Lindus Embedded

Ecryptfs can encrypt partitions or directores without its own partition. All the information needed to decrypt is contained on the files themselves, so it’s ideal for backup and restoration on any other system.

To create an encrypted folder:

sudo apt-get install ecryptfs-utils
mount -t ecryptfs /path/folder /path/folder

It will ask a few questions, starting with a passphrase. Enter the passphrase and leave all as defaults (aes, 16 bytes, no plaintext passthrough, no filename encryption, append signature to /root/.ecryptfs/sig-cache.txt).

mount will show (with your own ecryptfs_sig):

/path/folder on /path/folder type ecryptfs 

    (rw,ecryptfs_sig=bd28c38da9fc938b,ecryptfs_cipher=aes,ecryptfs_key_bytes=16,ecryptfs_unlink_sigs)

You follow the same steps to mount an existing encrypted folder. If you have never mounted that filesystem on the computer you are using you will see:

WARNING: Based on the contents of [/root/.ecryptfs/sig-cache.txt],
it looks like you have never mounted with this key
before. This could mean that you have typed your
passphrase wrong.
Would you like to proceed with the mount (yes/no)?

Just type yes, it’s a safe answer.

The following presentation contains information already present in Yocto’s documentation but ordered in an easier to make sense of way.

Since the kernel.org servers were hacked a browsable repository can be found here.

Ramdisk

A ramdisk.img (initrd) is a small partition image that is mounted read-only by the kernel at boot time. It only contains /init and a few config files. It is used to start init which will mount the rest of the system images properly and run the init procedure. A Ramdisk is a standard Linux feature.

ramdisk.img is included ($SDK_ROOT/tools/lib/images/ramdisk.img) or compiled ($SDK_ROOT/out/target/product/$PRODUT_NAME/ramdisk.img) by the google android sdk. It’s in a gzipped cpio file.

Modifying Android’s ramdisk image

To modify it, first copy it to your Linux machine and extract it as follows:

Make any changes you want to the extracted ramdisk.cpio, and remove the unneeded files. Recreate the ramdisk.cpio with command:

System and data images

The system.img is a partition image that will be mounted as / and thus contains all system binaries.
The userdata.img is a partition image that can be mounted as /data and thus contains all application-specific and user-specific data.

On the real device they have to be mounted by the init.rc script which runs from the ramdisk. They can be of different formats, usually YAFFS2, EXT4 or UBI-FS.

They are generated by the build system and can be used to flash to a real device. The emulator uses them differently (see below)

Android emulator images

• system.img is copied into a temporary file, which is used by the emulator session. So any change you make to / as root in the emulator are lost when the program exits
• userdata.img is only used when you use -wipe-data. Instead, it uses ~/.android/userdata-qemu.img (on Linux) as the persistent /data partition image. Using -wipe-data simply copies the content of userdata.img into userdata-qemu.img
• sdcard.img, if present or specified with -sdcard , corresponds to /sdcard
• cache.img if specified in the emulator with -cache corresponds to /cache. if none is specified, an empty temporary file is created at emulator startup and /cache will be mounted to it. this temp file is automatically cleaned up at exit.

The emulator will not modify system.img and userdata.img since they were generated as device images.

Extracting Android’s YAFFS2 images

A YAFFS2 file is identied as a “VMS Alpha executable” by Linux.

Download unyaffs from the unyaffs Google Project Hosting.

If the executable does not work on your system you can download the source and recompile it.

Copy the YAFF2 img files to the same folder and extract them:

Extracting JFFS2 images

Just for completion, here is how to extract a JFFS2 image.