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Genimage - The Image Creation Tool

genimage is a tool to generate multiple filesystem and flash/disk images from a given root filesystem tree. genimage is intended to be run in a fakeroot environment. It also supports creating flash/disk images out of different file-system images and files.

Configuration is done in a config file parsed by libconfuse. Options like the path to tools can be given via environment variables, the config file or from commandline switches.

The Configuration File

The config file of genimage uses a simple configuration language, provided by libconfuse. This supports nested sections, as well as simple key-value pairs.

Single-line comments can be introduced with # or //, multi-line comments look like /* … */ (as in C).

The config file is separated into the main sections image, flash and config, and provides an include primitive.

The image section

An image section describes a single filesystem or disk image to be built. It can be given multiple times to generate multiple images. An image can also have multiple partitions which refer to images themselves. Each image must have a type which can have different suboptions depending on the type.

Let's have a look at an example:

image nand-pcm038.img {
        flash {
        flashtype = "nand-64M-512"
        partition barebox {
                image = "barebox-pcm038.bin"
                size = 512K
        partition root {
                image = "root-nand.jffs2"
                size = 24M

This would generate a nand-pcm038.img which is a flash of type "nand-64M-512" The image contains two partitions, "barebox-pcm038.bin" and "root-nand.jffs2" which must refer to images described elsewhere in the config file. For example "root-nand.jffs2" partition could be described like this:

image root-nand.jffs2 {
        name = "root"
        jffs2 {}
        size = 24M
        mountpoint = "/"

In this case a single jffs2 image is generated from the root mountpoint.

Here are all options for images:

name:The name of this image. This is used for some image types to set the name of the image.
size:Size of this image in bytes. 'k', 'M' or 'G' can be used as suffix to specify the size in multiple of 1024 etc. The suffix 's' specifies a multiple of the (traditional) sector size of 512. If the image if filled from a mountpoint then '%' as suffix indicates a percentage. '200%' means the resulting filesystem should be about 50% filled. Note that is is only a rough estimate based on the original size of the content.
mountpoint:mountpoint if image refers to a filesystem image. The default is "/". The content of "${rootpath}${mountpoint}" will be used to fill the filesystem.
srcpath:If this is set, specified path will be directly used to fill the filesystem. Ignoring rootpath/mountpoint logic. Path might be absolute or relative to current working directory.
empty:If this is set to true, then the specified rootpath and mountpoint are ignored for this image and an empty filesystem is created. This option is only used for writeable filesystem types, such as extX, vfat, ubifs and jffs2. This defaults to false.
temporary:If this is set to true, the image is created in tmppath rather than outputpath. This can be useful for intermediate images defined in the configuration file which are not needed by themselves after the main image is created. This defaults to false.
exec-pre:Custom command to run before generating the image.
exec-post:Custom command to run after generating the image.
flashtype:refers to a flash section. Optional for non flash like images like hd images
partition:can be given multiple times and refers to a partition described below

Additionally each image can have one of the following sections describing the type of the image:

cpio, cramfs, ext2, ext3, ext4, file, flash, hdimage, iso, jffs2, qemu, squashfs, tar, ubi, ubifs, vfat.

Partition options:


The offset of this partition as a total offset to the beginning of the device.


The size of this partition in bytes. If the size and autoresize are both not set then the size of the partition image is used.


Alignment value to use for automatic computation of offset and size. Defaults to 1 for partitions not in the partition table, otherwise to the image's align value.


Used by dos partition tables to specify the partition type.


The image file this partition shall be filled with


Boolean specifying that all bytes of the partition should be explicitly initialized. Any bytes beyond the size of the specified image will be set to 0.


Boolean specifying that the partition should be resized automatically. For UBI volumes this means that the autoresize flag is set. Only one volume can have this flag. For hd images this can be used for the last partition. If set the partition will fill the remaining space of the image.


Boolean specifying whether to set the bootable flag.


