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![Zbigniew Jędrzejewski-Szmek](/assets/img/avatar_default.png)
This reverts commit 5e8ff010a1
.
This broke all the URLs, we can't have that. (And actually, we probably don't
_want_ to make the change either. It's nicer to have all the pages in one
directory, so one doesn't have to figure out to which collection the page
belongs.)
214 lines
11 KiB
Markdown
214 lines
11 KiB
Markdown
---
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title: File Descriptor Store
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category: Interfaces
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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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# The File Descriptor Store
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*TL;DR: The systemd service manager may optionally maintain a set of file
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descriptors for each service. Those file descriptors are under control of the
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service. Storing file descriptors in the manager makes is easier to restart
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services without dropping connections or losing state.*
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Since its inception `systemd` has supported the *socket* *activation*
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mechanism: the service manager creates and listens on some sockets (and similar
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UNIX file descriptors) on behalf of a service, and then passes them to the
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service during activation of the service via UNIX file descriptor (short: *fd*)
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passing over `execve()`. This is primarily exposed in the
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[.socket](https://www.freedesktop.org/software/systemd/man/systemd.socket.html)
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unit type.
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The *file* *descriptor* *store* (short: *fdstore*) extends this concept, and
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allows services to *upload* during runtime additional fds to the service
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manager that it shall keep on its behalf. File descriptors are passed back to
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the service on subsequent activations, the same way as any socket activation
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fds are passed.
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If a service fd is passed to the fdstore logic of the service manager it only
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maintains a duplicate of it (in the sense of UNIX
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[`dup(2)`](https://man7.org/linux/man-pages/man2/dup.2.html)), the fd remains
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also in possession of the service itself, and it may (and is expected to)
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invoke any operations on it that it likes.
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The primary use-case of this logic is to permit services to restart seamlessly
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(for example to update them to a newer version), without losing execution
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context, dropping pinned resources, terminating established connections or even
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just momentarily losing connectivity. In fact, as the file descriptors can be
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uploaded freely at any time during the service runtime, this can even be used
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to implement services that robustly handle abnormal termination and can recover
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from that without losing pinned resources.
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Note that Linux supports the
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[`memfd`](https://man7.org/linux/man-pages/man2/memfd_create.2.html) concept
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that allows associating a memory-backed fd with arbitrary data. This may
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conveniently be used to serialize service state into and then place in the
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fdstore, in order to implement service restarts with full service state being
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passed over.
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## Basic Mechanism
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The fdstore is enabled per-service via the
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[`FileDescriptorStoreMax=`](https://www.freedesktop.org/software/systemd/man/systemd.service.html#FileDescriptorStoreMax=)
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service setting. It defaults to zero (which means the fdstore logic is turned
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off), but can take an unsigned integer value that controls how many fds to
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permit the service to upload to the service manager to keep simultaneously.
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If set to values > 0, the fdstore is enabled. When invoked the service may now
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(asynchronously) upload file descriptors to the fdstore via the
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[`sd_pid_notify_with_fds()`](https://www.freedesktop.org/software/systemd/man/sd_pid_notify_with_fds.html)
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API call (or an equivalent re-implementation). When uploading the fds it is
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necessary to set the `FDSTORE=1` field in the message, to indicate what the fd
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is intended for. It's recommended to also set the `FDNAME=…` field to any
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string of choice, which may be used to identify the fd later.
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Whenever the service is restarted the fds in its fdstore will be passed to the
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new instance following the same protocol as for socket activation fds. i.e. the
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`$LISTEN_FDS`, `$LISTEN_PIDS`, `$LISTEN_FDNAMES` environment variables will be
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set (the latter will be populated from the `FDNAME=…` field mentioned
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above). See
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[`sd_listen_fds()`](https://www.freedesktop.org/software/systemd/man/sd_listen_fds.html)
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for details on receiving such fds in a service. (Note that the name set in
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`FDNAME=…` does not need to be unique, which is useful when operating with
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multiple fully equivalent sockets or similar, for example for a service that
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both operates on IPv4 and IPv6 and treats both more or less the same.).
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And that's already the gist of it.
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## Seamless Service Restarts
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A system service that provides a client-facing interface that shall be able to
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seamlessly restart can make use of this in a scheme like the following:
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whenever a new connection comes in it uploads its fd immediately into its
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fdstore. At appropriate times it also serializes its state into a memfd it
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uploads to the service manager — either whenever the state changed
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sufficiently, or simply right before it terminates. (The latter of course means
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that state only survives on *clean* restarts and abnormal termination implies the
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state is lost completely — while the former would mean there's a good chance the
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next restart after an abnormal termination could continue where it left off
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with only some context lost.)
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Using the fdstore for such seamless service restarts is generally recommended
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over implementations that attempt to leave a process from the old service
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instance around until after the new instance already started, so that the old
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then communicates with the new service instance, and passes the fds over
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directly. Typically service restarts are a mechanism for implementing *code*
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updates, hence leaving two version of the service running at the same time is
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generally problematic. It also collides with the systemd service manager's
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general principle of guaranteeing a pristine execution environment, a pristine
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security context, and a pristine resource management context for freshly
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started services, without uncontrolled "leftovers" from previous runs. For
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example: leaving processes from previous runs generally negatively affects
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lifecycle management (i.e. `KillMode=none` must be set), which disables large
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parts of the service managers state tracking, resource management (as resource
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counters cannot start at zero during service activation anymore, since the old
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processes remaining skew them), security policies (as processes with possibly
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out-of-date security policies – SElinux, AppArmor, any LSM, seccomp, BPF — in
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effect remain), and similar.
