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docs: various updates to the fdstore docs

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ispell made some suggestions which I applied.

Addresses: https://github.com/systemd/systemd/pull/29209#pullrequestreview-1632623460

Also adds a brief paragraph about initrd transitions. (Plymouth really
should start using the fdstore for pinning DRM objects, and stop trying
to survive the initrd→host transition)
This commit is contained in:
Lennart Poettering 2023-09-19 22:22:49 +02:00
parent c8d44d818f
commit 1df74d1cea
1 changed files with 39 additions and 20 deletions
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59
docs/FILE_DESCRIPTOR_STORE.md
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@ -1,5 +1,5 @@
---
title: The File Descriptor Store
title: File Descriptor Store
category: Interfaces
layout: default
SPDX-License-Identifier: LGPL-2.1-or-later
@ -8,9 +8,9 @@ SPDX-License-Identifier: 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, that are under control of the service and that
help making service restarts without losing connectivity or context easier to
implement.*
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
@ -32,12 +32,12 @@ maintains a duplicate of it (in the sense of UNIX
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
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
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
@ -58,7 +58,7 @@ 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()`](https://www.freedesktop.org/software/systemd/man/sd_pid_notify_with_fds.html)
API call (or an equivalent reimplementation). When uploading the fds it is
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.
@ -98,20 +98,20 @@ 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 "left-overs" from previous runs. For
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
life-cycle 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
# File Descriptor Store Life-cycle
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
behaviour can be turned off, by setting the `FDPOLL=0` field when uploading the
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
@ -119,7 +119,7 @@ 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 behaviour can be modified via the
This behavior can be modified via the
[`FileDescriptorStorePreserve=`](https://www.freedesktop.org/software/systemd/man/systemd.service.html#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
@ -141,12 +141,12 @@ 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 fd remains
pinned even if the service closes it. 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.
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
@ -175,9 +175,28 @@ 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 `FileDescriptorStorePresever=yes` (and referencing the unit
or by setting `FileDescriptorStorePreserve=yes` (and referencing the unit
continuously).
For further details see [Resource
Pass-Through](https://www.freedesktop.org/software/systemd/man/systemd-soft-reboot.service.html#Resource%20Pass-Through).
# initrd Transitions
The fdstore may also be used to hand over file descriptor to resources from the
initrd context to the main system. This is important as code running off the
initrd should generally not continue to run after the initrd to host
transition, since it pins the backing files from the initrd, and might run a
slightly different version of things 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