Boolean specifying whether to include this partition in the partition table. Defaults to true.


UUID string used by GPT partition tables to specify the partition id. Defaults to a random value.


String used by GPT partition tables to specify the partition type. Either a UUID or a shortcut:

  • L: Linux filesystem (0fc63daf-8483-4772-8e79-3d69d8477de4)
  • S: Swap (0657fd6d-a4ab-43c4-84e5-0933c84b4f4f)
  • H: Home (933ac7e1-2eb4-4f13-b844-0e14e2aef915)
  • U: EFI System (c12a7328-f81f-11d2-ba4b-00a0c93ec93b)
  • R: Linux RAID (a19d880f-05fc-4d3b-a006-743f0f84911e)
  • V: Linux LVM (e6d6d379-f507-44c2-a23c-238f2a3df928)
  • F: FAT32 / Basic Data Partition (ebd0a0a2-b9e5-4433-87c0-68b6b72699c7)

Defaults to L.

For each partition, its final alignment, offset and size are determined as follows:

  • If the align option is not present, it defaults to the value of the image's align option if the partition is in the partition table, otherwise to 1.
  • If the offset option is absent or zero, and in-partition-table is true, the partition is placed after the end of all previously defined partitions, with the final offset rounded up to the partition's align value.
  • Otherwise, the offset option is used as-is. Note that if absent, that option defaults to 0, so in practice one must specify an offset for any partition that is not in the partition table (with at most one exception, e.g. a bootloader).
  • If the partition has the autoresize flag set, its size is computed as the space remaining in the image from its offset (for a GPT image, space is reserved at the end for the backup GPT table), rounded down to the partition's align value. If the partition also has a size option, it is ensured that the computed value is not less than that size.
  • Otherwise, if the size option is present and non-zero, its value is used as-is.
  • Otherwise, if the partition has an image option, the size of that image, rounded up to the partition's align value, is used to determine the size of the partition.

The following sanity checks are done on these final values (in many cases, these will automatically be satisfied when the value has been determined via one of the above rules rather than given explicitly):

  • For a partition in the partition table, the partition's align value must be greater than or equal to the image's align value.
  • The partition's offset and size must both be multiples of its align.
  • The size must not be 0.
  • The partition must not overlap any other partition, or the areas occupied by the partition table.

The image configuration options


Generate android sparse images. The are typically used by fastboot. Sparse images encode "don't care" areas and areas that are filled with a single 32 bit value. As a result, they are often much smaller than raw disk images. Genimage assumes that all 'holes' in the input file are "don't care" areas. This is a reasonable assumption: Tools to generate filesystems typically operate on devices. So they only create holes in areas they don't care about. Genimage itself operates the same way when generating HD images.


image:The source image that will be converted.
block-size:The granularity that the sparse image uses to find "don't care" or "fill" blocks. The supported block sizes depend on the user. The default is 4k.


Generates cpio images.


format:archive format. Passed to the -H option to the cpio command. Valid options are bin, odc, newc, crc, tar, ustar, hpbin and hpodc
extraargs:Extra arguments passed to the cpio tool
compress:If given, pipe image through compression tool. Valid options are for example gzip, lzop or any other tool that compresses from stdin to stdout.


Generates cramfs images.


extraargs:Extra arguments passed to mkcramfs

ext2, ext3, ext4

Generates ext* images.


