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0d592a5e17
Since 56b2970 has proven to be a no-go for us, as it breaks existing links, let's embrace the trailing slash and use absolute links everywhere for our pages. This way we'll get around browser cleverly appending the relative link to the current location (since it ends with a slash), and given our docs/ layout is flat it's not much of a hassle either. Converted using this beauty: $ sed -ri 's/(\[.+\]\()([A-Z_]+\))/\1\/\2/g' *.md Resolves: #32088 (again) and #32310
129 lines
6.2 KiB
Markdown
129 lines
6.2 KiB
Markdown
---
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title: User Record Blob Directories
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category: Users, Groups and Home Directories
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layout: default
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SPDX-License-Identifier: LGPL-2.1-or-later
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---
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# User Record Blob Directories
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The blob directories are for storing binary or unstructured data that would
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otherwise be stored in [JSON User Records](/USER_RECORD). For instance,
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this includes image files such as the user's avatar picture. This data,
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like most of the user record, will be made publicly available to the
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system.
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The JSON User Record specifies the location of the blob directory via the
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`blobDirectory` field. If the field is unset, then there is no blob directory
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and thus no blob files to look for. Note that `blobDirectory` can exist in the
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`regular`, `perMachine`, and `status` sections. The blob directory is completely
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owned and managed by the service that owns the rest of the user record (as
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specified in the `service` field).
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For consistency, blob directories have certain restrictions placed on them
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that may be enforced by their owning service. Services implementing blob
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directories are free to ignore these restrictions, but software that wishes
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to store some of its data in blob directories must adhere to the following:
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* The directory only contains regular files; no sub-directories or any special
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files are permitted.
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* Filenames inside of the directory are restricted to
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[URI Unreserved Characters](https://www.rfc-editor.org/rfc/rfc3986#section-2.3)
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(alphanumeric, `-`, `.`, `_`, and `~`), and must not start with a dot.
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* The total size of the directory should not exceed 64M.
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* File ownership and permissions will not be preserved. The service may reset
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the mode of the files to 0644, and ownership to whatever it wishes.
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* Timestamps, xattrs, ACLs, or any other metadata on the files will not be preserved.
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Services are required to ensure that the directory and its contents are
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world-readable. Aside from this requirement, services are free to provide
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the directory and its contents in whatever manner they like, including but
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not limited to synthesizing the directory at runtime using external data
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or keeping around multiple copies. Thus, only the service that owns the
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directory is permitted to write to this directory in any way: for all
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other software the directory is strictly read-only.
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Services may choose to provide some way to change user records. Services
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that provide this functionality should support changing the blob directory also.
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Care must be taken to avoid exposing sensitive data to malicious clients. This
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includes but is not limited to disallowing symlinks and using file descriptors
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(excluding O_PATH!) to ensure that the client actually has permission to access
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the data it wants the service to publish.
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Services that make use of the `signature` section in the records they manage
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should enforce `blobManifest`. This ensures that the contents of the blob directory
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are part of the cryptographically signed data.
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## Known Files
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Various files in the blob directories have known semantic meanings.
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The following files are currently defined:
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`avatar` → An image file that should be used as the user's avatar picture.
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The exact file type and resolution of this image are left unspecified,
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and requirements will depend on the capabilities of the components that will
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display it. However, we suggest the use of commonly-supported picture formats
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(i.e. PNG or JPEG) and a resolution of 512 x 512. This image should not have any
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transparency. If missing, of an incompatible file type, or otherwise unusable,
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then the user does not have a profile picture and a default will be used instead.
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`login-background` → An image file that will be used as the user's background on the
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login screen (i.e. in GDM). The exact file type and resolution are left unspecified
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and are ultimately up to the components that will render this background image. This
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image should not have any transparency. If missing, of an incompatible file type, or
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otherwise unusable, a fallback background of some kind will be used.
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## Extending These Directories
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Like JSON User Records, the blob directories are intended to be extendable for
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various applications. In general, subsystems are free to introduce their own
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files, as long as:
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* The requirements listed above are all met.
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* Care is taken to avoid namespace clashes. Please prefix your file names with
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a short identifier of your project to avoid ambiguities and incompatibilities.
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* This specification is supposed to be a living specification. If you need
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additional files, please consider defining them upstream for inclusion in
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this specification. If they are reasonably universally useful, it would be
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best to list them here.
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## Examples
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The simplest way to define a user record is via the drop-in directories (as documented
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in [nss-systemd(8)](https://www.freedesktop.org/software/systemd/man/latest/nss-systemd.html)
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and [systemd-userdb.service(8)](https://www.freedesktop.org/software/systemd/man/latest/systemd-userdbd.service.html)).
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Such records can have blob directories by simply referring to some persistent
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place from the record, possibly next to the record itself. For instance,
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`/etc/userdb/grobie.user` may contain:
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```json
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{
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"userName": "grobie",
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"disposition": "regular",
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"homeDirectory": "/home/grobie",
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"blobDirectory": "/etc/userdb/grobie.blob/",
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}
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```
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In this case, `/etc/userdb/grobie.blob/` will be the blob directory for the
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user `grobie`.
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A more complicated case is a home directory managed by `systemd-homed.service`.
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When it manages a home directory, it maintains and synchronizes two separate
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blob directories: one belonging to the system in `/var/cache/systemd/home`,
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and another belonging to the home directory in `~/.identity-blob`. The system
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blob directory ensures that the blob data is available while the home directory
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is encrypted or otherwise unavailable, and the home blob directory ensures that
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the user account remains portable between systems. To implement this behavior,
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`systemd-homed.service` always sets `blobDirectory` to the system blob directory
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in the `binding` section of the user record (i.e. this is _not_ persisted to
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`~/.identity`). If some client tries to update the user record with a new blob
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directory, `systemd-homed.service` will copy the updated blob directory into both
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the system and home blob locations.
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