hope you'll alawys get the support your require.
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Oswald Boelcke
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Hi there! I am trying to learn the same thing as well! I feel down the rabbit hole a year ago trying to get "tools" to work, and was not successful, maybe someone can teach us.
I can however, share with you a theory of different way of doing what we are trying to do that might? work for you? Depends if you have working firmware or not.
If you are able to flash your firmware and then preinstall your apps, you can pull the partitions.
It seems not all the partitions are the same on each device, so you would have to poke around to figure out what all is what.
running command df - h in adb shell can give you some clues. If you are able to install magisk and watch it in twrp, it can give you another clue about your boot.img partition.
I have all the commands and stuff I worked out in this comment as well as what I have tried and worked out. Maybe this can work for us!
Just note your mmc blocks- what they actually are- is likely different than mine
Also maybe someone can elaborate more, I just learned how to do this yesterday and it worked for me to not bootloop with the modded boot.img, just trying to find the right partition for my system now.

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Thanks for your answer, I'm gonna try that, but I can't get the new system.img to boot on the device, I'm using a basic device that doesn't have TWRP.
Also, when I flash the edited system.img the device doesn't boot, I think its because of the size, any clue to fix that?

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I know there is a command with it, one of the tool kits that carries the individual exe or bins whatever os you are running had it in there but I cannot think what it was but it could capture the original file size and amend the new one, I remember manually doing it by hand and not with a all in one tool. Your line of thinking is right, maybe check out some of the old toolsets on here. That is likely where I found it.

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It was something like this, I tried running these in cmd and reading their help files but I didn't see what I was looking for, but some toolset like this on this site has the exe or bin (if linux) you need

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I used make_ext4fs but I'm still getting smaller size