trinadh465/platform_external_gptfdisk_aosp10_r33_cve-2021-0308Disk partitioning suite for GPT and MBR disks, offering interactive and scriptable tools to create, modify, and repair partition tables, including fixing corrupted MBR structures.
GPT fdisk (aka gdisk, cgdisk, and sgdisk) and FixParts by Roderick W. Smith, [email protected]
This package includes the source code for four related disk partitioning programs:
gdisk -- This program is modeled after Linux fdisk, but it operates on GUID Partition Table (GPT) disks rather than the Master Boot Record (MBR) disks that fdisk modifies. As such, gdisk is an interactive text-mode tool for manipulating partitions, but it does nothing to the contents of those partitions (usually filesystems, but sometimes swap space or other data).
cgdisk -- This program is modeled after Linux cfdisk, but it operates on GPT disks rather than the MBR disks that cfdisk modifies. As such, cgdisk is a curses-based text-mode tool for manipulating partitions, which is to say that it uses an interface that relies on arrow keys and a dynamic display rather than the command letters and a scrolling display like gdisk uses.
sgdisk -- This program is conceptually similar to the Linux sfdisk and FreeBSD gpt programs, but its operational details differ. It enables manipulation of GPT disks using command-line options, so it's suitable for use in scripts or by experts to perform specific tasks that might take several commands in gdisk to accomplish.
fixparts -- This program, unlike the preceding three, operates on MBR disks. It's intended to fix certain problems that can be created by various utilities. Specifically, it can fix mis-sized extended partitions and primary partitions located in the middle of extended partitions. It also enables changing primary vs. logical partition status (within limits of what's legal in the MBR scheme) and making a few other minor changes. It does NOT support creating new partitions; for that, you should use fdisk, parted, or some other tool.
More details about the abilities of these tools follows.
All four programs rely on the same set of underlying code base; they differ only in their control interfaces (defined in gdisk.cc, cgdisk.cc, sgdisk.cc, and fixparts.cc, respectively) and in which support code they use.
The gdisk program is intended as a (somewhat) fdisk-workalike program for GPT-partitioned disks, cgdisk is similarly a workalike for fdisk, and sgdisk provides most of gdisk's functionality in a more script-friendly program. Although libparted and programs that use it (GNU Parted, gparted, etc.) provide the ability to handle GPT disks, they have certain limitations that gdisk overcomes. Specific advantages of gdisk, cgdisk, and sgdisk include:
The ability to convert MBR-partitioned disks in-place to GPT format, without losing data
The ability to convert BSD disklabels in-place to create GPT partitions, without losing data
The ability to convert from GPT format to MBR format without data loss (gdisk and sgdisk only)
More flexible specification of filesystem type code GUIDs, which GNU Parted tends to corrupt
Clear identification of the number of unallocated sectors on a disk
A user interface that's familiar to long-time users of Linux fdisk and cfdisk (gdisk and cgdisk only)
The MBR boot loader code is left alone
The ability to create a hybrid MBR, which permits GPT-unaware OSes to access up to three GPT partitions on the disk (gdisk and sgdisk only)
Of course, GPT fdisk isn't without its limitations. Most notably, it lacks the filesystem awareness and filesystem-related features of GParted. You can't resize a partition's filesystem or create a partition with a filesystem already in place with gdisk, for instance. There's no GUI version of gdisk.
The GPT fdisk package provides three program files: the interactive text-mode gdisk, the curses-based interactive cgdisk, and the command-line-driven sgdisk. The first two are intended for use in manually partitioning disks or changing partitioning details; sgdisk is intended for use in scripts to help automate tasks such as disk cloning or preparing multiple disks for Linux installation.
This program's creation was motivated by cries for help I've seen in online forums from users who have found their partition tables to be corrupted by various buggy partitioning tools. Although most OSes can handle the afflicted disks fine, libparted-based tools (GParted, parted, most Linux installers, etc.) tend to flake out when presented with these disks. Typically, the symptom is a disk that appears to hold no partitions; however, sometimes the libparted tool presents partitions other than those that the OS sees.
I've observed four causes of these symptoms, three of which FixParts can correct:
Old GPT data -- If a disk is used as a GPT disk and then re-used as an MBR disk, the GPT data may be incompletely erased. This happens if the disk is repartitioned with fdisk or the Microsoft Windows installer, for instance. (Tools based on libparted correctly remove the old GPT data when converting from GPT to MBR format.) FixParts checks for this problem when it starts and offers to correct it. If you opt to erase the GPT data, this erasure occurs immediately, unlike other changes the program makes.
Mis-sized extended partitions -- Some tools create an extended partition that's too large, typically ending after the last sector of the disk. FixParts automatically corrects this problem (if you use the 'w' option to save the partition table).
Primary partitions inside an extended partition -- Some utilities create or move primary partitions to within the range covered by the extended partition. FixParts can usually correct this problem by turning the primary partition into a logical partition or by changing one or more other logical partitions into primaries. Such corrections aren't always possible, though, at least not without deleting or resizing other partitions.
Leftover RAID data -- If a disk is used in a RAID array and then re-used as a non-RAID disk, some utilities can become confused and fail to see the disk. FixParts can NOT correct this problem. You must destroy the old RAID data, or possibly remove the dmraid package from the system, to fix this problem.
When run, FixParts presents an fdisk-like interface, enabling you to adjust partition types (primary, logical, or omitted), change type codes, change the bootable flag, and so on. Although you can delete a partition (by omitting it), you can't create new partitions with the program. If you're used to partitioning disks, particularly with Linux fdisk, two unusual features of FixParts require elaboration:
No extended partitions -- Internally, FixParts reads the partition table and discards data on any extended partition(s) it finds. When you save the partition table, the program generates a new extended partition. This design means that the program automatically corrects many problems related to the extended partition. It also means that you'll see no evidence of extended partitions in the FixParts user interface, although it keeps track of the requirements and prevents you from creating illegal layouts, such as a primary between two logicals.
Partition numbering -- In most Linux tools, partitions 1-4 are primaries and partitions 5 and up are logicals. Although a legal partition table loaded into FixParts will initially conform to this convention, some types of damaged table might not, and various changes you make can also cause deviations. When FixParts writes the partition table, its numbering will be altered to conform to the standard MBR conventions, but you should use the explicit labeling of partitions as primary or logical rather than the partition numbers to determine a partition's status.
