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man: Clarify systemd-notify and sd_notify() PID documentation

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Let's clarify more explicitly that privileged calls to
systemd-notify --pid= and sd_pid_notify() effectively override any
configured NotifyAccess=main|exec for a service.

(cherry picked from commit bbe9e03f8066d1001497494ee862cf45f986b854)
This commit is contained in:
Daan De Meyer 2025-01-14 16:05:33 +01:00
parent b1fe0a92eb
commit 9b186fc8bc
2 changed files with 17 additions and 7 deletions
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9
man/sd_notify.xml
View File

@ -140,9 +140,12 @@
<para><function>sd_pid_notify()</function> and <function>sd_pid_notifyf()</function> are similar to
<function>sd_notify()</function> and <function>sd_notifyf()</function> but take a process ID (PID) to use
as originating PID for the message as first argument. This is useful to send notification messages on
behalf of other processes, provided the appropriate privileges are available. If the PID argument is
specified as 0, the process ID of the calling process is used, in which case the calls are fully
equivalent to <function>sd_notify()</function> and <function>sd_notifyf()</function>.</para>
behalf of other processes, provided the appropriate privileges are available. Effectively, this means
that a privileged invocation of <command>sd_pid_notify()</command> may circumvent
<varname>NotifyAccess=main</varname> or <varname>NotifyAccess=exec</varname> restrictions enforced for a
service. If the PID argument is specified as 0, the process ID of the calling process is used, in which
case the calls are fully equivalent to <function>sd_notify()</function> and
<function>sd_notifyf()</function>.</para>
<para><function>sd_pid_notify_with_fds()</function> is similar to <function>sd_pid_notify()</function>
but takes an additional array of file descriptors. These file descriptors are sent along the notification

15
man/systemd-notify.xml
View File

@ -125,12 +125,19 @@
argument is specified as <literal>self</literal>, the PID of the <command>systemd-notify</command>
command itself is used, and if <literal>parent</literal> is specified the calling process' PID is
used — even if it is the service manager. <option>--pid=auto</option> is equivalent to <command>systemd-notify
MAINPID=$PID</command>. For details about the semantics of this option see
--pid=$PID</command>. For details about the semantics of this option see
<citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
<para>If this switch is used in an <command>systemd-notify</command> invocation from a process that
shall become the new main process of a service — and which is not the process forked off by the
service manager (or the current main process) —, then it is essential to set
<para><command>systemd-notify</command> will first attempt to invoke <function>sd_notify()</function>
pretending to have the PID specified with <option>--pid=</option>. This will only succeed when
invoked with sufficient privileges. On failure, it will then fall back to invoking it under its own
PID. Effectively, this means that a privileged invocation of <command>systemd-notify --pid=</command>
may circumvent <varname>NotifyAccess=main</varname> or <varname>NotifyAccess=exec</varname>
restrictions enforced for a service.</para>
<para>If this switch is used in an unprivileged <command>systemd-notify</command> invocation from a
process that shall become the new main process of a service — and which is not the process forked off
by the service manager (or the current main process) —, then it is essential to set
<varname>NotifyAccess=all</varname> in the service unit file, or otherwise the notification will be
ignored for security reasons. See
<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>