Jonathan Levin, http://newosxbook.com/ - 6/12/13

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• What Type Of Dmg Is Considered Falling

For Fallout 4 on the PlayStation 4, a GameFAQs message board topic titled 'What perk is needed for shotgun dmg'.

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1. About

As part of writing HFSleuth, a 'bonus' tool for my book, I decided to implement DMG (disk image support). I realized, however, that the DMG file format (being Apple proprietary) was woefully undocumented. I briefly mention DMGs (pages 589-590), but due to the page constraints of an already large book, I had failed to delve into their format sufficiently. This article, therefore, is an attempt to rectify that shortcoming. The DMG file format has been painstakingly reverse-engineered by several[1,2], and this article/addendum aims to consolidate their hard work into a single document. HFSleuth can operate fully on all known DMG types (to date), and can serve as a complementary tool to Apple's hdiutil(1), or - as it is POSIX portable - even as a replacement for it, on non OS X systems. When set to verbose mode, HFSleuth also provides step by step information as it processes DMGs, and is used in the examples below.

2. The Disk Image file format

The first noteable fact about the DMG file format is, that there is no DMG file format. DMGs come in a variety of sub-formats, corresponding to the different tools which create them, and their compression schemes. The common denominator of most of these is the existence of a 512-byte trailer at the end of the file. This trailer is identifiable by a magic 32-bit value, 0x6B6F6C79, which is 'koly' in ASCII. As other references to this trailer call it the 'koly' block, we can do the same. Note, that 'most' is not 'all': images created with hdiutil(1), for example, can simply be raw dd(1)-like images of the disk layout, with no metadata. In those cases, however, there is nothing special or noteworthy about the file, which can be read as any disk would, by its partition table (commonly APM, or GPT). Images created with the DiscRecording.Framework contain the koly block. The koly block, when present, is formatted according to the following:

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The most important elements in the koly block are the fields pointing to the XML plist: This property list, embedded elsewhere in the DMG, contains the DMG block map table. Commonly, the plist is placed in the blocks leading up to the koly block, which fits the simple algorithm to create a DMG: First compress the image blocks, then place the XML plist, and finalize with the koly block. This is shown in figure 1: Using HFSleuth in verbose mode on a DMG will dump the KOLY header, as shown in the following output:
This method of creating DMGs also explains why commands such as 'file' have a hard time identifying the DMG file type: In the absence of a fixed header, a DMG can start with any type of data (disk or partition headers), which can be further compressed by myriad means. DMG files compressed with BZlib, for example, start with a BZ2 header. They cannot be opened with bunzip2, however, since compression methods are intermingled, and bunzip2 will discard blocks which do not start with a bz2 header.
DMGs compressed with zlib often incorrectly appear as 'VAX COFF', due to the zlib header. The XML Property list (which is uncompressed and easily viewable by seeking to the DOCTYPE declaration using more(1) or using tail(1)) is technically the resource fork of the DMG. The property list file contains, at a minimum, a 'blkx' key, though it may contain other key/values, most commonly 'plst', and sometimes a service level agreement (SLA) which will be displayed by the OS (specifically, /System/Library/PrivateFrameworks/DiskImages.framework/Versions/A/Resources/DiskImages UI Agent.app/Contents/MacOS/DiskImages UI Agent) as a pre-requisite to attaching the DMG*. Due to XML parser restrictions, data in the property list is 7-bit. This forces all binary (8-bit) data to be encoded using Base-64 encoding (a wiser choice would have been using CDATA blocks). The output of such a property list is shown below: A detailed discussion of both APM and GPT can be found in chapter 15 of the book[3], as well as Apple's notes on APM[4] and GPT[5]. What makes the blxx data useful, however, is that it allows an implementation to skip past the partition table data, and isolate the partition of interest directly from the DMG. The 'data' in the blxx header is a structure, which (like its sibling, koly) is also identifiable by a fixed signature - in this case 'mish'. In Base-64 this encodes as 'bWlza', which is readily evident in the previous listing. The mish block is formatted like this:In other words, for each entry, the chunk of SectorCount sectors, starting at SectorNumber are stored at CompressedLength bytes, at offset CompressedOffset in the data fork. When expanded, each such chunk will take SectorCount * SECTOR_SIZE bytes. Each chunk of blocks in a given entry is stored using the same compression, but different entries can contain different compression methods. The various block chunk entry types are shown below:Running HFSleuth on a DMG in verbose and debug mode will produce detailed output of the decompression, demonstrating the above:Note in the example above the mix of Zlib and RAW compression methods: Zlib uses highly efficient compression algorithms, but sometimes it just makes sense to leave data in raw form (e.g. chunks 829 and 832). In these cases, the 'compressed' size is actually the same as the uncompressed size. It's also worth noting that (though it is commonly the case) there is no guarantee that the blocks are compressed in order.

