systemd/docs/USER_RECORD_BLOB_DIRS.md

title	category	layout	SPDX-License-Identifier
User Record Blob Directories	Users, Groups and Home Directories	default	LGPL-2.1-or-later

User Record Blob Directories

The blob directories are for storing binary or unstructured data that would otherwise be stored in JSON User Records. For instance, this includes image files such as the user's avatar picture. This data, like most of the user record, will be made publicly available to the system.

The JSON User Record specifies the location of the blob directory via the blobDirectory field. If the field is unset, then there is no blob directory and thus no blob files to look for. Note that blobDirectory can exist in the regular, perMachine, and status sections. The blob directory is completely owned and managed by the service that owns the rest of the user record (as specified in the service field).

For consistency, blob directories have certain restrictions placed on them that may be enforced by their owning service. Services implementing blob directories are free to ignore these restrictions, but software that wishes to store some of its data in blob directories must adhere to the following:

• The directory only contains regular files; no sub-directories or any special files are permitted.

• Filenames inside of the directory are restricted to URI Unreserved Characters (alphanumeric, -, ., _, and ~), and must not start with a dot.

• The total size of the directory should not exceed 64M.

• File ownership and permissions will not be preserved. The service may reset the mode of the files to 0644, and ownership to whatever it wishes.

• Timestamps, xattrs, ACLs, or any other metadata on the files will not be preserved.

Services are required to ensure that the directory and its contents are world-readable. Aside from this requirement, services are free to provide the directory and its contents in whatever manner they like, including but not limited to synthesizing the directory at runtime using external data or keeping around multiple copies. Thus, only the service that owns the directory is permitted to write to this directory in any way: for all other software the directory is strictly read-only.

Services may choose to provide some way to change user records. Services that provide this functionality should support changing the blob directory also. Care must be taken to avoid exposing sensitive data to malicious clients. This includes but is not limited to disallowing symlinks and using file descriptors (excluding O_PATH!) to ensure that the client actually has permission to access the data it wants the service to publish.

Services that make use of the signature section in the records they manage should enforce blobManifest. This ensures that the contents of the blob directory are part of the cryptographically signed data.

Known Files

Various files in the blob directories have known semantic meanings. The following files are currently defined:

avatar → An image file that should be used as the user's avatar picture. The exact file type and resolution of this image are left unspecified, and requirements will depend on the capabilities of the components that will display it. However, we suggest the use of commonly-supported picture formats (i.e. PNG or JPEG) and a resolution of 512 x 512. This image should not have any transparency. If missing, of an incompatible file type, or otherwise unusable, then the user does not have a profile picture and a default will be used instead.

login-background → An image file that will be used as the user's background on the login screen (i.e. in GDM). The exact file type and resolution are left unspecified and are ultimately up to the components that will render this background image. This image should not have any transparency. If missing, of an incompatible file type, or otherwise unusable, a fallback background of some kind will be used.

Extending These Directories

Like JSON User Records, the blob directories are intended to be extendable for various applications. In general, subsystems are free to introduce their own files, as long as:

• The requirements listed above are all met.

• Care is taken to avoid namespace clashes. Please prefix your file names with a short identifier of your project to avoid ambiguities and incompatibilities.

• This specification is supposed to be a living specification. If you need additional files, please consider defining them upstream for inclusion in this specification. If they are reasonably universally useful, it would be best to list them here.

Examples

The simplest way to define a user record is via the drop-in directories (as documented in nss-systemd(8) and systemd-userdb.service(8)). Such records can have blob directories by simply referring to some persistent place from the record, possibly next to the record itself. For instance, /etc/userdb/grobie.user may contain:

{
        "userName": "grobie",
        "disposition": "regular",
        "homeDirectory": "/home/grobie",
        "blobDirectory": "/etc/userdb/grobie.blob/",
}

In this case, /etc/userdb/grobie.blob/ will be the blob directory for the user grobie.

A more complicated case is a home directory managed by systemd-homed.service. When it manages a home directory, it maintains and synchronizes two separate blob directories: one belonging to the system in /var/cache/systemd/home, and another belonging to the home directory in ~/.identity-blob. The system blob directory ensures that the blob data is available while the home directory is encrypted or otherwise unavailable, and the home blob directory ensures that the user account remains portable between systems. To implement this behavior, systemd-homed.service always sets blobDirectory to the system blob directory in the binding section of the user record (i.e. this is not persisted to ~/.identity). If some client tries to update the user record with a new blob directory, systemd-homed.service will copy the updated blob directory into both the system and home blob locations.