systemd/docs/FILE_DESCRIPTOR_STORE.md

title	category	layout	SPDX-License-Identifier
File Descriptor Store	Interfaces	default	LGPL-2.1-or-later

The File Descriptor Store

TL;DR: The systemd service manager may optionally maintain a set of file descriptors for each service. Those file descriptors are under control of the service. Storing file descriptors in the manager makes is easier to restart services without dropping connections or losing state.

Since its inception systemd has supported the socket activation mechanism: the service manager creates and listens on some sockets (and similar UNIX file descriptors) on behalf of a service, and then passes them to the service during activation of the service via UNIX file descriptor (short: fd) passing over execve(). This is primarily exposed in the .socket unit type.

The file descriptor store (short: fdstore) extends this concept, and allows services to upload during runtime additional fds to the service manager that it shall keep on its behalf. File descriptors are passed back to the service on subsequent activations, the same way as any socket activation fds are passed.

If a service fd is passed to the fdstore logic of the service manager it only maintains a duplicate of it (in the sense of UNIX dup(2)), the fd remains also in possession of the service itself, and it may (and is expected to) invoke any operations on it that it likes.

The primary use-case of this logic is to permit services to restart seamlessly (for example to update them to a newer version), without losing execution context, dropping pinned resources, terminating established connections or even just momentarily losing connectivity. In fact, as the file descriptors can be uploaded freely at any time during the service runtime, this can even be used to implement services that robustly handle abnormal termination and can recover from that without losing pinned resources.

Note that Linux supports the memfd concept that allows associating a memory-backed fd with arbitrary data. This may conveniently be used to serialize service state into and then place in the fdstore, in order to implement service restarts with full service state being passed over.

Basic Mechanism

The fdstore is enabled per-service via the FileDescriptorStoreMax= service setting. It defaults to zero (which means the fdstore logic is turned off), but can take an unsigned integer value that controls how many fds to permit the service to upload to the service manager to keep simultaneously.

If set to values > 0, the fdstore is enabled. When invoked the service may now (asynchronously) upload file descriptors to the fdstore via the sd_pid_notify_with_fds() API call (or an equivalent re-implementation). When uploading the fds it is necessary to set the FDSTORE=1 field in the message, to indicate what the fd is intended for. It's recommended to also set the FDNAME=… field to any string of choice, which may be used to identify the fd later.

Whenever the service is restarted the fds in its fdstore will be passed to the new instance following the same protocol as for socket activation fds. i.e. the $LISTEN_FDS, $LISTEN_PIDS, $LISTEN_FDNAMES environment variables will be set (the latter will be populated from the FDNAME=… field mentioned above). See sd_listen_fds() for details on receiving such fds in a service. (Note that the name set in FDNAME=… does not need to be unique, which is useful when operating with multiple fully equivalent sockets or similar, for example for a service that both operates on IPv4 and IPv6 and treats both more or less the same.).

And that's already the gist of it.

Seamless Service Restarts

A system service that provides a client-facing interface that shall be able to seamlessly restart can make use of this in a scheme like the following: whenever a new connection comes in it uploads its fd immediately into its fdstore. At appropriate times it also serializes its state into a memfd it uploads to the service manager — either whenever the state changed sufficiently, or simply right before it terminates. (The latter of course means that state only survives on clean restarts and abnormal termination implies the state is lost completely — while the former would mean there's a good chance the next restart after an abnormal termination could continue where it left off with only some context lost.)

Using the fdstore for such seamless service restarts is generally recommended over implementations that attempt to leave a process from the old service instance around until after the new instance already started, so that the old then communicates with the new service instance, and passes the fds over directly. Typically service restarts are a mechanism for implementing code updates, hence leaving two version of the service running at the same time is generally problematic. It also collides with the systemd service manager's general principle of guaranteeing a pristine execution environment, a pristine security context, and a pristine resource management context for freshly started services, without uncontrolled "leftovers" from previous runs. For example: leaving processes from previous runs generally negatively affects lifecycle management (i.e. KillMode=none must be set), which disables large parts of the service managers state tracking, resource management (as resource counters cannot start at zero during service activation anymore, since the old processes remaining skew them), security policies (as processes with possibly out-of-date security policies – SElinux, AppArmor, any LSM, seccomp, BPF — in effect remain), and similar.