use-mke2fs:If set to true, then mke2fs is used to create the image. Otherwise, genext2fs is used. Defaults to false.
mke2fs-conf:mke2fs.conf that should be used. If unspecified, the system default is used.
extraargs:Extra arguments passed to genext2fs or mke2fs.
features:Filesystem features. Passed to the -O option of tune2fs. This is a comma separated list of enabled or disabled features. See man ext4 for features. For genext2fs all feature are specified. Default for ext3 images is has_journal. Default for ext4 images is extents,uninit_bg,dir_index,has_journal. For mke2fs these features are added in addition to the default features of the ext type. Already enabled features can be disabled by prefixing the feature with ^.
label:Specify the volume-label. Passed to the -L option of tune2fs
fs-timestamp:Sets different timestamps in the image. Sets the given timestamp using the debugfs commands set_current_time, set_super_value mkfs_time and set_super_value lastcheck
root-owner:User and group IDs for the root directory. Defaults to 0:0. Only valid with mke2fs.
usage-type:Specify the usage type for the filesystem. Only valid with mke2fs. More details can be found in the mke2fs man-page.


This represents a pre-existing image which will be used as-is. When a partition section references an image that is not defined elsewhere in the configuration file, a file rule is implicitly generated. It is up to the user to ensure that the image exists in the input directory, or to use an absolute path to the image.

It is possible to add a file image explicitly, which allows one to provide genimage with some information about the image which can not be deduced automatically. Currently, one such option exists:

holes:A list of "(<start>;<end>)" pairs specifying ranges of the file that do not contain meaningful data, and which can therefore be allowed to overlap other partitions or image metadata.

For example:

image foo {
        hdimage {
                partition-table-type = "gpt"
                gpt-location = 64K

        partition bootloader {
                in-partition-table = false
                offset = 0
                image = "/path/to/bootloader.img"

        partition rootfs {
                offset = 1M
                image = "rootfs.ext4"

image /path/to/bootloader.img {
        file {
                holes = {"(440; 1K)", "(64K; 80K)"}

This tells genimage that despite the bootloader partition overlapping both the last 72 bytes of the MBR (where the DOS partition table is located) and the GPT header occupying the sector starting at offset 512, this is all OK because bootloader.img does not contain useful data in that range. Further, in this example, the bootloader image has been carefully crafted to also allow placing the GPT array at offset 64K (the GPT header is always at offset 512).

If the bootloader image is not declared explicitly and only used once then the holes can also be configured in the partition. This simplifies the config file for simple use-cases.

For example:

image bar {
        hdimage {}

        partition bootloader {
                in-partition-table = false
                offset = 0
                image = "/path/to/bootloader.img"
                holes = {"(440; 512)"}

        partition rootfs {
                offset = 1M
                image = "rootfs.ext4"


Generates U-Boot FIT images.


its:String option holding the path of the input its file
keydir:String option holding the directory containing the keys used for signing.


Generates flash images. These are basically the partition contents padded to the partition sizes concatenated together. There is no partition table. Needs a valid flashtype where the flash parameters are read from.


Generates DOS partition images.



Partition alignment. Defaults to 512 bytes


Boolean. If true, writes a partition table. If false, no partition table is generated. Defaults to true. Deprecated: use partition-table-type instead.


Define what kind of partition table should be used. Valid options are:

  • none: No partition table at all. In this case, the in-partition-table option for each partition is ignored.
  • mbr: Legacy DOS/MBR partition table
  • gpt: GUID Partition Table
  • hybrid: A hybrid MBR/GPT partition table. Partitions with an explicit partition-type will be placed in in the MBR table. At most 3 such partitions are allowed. This limit does not effect the maximum number of GPT partition entries in the same image.

Number of the extended partition. Contains the number of the extended partition between 1 and 4 or 0 for automatic. Defaults to 0.


32 bit integer used as disk signature (offset 440 in the MBR). Using a special value random will result in using random 32 bit number.


Boolean. If true, a GPT type partition table is written. If false a DOS type partition table is written. Defaults to false. Deprecated: use partition-table-type instead.


Location of the GPT table. Occasionally useful for moving the GPT table away from where a bootloader is placed due to hardware requirements. All partitions in the table must begin after this table. Regardless of this setting, the GPT header will still be placed at 512 bytes (sector 1). Defaults to 1024 bytes (sector 2).


Boolean. If true, then the backup partition table at the end of the image is not written.


