Ramdisk changes not reflected on Android filesystem

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LOLgrep

New member
Jul 28, 2022
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Hey all,

I am learning how Android works and am trying to figure out how I can update the Android filesystem by extracting a ramdisk from normal boot.img, adding some files, then flashing it back. So far, I have been unsuccessful in doing this and am hoping to figure out why. Here's the steps below I have taken:

Using a Google Pixel 4a, Android 11, kernel v 4.14 (i.e. not GKI)

High level:
Extract ramdisk.cpio from boot.img using magiskboot via adb on device, modify extract contents, sent back up to magiskboot, repackaged, then flashed via fastboot.

Detailed steps:

Grab ramdisk.cpio
Code:
$ # obtain the ramdisk.cpio from magiskboot
$ adb -d shell  "cd ${BOOT_IMG_PATH}; ./magiskboot unpack boot.img"
$ adb -d pull /${BOOT_IMG_PATH}/ramdisk.cpio /tmp/


$ # attempt to modify the filesystem
$ mkdir /tmp/rd && cd rd
$ cpio -i < /tmp/ramdisk.cpio
$ touch yolo
$ echo "why doest this work" > system/wtf.txt
$ echo "why doest this work" > sys/wtf.txt
$ echo "why doest this work" > vendor/wtf.txt

#patch this directory back up and send to magiskboot
$ find . | cpio -oH new > /tmp/new.ramdisk.cpio
$ adb -d push /tmp/new.ramdisk.cpio ${BOOT_IMG_PATH}/ramdisk.cpio
$ adb -d shell "cd ${BOOT_IMG_PATH}; ./magiskboot repack boot.img
$ adb -d pull /${BOOT_IMG_PATH}/new-boot.img /tmp/

# apply this modifyied boot.img
$ adb reboot bootloader
fastboot flash boot /tmp/new-boot.img
fastboot reboot

After doing this, I'll adb back in to verify:
Code:
adb -d shell "find / -name "wtf.txt" 2>/dev/null
# silence.... always silence... no file change


* I am aware that Wu modifies the extracted dtb file from boot.img with a "magiskboot dtb dtb patch" command but that doesn't seem to apply to my particular boot.img as the fstab doesn't seem to be around
* I am aware that vbmeta and codesigning, I have disabled vbmeta via fastboot
* I am aware that there's A/B slots for flashing. I have tried flashing both slots to make sure the updated ramdisk is seen
* I am aware of magiskboot's kernel patch from skip_initramfs -> want_initramfs. I could use some clarification on this if it pertains to my problem
* My Android device uses "mount method C" from Wu's great writeup https://github.com/topjohnwu/Magisk/blob/master/docs/boot.md. That is, it's init's job to mount everything on my device. I guess I feel confused as to why init wouldn't mount the additional files that I've added to the ramdisk


Extremely grateful for help or guidance on what I've overlooked. Thanks y'all
 

Renate

Recognized Contributor / Inactive Recognized Dev
Feb 3, 2012
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Nexus 7 (2013)
Moto E5
You should probably examine your modified boot file to see if the new stuff is in there.
I don't use your tools or even deal with cpio as a file type.
Code:
C:\>echo Hello > sbin\bogus
C:\>imgutil /i boot.img sbin/bogus
C:\>imgutil /l boot.img
...
sbin/bogus
...