Ramdisk changes not reflected on Android filesystem

[LOLgrep]

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

$ # 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:

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]

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.

C:\>echo Hello > sbin\bogus
C:\>imgutil /i boot.img sbin/bogus
C:\>imgutil /l boot.img
...
sbin/bogus
...