File Descriptor Store Lifecycle

By default any file descriptor stored in the fdstore for which a POLLHUP or POLLERR is seen is automatically closed and removed from the fdstore. This behavior can be turned off, by setting the FDPOLL=0 field when uploading the fd via sd_notify_with_fds().

The fdstore is automatically closed whenever the service is fully deactivated and no jobs are queued for it anymore. This means that a restart job for a service will leave the fdstore intact, but a separate stop and start job for it — executed synchronously one after the other — will likely not.

This behavior can be modified via the FileDescriptorStorePreserve= setting in service unit files. If set to yes the fdstore will be kept as long as the service definition is loaded into memory by the service manager, i.e. as long as at least one other loaded unit has a reference to it.

The systemctl clean --what=fdstore … command may be used to explicitly clear the fdstore of a service. This is only allowed when the service is fully deactivated, and is hence primarily useful in case FileDescriptorStorePreserve=yes is set (because the fdstore is otherwise fully closed anyway in this state).

Individual file descriptors may be removed from the fdstore via the sd_notify() mechanism, by sending an FDSTOREREMOVE=1 message, accompanied by an FDNAME=… string identifying the fds to remove. (The name does not have to be unique, as mentioned, in which case all matching fds are closed). Generally it's a good idea to send such messages to the service manager during initialization of the service whenever an unrecognized fd is received, to make the service robust for code updates: if an old version uploaded an fd that the new version doesn't recognize anymore it's good idea to close it both in the service and in the fdstore.

Note that storing a duplicate of an fd in the fdstore means the resource pinned by the fd remains pinned even if the service closes its duplicate of the fd. This in particular means that peers on a connection socket uploaded this way will not receive an automatic POLLHUP event anymore if the service code issues close() on the socket. It must accompany it with an FDSTOREREMOVE=1 notification to the service manager, so that the fd is comprehensively closed.

Access Control

Access to the fds in the file descriptor store is generally restricted to the service code itself. Pushing fds into or removing fds from the fdstore is subject to the access control restrictions of any other sd_notify() message, which is controlled via NotifyAccess=.

By default only the main service process hence can push/remove fds, but by setting NotifyAccess=all this may be relaxed to allow arbitrary service child processes to do the same.

Soft Reboot

The fdstore is particularly interesting in soft reboot scenarios, as per systemctl soft-reboot (which restarts userspace like in a real reboot, but leaves the kernel running). File descriptor stores that remain loaded at the very end of the system cycle — just before the soft-reboot – are passed over to the next system cycle, and propagated to services they originate from there. This enables updating the full userspace of a system during runtime, fully replacing all processes without losing pinning resources, interrupting connectivity or established connections and similar.

This mechanism can be enabled either by making sure the service survives until the very end (i.e. by setting DefaultDependencies=no so that it keeps running for the whole system lifetime without being regularly deactivated at shutdown) or by setting FileDescriptorStorePreserve=yes (and referencing the unit continuously).

For further details see Resource Pass-Through.

Initrd Transitions

The fdstore may also be used to pass file descriptors for resources from the initrd context to the main system. Restarting all processes after the transition is important as code running in the initrd should generally not continue to run after the switch to the host file system, since that pins backing files from the initrd, and the initrd might contain different versions of programs than the host.

Any service that still runs during the initrd→host transition will have its fdstore passed over the transition, where it will be passed back to any queued services of the same name.

The soft reboot cycle transition and the initrd→host transition are semantically very similar, hence similar rules apply, and in both cases it is recommended to use the fdstore if pinned resources shall be passed over.

Debugging

The systemd-analyze tool may be used to list the current contents of the fdstore of any running service.

The systemd-run tool may be used to quickly start a testing binary or similar as a service. Use -p FileDescriptorStoreMax=4711 to enable the fdstore from systemd-run's command line. By using the -t switch you can even interactively communicate via processes spawned that way, via the TTY.