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4f57f77267
Fixes #17910: we didn't clearly explain that coredumps may exist without journal entries, and vice versa. Also, make the examples more concrete, and use '$' instead of '#' to avoid suggesting that running as root is required. The text is extended a bit in various places. In the description of systemd-coredump, the details of executor separation are split out to a separate subsection, since they are rather detailed and not necessary to understand for normal use.
165 lines
9.6 KiB
XML
165 lines
9.6 KiB
XML
<?xml version='1.0'?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="systemd-coredump" conditional='ENABLE_COREDUMP'
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>systemd-coredump</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd-coredump</refentrytitle>
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<manvolnum>8</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd-coredump</refname>
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<refname>systemd-coredump.socket</refname>
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<refname>systemd-coredump@.service</refname>
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<refpurpose>Acquire, save and process core dumps</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>/usr/lib/systemd/systemd-coredump</filename></para>
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<para><filename>/usr/lib/systemd/systemd-coredump</filename> <option>--backtrace</option></para>
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<para><filename>systemd-coredump@.service</filename></para>
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<para><filename>systemd-coredump.socket</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><filename>systemd-coredump@.service</filename> is a system service to process core dumps. It will
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log a summary of the event to
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<citerefentry><refentrytitle>systemd-journald.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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including information about the process identifier, owner, the signal that killed the process, and the
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stack trace if possible. It may also save the core dump for later processing.</para>
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<para>The behavior of a specific program upon reception of a signal is governed by a few
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factors which are described in detail in
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<citerefentry project='man-pages'><refentrytitle>core</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
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In particular, the core dump will only be processed when the related resource limits are sufficient.
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</para>
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<para>Core dumps can be written to the journal or saved as a file. In both cases, they can be retrieved
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for further processing, for example in
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<citerefentry project='man-pages'><refentrytitle>gdb</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
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See <citerefentry><refentrytitle>coredumpctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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in particular the <command>list</command> and <command>debug</command> verbs.</para>
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<para>By default, <command>systemd-coredump</command> will log the core dump to the journal, including a
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backtrace if possible, and store the core dump (an image of the memory contents of the process) itself in
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an external file in <filename>/var/lib/systemd/coredump</filename>. These core dumps are deleted after a
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few days by default; see <filename>/usr/lib/tmpfiles.d/systemd.conf</filename> for details. Note that the
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removal of core files from the file system and the purging of journal entries are independent, and the
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core file may be present without the journal entry, and journal entries may point to since-removed core
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files. Metadata is attached to core files in the form of extended attributes, so the core files may be
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useful even without the full metadata available in the journal entry.
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</para>
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<refsect2>
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<title>Invocation of <command>systemd-coredump</command></title>
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<para>The <command>systemd-coredump</command> executable does the actual work. It is invoked twice:
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once as the handler by the kernel, and the second time in the
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<filename>systemd-coredump@.service</filename> to actually write the data to the journal and process
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and save the core file.</para>
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<para>When the kernel invokes <command>systemd-coredump</command> to handle a core dump, it runs in
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privileged mode, and will connect to the socket created by the
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<filename>systemd-coredump.socket</filename> unit, which in turn will spawn an unprivileged
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<filename>systemd-coredump@.service</filename> instance to process the core dump. Hence
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<filename>systemd-coredump.socket</filename> and <filename>systemd-coredump@.service</filename> are
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helper units which do the actual processing of core dumps and are subject to normal service
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management.</para>
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<para>It is also possible to invoke <command>systemd-coredump</command> with
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<option>--backtrace</option> option. In this case, <command>systemd-coredump</command> expects a
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journal entry in the journal
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<ulink url="https://www.freedesktop.org/wiki/Software/systemd/export">Journal Export Format</ulink>
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on standard input. The entry should contain a <varname>MESSAGE=</varname> field and any additional
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metadata fields the caller deems reasonable. <command>systemd-coredump</command> will append additional
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metadata fields in the same way it does for core dumps received from the kernel. In this mode, no core
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dump is stored in the journal.</para>
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</refsect2>
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</refsect1>
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<refsect1>
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<title>Configuration</title>
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<para>For programs started by <command>systemd</command>, process resource limits can be set by directive
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<varname>LimitCORE=</varname>, see
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<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
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</para>
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<para>In order to be used by the kernel to handle core dumps,
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<command>systemd-coredump</command> must be configured in
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<citerefentry project='man-pages'><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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parameter <varname>kernel.core_pattern</varname>. The syntax of this parameter is explained in
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<citerefentry project='man-pages'><refentrytitle>core</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
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systemd installs the file <filename>/usr/lib/sysctl.d/50-coredump.conf</filename> which configures
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<varname>kernel.core_pattern</varname> accordingly. This file may be masked or overridden to use a different
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setting following normal
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<citerefentry><refentrytitle>sysctl.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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rules. If the sysctl configuration is modified, it must be updated in the kernel before it
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takes effect, see
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<citerefentry project='man-pages'><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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and
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<citerefentry><refentrytitle>systemd-sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
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</para>
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<para>In order to be used in the <option>--backtrace</option> mode, an appropriate backtrace
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handler must be installed on the sender side. For example, in case of
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<citerefentry project='die-net'><refentrytitle>python</refentrytitle><manvolnum>1</manvolnum></citerefentry>, this
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means a <varname>sys.excepthook</varname> must be installed, see
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<ulink url="https://github.com/keszybz/systemd-coredump-python">systemd-coredump-python</ulink>.
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</para>
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<para>The behavior of <command>systemd-coredump</command> itself is configured through the configuration file
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<filename>/etc/systemd/coredump.conf</filename> and corresponding snippets
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<filename>/etc/systemd/coredump.conf.d/*.conf</filename>, see
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<citerefentry><refentrytitle>coredump.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>. A new
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instance of <command>systemd-coredump</command> is invoked upon receiving every core dump. Therefore, changes
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in these files will take effect the next time a core dump is received.</para>
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<para>Resources used by core dump files are restricted in two ways. Parameters like maximum size of acquired
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core dumps and files can be set in files <filename>/etc/systemd/coredump.conf</filename> and snippets mentioned
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above. In addition the storage time of core dump files is restricted by <command>systemd-tmpfiles</command>,
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corresponding settings are by default in <filename>/usr/lib/tmpfiles.d/systemd.conf</filename>. The default is
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to delete core dumps after a few days; see the above file for details.</para>
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<refsect2>
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<title>Disabling coredump processing</title>
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<para>To disable potentially resource-intensive processing by <command>systemd-coredump</command>, set
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<programlisting>Storage=none ProcessSizeMax=0</programlisting> in
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<citerefentry><refentrytitle>coredump.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
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</para>
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</refsect2>
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</refsect1>
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<refsect1>
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<title>Usage</title>
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<para>Data stored in the journal can be viewed with
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<citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> as usual.
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<citerefentry><refentrytitle>coredumpctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> can be
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used to retrieve saved core dumps independent of their location, to display information and to process
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them e.g. by passing to the GNU debugger (gdb).</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>coredump.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>coredumpctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-journald.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-tmpfiles</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>core</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sysctl.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-sysctl.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
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</para>
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</refsect1>
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</refentry>
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