3. Mounting DMGs

DMGs can be mounted, just like any other file system, though technically this is what is known as a 'loopback' mount (i.e. a mount backed by a local file, rather than a device file). To mount a DMG, the system uses the DiskImages kernel extension (KExt), also known as the IOHDIXController.kext. This is clearly visible in both OS X and iOS, using kextstat (or jkextstat, in the latter):The kext is provided with a number of 'PlugIn' kexts, namely:

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• AppleDiskImagesCryptoEncoding.kext
• AppleDiskImagesKernelBacked.kext
• AppleDiskImagesReadWriteDiskImage.kext - for UDRO/UDRW
• AppleDiskImagesFileBackingStore.kext
• AppleDiskImagesPartitionBackingStore.kext - Uses the Apple GUID 444D4700-0000-11AA-AA11-00306543ECAC
• AppleDiskImagesSparseDiskImage.kext - for UDSP
• AppleDiskImagesHTTPBackingStore.kext - Allows DMGs to reside on a remote HTTP server. Uses a 'KDISocket' with HTTP/1.1 partial GETs (206) to get the chunks it needs from a DMG
• AppleDiskImagesRAMBackingStore.kext
• AppleDiskImagesUDIFDiskImage.kext

The attachment of a DMG starts in user mode, by an I/O Kit call to IOHDIXController, preparing a dictionary with the following keys:

• hdik-unique-identifier - A UUID created by the caller (e.g. CFUUIDCreate())
• image-path - the path to the DMG in question

Some types of disk images (sparse, uncompressed, and z-Lib compressed) are natively supported by the kernel and can be mounted directly by it. A good example is the DeveloperDiskImage.dmg found in the iOS SDK. More often than not, however, mounting resorts to user-mode helper processes. This, in fact, is default on OS X (q.v. hdiutil -nokernel vs. hdiutil -kernel). When attaching a DMG, the DiskImages private framework spawns diskimages-helper and hdiejectd. The former is started with a -uuid argument per invocation, allowing the mounting of the same DMG multiple times. If the process is stopped, filesystem operations on its contents will likewise hang (with the exception of those already cached by VFS). You can demonstrate this with a simple experiment by mounting a DMG, sending the corresponding diskimages helper a STOP signal, and performing a filesystem intensive operation, such as an ls -lR, witnessing the hang, then sending a CONT. Further inspection in GDB with a breakpoint on mach_msg will enable you to peek at the Mach messages which are passed between the process and the kernel over the I/O Kit interface. This will show a backtrace similar to:Looking at the arguments to DI_kextDriveGetRequest (specifically, $rdx+ 0x20), will reveal a pointer to the data returned from the DMG file by the diskimages-helper. Apple provides extensive support for DMGs, which is only natural given their role in everything, from aspects of OS installation to software distribution. The DMG support is provided by the DiskImages project, which contains both the user mode (hdid, hdiutil) and kernel mode (kexts) required for operation. Lamentably, Apple keeps this as one of the non-open source projects in Darwin.

• hdid
• hdiutil
• DiskImages.framework - The private framework lending support to both the above tools, communicating with the KExts (below), as well as the user mode helper processes for mounting images (diskimages-helper and hdiejectd)
• IOHDIXController.kext

Answer: Advantages of using per-block compression, rather than a single compression algorithm for entire file:

1. Optimize compression for type of data: For example, discard blocks of zeros rather than compressing them, or even leaving data uncompressed
2. Allow an implementation to selectively decompress chunks, rather than the whole image, which may take a lot of filesystem space and/or memory (especially in kernel-mode).

References:

1. DMG2IMG:http://vu1tur.eu.org
   
2. DMG2ISO:at sourceforge.net