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## File Descriptor Store Lifecycle
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By default any file descriptor stored in the fdstore for which a `POLLHUP` or
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`POLLERR` is seen is automatically closed and removed from the fdstore. This
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behavior can be turned off, by setting the `FDPOLL=0` field when uploading the
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fd via `sd_notify_with_fds()`.
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The fdstore is automatically closed whenever the service is fully deactivated
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and no jobs are queued for it anymore. This means that a restart job for a
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service will leave the fdstore intact, but a separate stop and start job for
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it — executed synchronously one after the other — will likely not.
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This behavior can be modified via the
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[`FileDescriptorStorePreserve=`](https://www.freedesktop.org/software/systemd/man/systemd.service.html#FileDescriptorStorePreserve=)
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setting in service unit files. If set to `yes` the fdstore will be kept as long
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as the service definition is loaded into memory by the service manager, i.e. as
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long as at least one other loaded unit has a reference to it.
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The `systemctl clean --what=fdstore …` command may be used to explicitly clear
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the fdstore of a service. This is only allowed when the service is fully
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deactivated, and is hence primarily useful in case
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`FileDescriptorStorePreserve=yes` is set (because the fdstore is otherwise
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fully closed anyway in this state).
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Individual file descriptors may be removed from the fdstore via the
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`sd_notify()` mechanism, by sending an `FDSTOREREMOVE=1` message, accompanied
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by an `FDNAME=…` string identifying the fds to remove. (The name does not have
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to be unique, as mentioned, in which case *all* matching fds are
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closed). Generally it's a good idea to send such messages to the service
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manager during initialization of the service whenever an unrecognized fd is
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received, to make the service robust for code updates: if an old version
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uploaded an fd that the new version doesn't recognize anymore it's good idea to
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close it both in the service and in the fdstore.
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Note that storing a duplicate of an fd in the fdstore means the resource pinned
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by the fd remains pinned even if the service closes its duplicate of the
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fd. This in particular means that peers on a connection socket uploaded this
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way will not receive an automatic `POLLHUP` event anymore if the service code
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issues `close()` on the socket. It must accompany it with an `FDSTOREREMOVE=1`
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notification to the service manager, so that the fd is comprehensively closed.
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## Access Control
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Access to the fds in the file descriptor store is generally restricted to the
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service code itself. Pushing fds into or removing fds from the fdstore is
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subject to the access control restrictions of any other `sd_notify()` message,
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which is controlled via
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[`NotifyAccess=`](https://www.freedesktop.org/software/systemd/man/systemd.service.html#NotifyAccess=).
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By default only the main service process hence can push/remove fds, but by
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setting `NotifyAccess=all` this may be relaxed to allow arbitrary service
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child processes to do the same.
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## Soft Reboot
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The fdstore is particularly interesting in [soft
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reboot](https://www.freedesktop.org/software/systemd/man/systemd-soft-reboot.service.html)
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scenarios, as per `systemctl soft-reboot` (which restarts userspace like in a
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real reboot, but leaves the kernel running). File descriptor stores that remain
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loaded at the very end of the system cycle — just before the soft-reboot – are
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passed over to the next system cycle, and propagated to services they originate
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from there. This enables updating the full userspace of a system during
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runtime, fully replacing all processes without losing pinning resources,
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interrupting connectivity or established connections and similar.
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This mechanism can be enabled either by making sure the service survives until
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the very end (i.e. by setting `DefaultDependencies=no` so that it keeps running
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for the whole system lifetime without being regularly deactivated at shutdown)
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or by setting `FileDescriptorStorePreserve=yes` (and referencing the unit
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continuously).
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For further details see [Resource
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Pass-Through](https://www.freedesktop.org/software/systemd/man/systemd-soft-reboot.service.html#Resource%20Pass-Through).
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## Initrd Transitions
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The fdstore may also be used to pass file descriptors for resources from the
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initrd context to the main system. Restarting all processes after the
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transition is important as code running in the initrd should generally not
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continue to run after the switch to the host file system, since that pins
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backing files from the initrd, and the initrd might contain different versions
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of programs than the host.
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Any service that still runs during the initrd→host transition will have its
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fdstore passed over the transition, where it will be passed back to any queued
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services of the same name.
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The soft reboot cycle transition and the initrd→host transition are
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semantically very similar, hence similar rules apply, and in both cases it is
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recommended to use the fdstore if pinned resources shall be passed over.
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## Debugging
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The
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[`systemd-analyze`](https://www.freedesktop.org/software/systemd/man/systemd-analyze.html#systemd-analyze%20fdstore%20%5BUNIT...%5D)
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tool may be used to list the current contents of the fdstore of any running
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service.
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The
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[`systemd-run`](https://www.freedesktop.org/software/systemd/man/systemd-run.html)
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tool may be used to quickly start a testing binary or similar as a service. Use
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`-p FileDescriptorStore=4711` to enable the fdstore from `systemd-run`'s
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command line. By using the `-t` switch you can even interactively communicate
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via processes spawned that way, via the TTY.
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