SYSTEMD-GPT-AUTO-GENERATOR(8)

NAME
systemd-gpt-auto-generator - Generator for automatically discovering and mounting root, /home/, /srv/, /var/ and /var/tmp/ partitions, as well as discovering and enabling swap partitions, based on GPT partition type GUIDs

SYNOPSIS

/usr/lib/systemd/system-generators/systemd-gpt-auto-generator

DESCRIPTION

systemd-gpt-auto-generator is a unit generator that automatically discovers the root partition, /home/, /srv/, /var/, /var/tmp/, the EFI System Partition (ESP), the Extended Boot Loader Partition (XBOOTLDR), and swap partitions and creates mount and swap units for them, based on the partition type GUIDs of GUID partition tables (GPT). See UEFI Specification[1], chapter 5 for more details. It implements the UAPI.2 Discoverable Partitions Specification[2].

Note that this generator has no effect on non-GPT systems. It will also not create mount point configuration for directories which already contain files or if the mount point is explicitly configured in fstab(5). Additionally no unit will be created for the ESP or the XBOOTLDR partition if mount entries are found in the /boot/ or /efi/ hierarchies in fstab(5).

If the units this generator creates are overridden, for example by units in directories with higher precedence, drop-ins and additional dependencies created by this generator might still be used.

When run in the initial RAM disk (initrd) this generator can automatically search for the root file system. Specifically:

    • It will look for the root partition on the same physical disk where the EFI System Partition (ESP) is located. Note that support from the boot loader is required for this to work: the EFI variable LoaderDevicePartUUID of the 4a67b082-0a4c-41cf-b6c7-440b29bb8c4f vendor UUID is used to determine from which partition (and hence disk) the system was booted. If the boot loader does not set this variable, this generator will not be able to detect the root partition. See the Boot Loader Interface[3] for details.

    • Alternatively, it will look for the root file system on a loopback block device whose ".lo_name" field is set to one of the literal strings "rootdisk" or "rootdisk.raw". This field can be set via losetup's --loop-ref= string. For images downloaded via systemd-import-generator(8) make sure to set the "blockdev" option and set the local name string to "rootdisk" to achieve this effect. Note that discovery of the root file system on loopback block devices like this is only done if "root=gpt-auto" or "root=dissect" is specified explicitly on the kernel command line, unlike the discovery based on the boot loader reported ESP which is also enabled if no "root=" parameter is specified at all. (The latter relies on systemd-udevd.service's /dev/gpt-auto-root block device symlink generation).

When run on the host system (i.e. after successfully transitioning out of the initrd into the root filesystem) this generator will look for all other partitions on the same physical disk as the root partition. For this discovery, boot loader support is not required. Moreover, it is not required that the root partition was automatically discovered by the initrd (as described above) for the discovery of the non-root file partitions to take place. Or in other words: automatic discovery of the root file system and of the non-root file systems are independent operations, that do not rely on each other, and are done during two distinct phases of the boot process (one in the initrd, the other after). These partitions will not be searched for on systems where the root file system is distributed on multiple disks, for example via btrfs RAID.

The root partition can be configured explicitly by symlinking /run/systemd/volatile-root to /dev/block/$major:$minor. This is especially useful if the root mount has been replaced by some form of volatile file system (overlayfs).

systemd-gpt-auto-generator is useful for centralizing file system configuration in the partition table and making configuration in /etc/fstab or on the kernel command line unnecessary.

This generator looks for the partitions based on their partition type GUID. The following partition type GUIDs are identified:

