First, an introduction:
The Image Update system allows the OEM (us!
) to issue updates to a "Live" filesystem - without disrupting user data. This allows, for example, a buggy driver to be updated after the phone has been shipped, or a software package to be updated to the latest version, with minimal knowledge on the user's part. The system validates all updates against an internal list of certificates, and refuses the update if a match is not obtained. This system can also be used to deploy entirely new software to the device (such as support for another locale, input method editor, application support for a new feature the carrier is rolling out, etc.)
Potential usage scenarios for this system
A central server could be maintained for all SYS/OEM updates - each ROM Chef would need to maintain a list of original packages, any updated package(s), and download URL's for each updated package. The user would then receive these updates through the built-in AutoUpdate facility in Windows Mobile, which can check periodically, or on-demand. Each Chef could maintain seperate download servers from the update server to minimize server load.
Alternatively, a chef could provide .cab.pkg updates in his or her ROM thread, on their own web site, etc., and the user could download these and install them at will. These packages can optionally be authenticated to be coming from the Chef, if the Chef wants to ensure updates are coming from him only. A public certificate could also be used to allow users to issue updates as well.
The more technical Summary
Image Update allows an OEM to issue updates to the OEM's, XIP, SYS, (possibly) Radio, or any combination of these. The update can be pushed to the user via a specially formatted SMS or by manual execution. There are at least 2 levels of certificate checking involved in the process, I believe against \SYS\Metadata\DefaultCerts.dat. The system reboots into the ULDR to apply the update, because the filesystem cannot be modified while actively mounted. The ULDR provides a minimal operating enviornment to facilitate this.
How does a Chef need to prepare a ROM for Image Updating?
The Chef would need to use a ROM Kitchen that leaves the .dsm and .rgu file structure intact (i.e. an "unprotected" ROM) - All .dsm's in this ROM would need to be properly formatted with Package Name, versioning info, etc. during the cooking process, in order to facilitate version checking, etc. Each .dsm would need to be signed with a certificate the Chef would use to validate the update as coming from him or her. (Alternatively a public certificate could be used like SDKCerts if the Chef chooses not to maintain direct control over updates, and allow other users to create updates as well)
The Chef also needs to ensure not to Reduce the ULDR partition or remove it; this would cripple the Update Loader and prevent the Image Update system from functioning.
The .cab.pkg format
At the root of a package, the .dsm defines the Package structure (all files, registry entries, etc.) It contains version info, certificates, and other data. A ROM consists of a number of these packages, in an area of flash memory that is not user-writable. When someone wants to issue an update using the ImageUpdate system, they create a matching .dsm, same guid, with a newer version number, same internal package name, same processor ID, os version, etc., there is also a flag that can be marked as an update package or a new package - in this .dsm they define the files that will make up the new, updated package. Here they can add or remove files. One of the files defined by the package is optionally an .rgu, and this defines the registry entries associated with the package. Also optionally included is a provxml to facilitate file operations (copy/replace/delete/rename/etc.) and further registry or metabase operations. This collection of files is rolled up into a ".cab.pkg" archive by a program like cabarc. Once in a pkg.cab format, the package is signed.
A .cab.pkg is referred to as a "Canonical Package" and multiple Canonical Packages can be rolled up into a single "Super Package" to facilitate updating multiple Packages at the same time.
Inside this .cab.pkg is where the "MNGE" file format comes in to play. Essentially, this format is Microsoft's way of storing an XIP Module in the filesystem. (Their equivalent of our imageinfo.bin + s000, s001, etc.) - The "MNGE" format is simply a 4-byte MNGE header, followed by the imageinfo.bin, followed by the s00x sections attached to the end. When the Image Update system processes a file in this MNGE format, it is added to the IMGFS as an XIP Module.
Deploying a .cab.pkg to a device
Now there are several ways to deploy this package to the device, the primary way is an OMA-DM SMS message, which triggers the Image Update system to initiate a download from a server defined in the SMS message. The server can be either a normal HTTP server or a secure HTTPS server. The Image Update system will prompt the user to plug in the usb cable to download over the users home connection if the user has not already authorized the Auto Update system to utilize their GPRS data connection.
Another way for a .cab.pkg update to be pushed to the system is simply through a file copy operation, be it ActiveSync, SD Card, Bluetooth, or otherwise. Once on the device, the .cab.pkg is executed by the user the same way a .cab would be.
The Update Agent
Initiated by either a completed download from push SMS, or user-executed, the "Update Agent" program (which is part of the \SYS\FWUPDATE Package) attempts to validate the Certificates, Package dependancies, and other info contained in the .cab.pkg and .dsm. Once validated, the "Update Agent" sets a flag that the bootloader reads, the flag is a boolean, off = boot into normal OS, on = boot into ULDR - so then the system reboots, the flag is read, and you load into...
The Update Loader
The "Update Loader" or "ULDR" which is a minimal kernel configuration, that provides just enough driver support to display info on screen, respond to user input, and read/write from the internal flash (NAND or NOR)
From here the ULDR does further validation on the .cab.pkg, and applies it to the filesystem. If there are any modules in the package it dynamically relocates the memory map to make sure there are no overlaps. This is why it's important that reloc's not be removed from your ROM - ULDR will fail in this case.
The End Result
Once the ULDR has completed updating, the device is again rebooted, back into the full system, where the now-updated packages are now a part of the IMGFS, any updated files are processed (.rgu, .provxml, etc.) - The package is now a full part of the ROM.
The new .dsm replaces the old .dsm (along with the other files in the package) and now a future update will be checked against this new package.
If the update was pushed via OMA-DM SMS, or AutoUpdate, the device Pushes a notification to the OMA-DM server notifying it of the update status.
What's missing right now to implent the ImageUpdate system?
We need a Kitchen that's properly configured to allow us to create versioning info, proper package names, and insert this along with a certificate (or multiple certificates) into the .dsm's.
We also need the Kitchen to be able to modify \SYS\Metadata\DefaultCerts.dat with the certificates used, so that it passes authentication. Alternatively the authentication checking could be patched out. (this one is easily doable at build-time)
We need a program that can convert from a standard file to an MNGE format, so we can implement modules in our .cab.pkg's. (done it seems, thanks ervius!)
We (optionally) need a properly configured web server that supports HTTP/HTTPS, can communicate the proper xml configuration data, and can be updated with new packages by Chefs. (this one's a ways off)
We (optionally) need a program to convert from MNGE format to a standard file to facilitate extracting modules from .cab.pkg's. (working hard on that)
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