UUID string used as disk id in GPT partitioning. Defaults to a random value.


If this is set to true, then the image file will be filled up to the end of the last partition. This might make the file bigger. This is necessary if the image will be processed by such tools as libvirt, libguestfs or parted.


Generates an ISO image.


boot-image:Path to the El Torito boot image. Passed to the -b option of genisofs
bootargs:Bootargs for the El Torito boot image. Defaults to -no-emul-boot -boot-load-size 4 -boot-info-table -c -hide
extraargs:Extra arguments passed to genisofs
input-charset:The input charset. Passed to the -input-charset option of genisofs. Defaults to default
volume-id:Volume ID. Passed to the -V option of genisofs


Generates a JFFS image. Needs a valid flashtype where the flash parameters are read from.


extraargs:Extra arguments passed to mkfs.jffs2


Generates a QEMU image. Needs at least one valid partition.


format:A valid qemu-img like qcow, qcow2, parallels, vdi, vhdx or vmdk. Check qemu-img convert --help for the complete list of possible values. Defaults to qcow2.
extraargs:Extra arguments passed to qemu-img convert


Generates a squashfs image.


extraargs:Extra arguments passed to mksquashfs
compression:compression type for the image. Possible values are gzip (default), lzo, xz or none
block-size:Block size. Passed to the -b option of mksquashfs. Defaults to 4096.


Generates a RAUC update bundle.


extraargs:Extra arguments passed to RAUC
file:Specify a file to be added into the RAUC bundle. Usage is: file foo { image = "bar" } which adds a file "foo" in the RAUC bundle from then input file "bar"
files:A list of filenames added into the RAUC bundle. Like file above, but without the ability to add the files under different name.
key:Path to the key file or PKCS#11 URI. Passed to the --key option of RAUC
cert:Path to the certificate file or PKCS#11 URI. Passed to the --cert option of RAUC
keyring:Optional path to the keyring file. Passed to the --keyring option of RAUC
manifest:content of the manifest file


Generates a tar image. The image will be compressed as defined by the filename suffix.


Generates an UBI image. Needs a valid flashtype where the flash parameters are read from.


extraargs:Extra arguments passed to ubinize


Generates a UBIFS image. Needs a valid flashtype where the flash parameters are read from.


extraargs:Extra arguments passed to mkubifs
max-size:Maximum size of the UBIFS image


Generates a VFAT image.


extraargs:Extra arguments passed to mkdosfs
label:Specify the volume-label. Passed to the -n option of mkdosfs
file:Specify a file to be added into the filesystem image. Usage is: file foo { image = "bar" } which adds a file "foo" in the filesystem image from the input file "bar"
files:A list of filenames added into the filesystem image. Like file above, but without the ability to add the files under different name.

Note: If no content is specified with file or files then rootpath and mountpoint are used to provide the content.


Generates a Firmware Image Package (FIP). A format used to bundle firmware to be loaded by ARM Trusted Firmware.


extraargs:Extra arguments passed to fiptool
fw-config:Firmware Configuration (device tree), usually provided by BL2 (Trusted Firmware)
nt-fw:Non-Trusted Firmware (BL33)
hw-config:Hardware Configuration (device tree), passed to BL33
tos-fw:Trusted OS (BL32) binaries. Second and third binary are used as extra1 and extra2 binaries if specified. Example: tos-fw = {"tee-header_v2.bin", "tee-pager_v2.bin", "tee-pageable_v2.bin"}
scp-fwu-cfg:SCP Firmware Updater Configuration FWU SCP_BL2U
ap-fwu-cfg:AP Firmware Updater Configuration BL2U
fwu:Firmware Updater NS_BL2U
fwu-cert:Non-Trusted Firmware Updater certificate
tb-fw:Trusted Boot Firmware BL2
scp-fw:SCP Firmware SCP_BL2
soc-fw:EL3 Runtime Firmware BL31
rot-cert:Root Of Trust key certificate
trusted-key-cert:Trusted key certificate
scp-fw-key-cert:SCP Firmware key certificate
soc-fw-key-cert:SoC Firmware key certificate
tos-fw-key-cert:Trusted OS Firmware key certificate
nt-fw-key-cert:Non-Trusted Firmware key certificate
tb-fw-cert:Trusted Boot Firmware BL2 certificate
scp-fw-cert:SCP Firmware content certificate
soc-fw-cert:SoC Firmware content certificate
tos-fw-cert:Trusted OS Firmware content certificate
nt-fw-cert:Non-Trusted Firmware content certificate
sip-sp-cert:SiP owned Secure Partition content certificate
plat-sp-cert:Platform owned Secure Partition content certificate