Table 1. Partition Type GUIDs
Partition Type | GUID | Name | Mount Point | Explanation
SD_GPT_ROOT_X86_64 | 4f68bce3-e8cd-4db1-96e7-fbcaf984b709 | Root Partition (x86-64) | / | The first partition with this type UUID, located on the same disk as the ESP used for booting, is used as the root file system / on AMD64 / 64-bit x86 systems.
SD_GPT_ROOT_ARM64 | b921b045-1df0-41c3-af44-4c6f280d3fae | Root Partition (64-bit ARM) | / | The first partition with this type UUID, located on the same disk as the ESP used for booting, is used as the root file system / on AArch64 / 64-bit ARM systems.
SD_GPT_ROOT_ALPHA SD_GPT_ROOT_ARC SD_GPT_ROOT_ARM SD_GPT_ROOT_ARM64 SD_GPT_ROOT_IA64 SD_GPT_ROOT_LOONGARCH64 SD_GPT_ROOT_MIPS SD_GPT_ROOT_MIPS64 SD_GPT_ROOT_MIPS_LE SD_GPT_ROOT_MIPS64_LE SD_GPT_ROOT_PARISC SD_GPT_ROOT_PPC SD_GPT_ROOT_PPC64 SD_GPT_ROOT_PPC64_LE SD_GPT_ROOT_RISCV32 SD_GPT_ROOT_RISCV64 SD_GPT_ROOT_S390 SD_GPT_ROOT_S390X SD_GPT_ROOT_TILEGX SD_GPT_ROOT_X86 SD_GPT_ROOT_X86_64 SD_GPT_USR_ALPHA SD_GPT_USR_ARC SD_GPT_USR_ARM SD_GPT_USR_IA64 SD_GPT_USR_LOONGARCH64 SD_GPT_USR_MIPS_LE SD_GPT_USR_MIPS64_LE SD_GPT_USR_PARISC SD_GPT_USR_PPC SD_GPT_USR_PPC64 SD_GPT_USR_PPC64_LE SD_GPT_USR_RISCV32 SD_GPT_USR_RISCV64 SD_GPT_USR_S390 SD_GPT_USR_S390X SD_GPT_USR_TILEGX SD_GPT_USR_X86 | ... | Root partitions for other architectures | / | The first partition with the type UUID matching the architecture, located on the same disk as the ESP used for booting, is used as the root file system /. For the full list and constant values, see UAPI.2 Discoverable Partitions Specification[2].
SD_GPT_HOME | 933ac7e1-2eb4-4f13-b844-0e14e2aef915 | Home Partition | /home/ | The first partition with this type UUID on the same disk as the root partition is mounted to /home/.
SD_GPT_SRV | 3b8f8425-20e0-4f3b-907f-1a25a76f98e8 | Server Data Partition | /srv/ | The first partition with this type UUID on the same disk as the root partition is mounted to /srv/.
SD_GPT_VAR | 4d21b016-b534-45c2-a9fb-5c16e091fd2d | Variable Data Partition | /var/ | The first partition with this type UUID on the same disk as the root partition is mounted to /var/ — under the condition its partition UUID matches the first 128 bit of the HMAC-SHA256 of the GPT type uuid of this partition keyed by the machine ID of the installation stored in machine-id(5). This can be generated using systemd-id128(1).
SD_GPT_TMP | 7ec6f557-3bc5-4aca-b293-16ef5df639d1 | Temporary Data Partition | /var/tmp/ | The first partition with this type UUID on the same disk as the root partition is mounted to /var/tmp/.
SD_GPT_SWAP | 0657fd6d-a4ab-43c4-84e5-0933c84b4f4f | Swap | n/a | All partitions with this type UUID on the same disk as the root partition are used as swap.
SD_GPT_ESP | c12a7328-f81f-11d2-ba4b-00a0c93ec93b | EFI System Partition (ESP) | /efi/ or /boot/ | The first partition with this type UUID located on the same disk as the root partition is mounted to /boot/ or /efi/, see below.
SD_GPT_XBOOTLDR | bc13c2ff-59e6-4262-a352-b275fd6f7172 | Extended Boot Loader Partition | /boot/ | The first partition with this type UUID located on the same disk as the root partition is mounted to /boot/, see below.

This generator understands the following attribute flags for partitions:

Table 2. Partition Attribute Flags
Flag | Value | Applicable to | Explanation
SD_GPT_FLAG_READ_ONLY | 0x1000000000000000 | /, /home/, /srv/, /var/, /var/tmp/, Extended Boot Loader Partition | Partition is mounted read-only
SD_GPT_FLAG_NO_AUTO | 0x8000000000000000 | /, /home/, /srv/, /var/, /var/tmp/, Extended Boot Loader Partition | Partition is not mounted automatically
SD_GPT_FLAG_NO_BLOCK_IO_PROTOCOL | 0x0000000000000002 | EFI System Partition (ESP) | Partition is not mounted automatically

The /home/, /srv/, /var/, /var/tmp/ and swap partitions may be encrypted in LUKS format. In this case, a device mapper device is set up under the names /dev/mapper/home, /dev/mapper/srv, /dev/mapper/var, /dev/mapper/tmp or /dev/mapper/swap. Note that this might create conflicts if the same partition is listed in /etc/crypttab with a different device mapper device name.

When systemd is running in the initrd the / partition may be encrypted with LUKS as well. In this case, a device mapper device is set up under the name /dev/mapper/root, and a sysroot.mount is set up that mounts the device under /sysroot. For more information, see bootup(7).

Mount and automount units for the EFI System Partition (ESP) and Extended Boot Loader Partition (XBOOTLDR) are generated on EFI systems. If the disk contains an XBOOTLDR partition, as defined in the UAPI.1 Boot Loader Specification[4], it is made available at /boot/. This generator creates an automount unit; the mount will only be activated on-demand when accessed. The mount point will be created if necessary.

The ESP is mounted to /boot/ if that directory exists and is not used for XBOOTLDR, and otherwise to /efi/. Same as for /boot/, an automount unit is used. The mount point will be created if necessary.

No configuration is created for mount points that are configured in fstab(5) or when the target directory contains files.

When using this generator in conjunction with btrfs file systems, make sure to set the correct default subvolumes on them, using btrfs subvolume set-default.

If the system was booted via systemd-stub(7) and the stub reported to userspace that the kernel image was measured to a TPM2 PCR, then any discovered root and /var/ volume identifiers (and volume encryption keys, in case they are encrypted) will be automatically measured into PCR 15 on activation, via systemd-pcrfs@.service(8). Moreover, information about the LUKS key slot used to unlock the volume is measured into NvPCR "cryptsetup". Finally, if the root or /usr/ partition is protected via Verity its root hash and the serial/issuer of the key used for the provided root hash signature (if any) are measured into the NvPCR "verity".

Mount constraint metadata contained in the file systems is validated by pulling in systemd-validatefs@.service(8) for generated mounts.

systemd-gpt-auto-generator implements systemd.generator(7).

KERNEL COMMAND LINE

systemd-gpt-auto-generator understands the following kernel command line parameters:

systemd.gpt_auto, rd.systemd.gpt_auto
    Those options take an optional boolean argument, and default to yes. The generator is enabled by default, and a false value may be used to disable it (e.g. "systemd.gpt_auto=0").

    Added in version 242.

systemd.image_policy=
    Takes an image dissection policy string as argument (as per systemd.image-policy(7)), and allows enforcing a policy on dissection and use of the automatically discovered GPT partition table entries.

    Note that the specified image policy is not taken into account for automatic root or /usr/ file system discovery unless root=dissect/mount.usr=dissect (or root=dissect-force) are specified. (The policy will always be applied to the other auto-discoverable partition types.)

    Added in version 254.

systemd.image_filter=
    Takes an image dissection filter string as argument (as per systemd.image-filter(7)), and allows enforcing a set of globbing patterns on the partition matching of the automatically discovered GPT partition table entries.

    Note that the specified image filter is not taken into account for automatic root or /usr/ file system discovery unless root=dissect/mount.usr=dissect (or root=dissect-force) are specified. (The filter will always be applied to the other auto-discoverable partition types.)

    Added in version 258.

root=, rootfstype=, rootflags=
    When root= is used with the special value "gpt-auto", basic automatic discovery of the root partition based on the GPT partition type is enabled. Use of the root partition is delayed until factory reset mode is left, in case it is enabled during the current boot. See Factory Reset[5] for more information on that. If "gpt-auto-force" is specified automatic discovery of the root partition is enabled, ignoring any factory reset mode.

    If root= is set to the special value "dissect" full automatic discovery of the root partition based on GPT partition information is enabled. This is a superset of root=gpt-auto, as it automatically configures Verity partitions (including signature-based setup) following the logic defined for that in the UAPI.2 Discoverable Partitions Specification[6]. Moreover it takes the configured image policy and image filter into account for all partition types, including the root file system. "root=dissect" will wait for the factory reset phase to be completed if it is in effect before activating the root file system. Use "root=dissect-force" to ignore the factory reset phase and activate the root file system immediately.

    Any other value (i.e. besides "gpt-auto", "gpt-auto-force", "dissect", "dissect-force") disables automatic root file system discovery.

    If root= is not specified at all on the kernel command line automatic discovery of the root partition via the ESP reported by the boot loader is also enabled (taking factory reset state into account, i.e. equivalent to "root=gpt-auto"), however in this case discovery based on the loopback block device ".lo_name" field is not enabled.

    The rootfstype= and rootflags= options are used to select the file system type and options when the root file system is automatically discovered.

    Added in version 242.

mount.usr=, mount.usrfstype=, mount.usrflags=
    Similar to root=, rootfstype=, rootflags= (see above), but applies to the /usr/ partition instead. Note that the "gpt-auto", "gpt-auto-force", "dissect-force" settings that root= understands are not supported by mount.usr= (however "dissect" is).

    Also note that automatic partition discovery for /usr/ must be enabled explicitly, unlike the discovery for the root file system, which is enabled if no root= parameter is passed at all.

    Added in version 258.

rw, ro
    Mount the root partition read-write or read-only initially.

    Note that unlike most kernel command line options these settings do not override configuration in the file system, and the file system may be remounted later. See systemd-remount-fs.service(8).

    Added in version 242.

systemd.swap=
    Takes a boolean argument or enables the option if specified without an argument. If disabled, automatic discovery of swap partition(s) based on GPT partition type is disabled. Defaults to enabled.

    Added in version 254.

SEE ALSO

systemd(1), systemd.mount(5), systemd.swap(5), systemd-fstab-generator(8), systemd-cryptsetup@.service(8), systemd-pcrfs@.service(8), systemd-validatefs@.service(8), machine-id(5), cryptsetup(8), fstab(5), btrfs(8)

NOTES

1.  UEFI Specification
    https://uefi.org/specifications

2.  UAPI.2 Discoverable Partitions Specification
    https://uapi-group.org/specifications/specs/discoverable_partitions_specification

3.  Boot Loader Interface
    https://systemd.io/BOOT_LOADER_INTERFACE

4.  UAPI.1 Boot Loader Specification
    https://uapi-group.org/specifications/specs/boot_loader_specification

5.  Factory Reset
    https://systemd.io/FACTORY_RESET

6.  UAPI.2 Discoverable Partitions Specification
    https://uapi-group.org/specifications/specs/discoverable_partitions_specification/