The Flash Section

The flash section can be given multiple times and each section describes a flash chip. The option names are mostly derived from the UBI terminology. There are the following options:

pebsize:The size of a physical eraseblock in bytes
lebsize:The size of a logical eraseblock in bytes (for ubifs)
numpebs:Number of physical eraseblocks on this device. The total size of the device is determined by pebsize * numpebs
minimum-io-unit-size:The minimum size in bytes accessible on this device
vid-header-offset:offset of the volume identifier header
sub-page-size:The size of a sub page in bytes.

Several flash related image types need a valid flash section. From the image types the flash type section is referred to using the flashtype option which contains the name of the flash type to be used.

For more information of the meaning of these values see the ubi(fs) and mtd FAQs:

Example flash section:

flash nand-64M-512 {
        pebsize = 16384
        lebsize = 15360
        numpebs = 4096
        minimum-io-unit-size = 512
        vid-header-offset = 512
        sub-page-size = 512
image jffs2 {
        flashtype = "nand-64M-512"

The config section

In this section the global behaviour of the program is described. Except as noted below, all options here can be given from either environment variables, the config file or command line switches. For instance, a config option foo can be passed as a --foo command line switch or as a GENIMAGE_FOO environment variable.

config:default: genimage.cfg Path to the genimage config file.
loglevel:default: 1 genimage log level.
outputpath:default: images Mandatory path where all images are written to (must exist).
inputpath:default: input This mandatory path is searched for input images, for example bootloader binaries, kernel images (must exist).
rootpath:default: root Mandatory path to the root filesystem (must exist).
tmppath:default: tmp Optional path to a temporary directory. There must be enough space available here to hold a copy of the root filesystem.
includepath:Colon-separated list of directories to search for files included via the include function. The current directory is searched after these. Thus, if this option is not given, only the current directory is searched. This has no effect when given in the config file.
configdump:File to write the final configuration to. This includes the results of all include directives, expansions of environment variables and application of default values - think gcc -E. Use - for stdout.
cpio:path to the cpio program (default cpio)
dd:path to the dd program (default dd)
e2fsck:path to the e2fsck program (default e2fsck)
genext2fs:path to the genext2fs program (default genext2fs)
genisoimage:path to the genisoimage program (default genisoimage)
mcopy:path to the mcopy program (default mcopy)
mmd:path to the mmd program (default mmd)
mkcramfs:path to the mkcramfs program (default mkcramfs)
mkdosfs:path to the mkdosfs program (default mkdosfs)
mkfsjffs2:path to the mkfs.jffs2 program (default mkfs.jffs2)
mkfsubifs:path to the mkfs.ubifs program (default mkfs.ubifs)
mksquashfs:path to the mksquashfs program (default mksquashfs)
qemu-img:path to the qemu-img program (default qemu-img)
tar:path to the tar program (default tar)
tune2fs:path to the tune2fs program (default tune2fs)
ubinize:path to the ubinize program (default ubinize)

Include Configurations Fragments

To include a "foo.cfg" config file, use the following statement:


This allows to re-use, for example flash configuration files, across different image configurations.