man `8 systemd-coredump`

SYSTEMD-COREDUMP(8) systemd-coredump SYSTEMD-COREDUMP(8)

NAME
systemd-coredump, systemd-coredump.socket, systemd-coredump@.service -
Acquire, save and process core dumps

SYNOPSIS
/usr/lib/systemd/systemd-coredump

/usr/lib/systemd/systemd-coredump --backtrace

systemd-coredump@.service

systemd-coredump.socket

DESCRIPTION
systemd-coredump@.service is a system service to process core dumps. It
will log a summary of the event to systemd-journald.service(8),
including information about the process identifier, owner, the signal
that killed the process, and the stack trace if possible. It may also
save the core dump for later processing. See the "Information about the
crashed process" section below.

The behavior of a specific program upon reception of a signal is
governed by a few factors which are described in detail in core(5). In
particular, the core dump will only be processed when the related
resource limits are sufficient.

Core dumps can be written to the journal or saved as a file. In both
cases, they can be retrieved for further processing, for example in
gdb(1). See coredumpctl(1), in particular the list and debug verbs.

By default, systemd-coredump will log the core dump to the journal,
including a backtrace if possible, and store the core dump (an image of
the memory contents of the process) itself in an external file in
/var/lib/systemd/coredump. These core dumps are deleted after a few
days by default; see /usr/lib/tmpfiles.d/systemd.conf for details. Note
that the removal of core files from the file system and the purging of
journal entries are independent, and the core file may be present
without the journal entry, and journal entries may point to
since-removed core files. Some metadata is attached to core files in
the form of extended attributes, so the core files are useful for some
purposes even without the full metadata available in the journal entry.

Invocation of systemd-coredump
The systemd-coredump executable does the actual work. It is invoked
twice: once as the handler by the kernel, and the second time in the
systemd-coredump@.service to actually write the data to the journal and
process and save the core file.

When the kernel invokes systemd-coredump to handle a core dump, it runs
in privileged mode, and will connect to the socket created by the
systemd-coredump.socket unit, which in turn will spawn an unprivileged
systemd-coredump@.service instance to process the core dump. Hence
systemd-coredump.socket and systemd-coredump@.service are helper units
which do the actual processing of core dumps and are subject to normal
service management.

It is also possible to invoke systemd-coredump with --backtrace option.
In this case, systemd-coredump expects a journal entry in the journal
Journal Export Format[1] on standard input. The entry should contain a
MESSAGE= field and any additional metadata fields the caller deems
reasonable. systemd-coredump will append additional metadata fields in
the same way it does for core dumps received from the kernel. In this
mode, no core dump is stored in the journal.

CONFIGURATION
For programs started by systemd, process resource limits can be set by
directive LimitCORE=, see systemd.exec(5).

In order to be used by the kernel to handle core dumps,
systemd-coredump must be configured in sysctl(8) parameter
kernel.core_pattern. The syntax of this parameter is explained in
core(5). systemd installs the file /usr/lib/sysctl.d/50-coredump.conf
which configures kernel.core_pattern accordingly. This file may be
masked or overridden to use a different setting following normal
sysctl.d(5) rules. If the sysctl configuration is modified, it must be
updated in the kernel before it takes effect, see sysctl(8) and
systemd-sysctl(8).

In order to be used in the --backtrace mode, an appropriate backtrace
handler must be installed on the sender side. For example, in case of
python(1), this means a sys.excepthook must be installed, see
systemd-coredump-python[2].

The behavior of systemd-coredump itself is configured through the
configuration file /etc/systemd/coredump.conf and corresponding
snippets /etc/systemd/coredump.conf.d/*.conf, see coredump.conf(5). A
new instance of systemd-coredump is invoked upon receiving every core
dump. Therefore, changes in these files will take effect the next time
a core dump is received.

Resources used by core dump files are restricted in two ways.
Parameters like maximum size of acquired core dumps and files can be
set in files /etc/systemd/coredump.conf and snippets mentioned above.
In addition the storage time of core dump files is restricted by
systemd-tmpfiles, corresponding settings are by default in
/usr/lib/tmpfiles.d/systemd.conf. The default is to delete core dumps
after a few days; see the above file for details.

Disabling coredump processing
To disable potentially resource-intensive processing by
systemd-coredump, set

Storage=none ProcessSizeMax=0

in coredump.conf(5).

INFORMATION ABOUT THE CRASHED PROCESS
coredumpctl(1) can be used to retrieve saved core dumps independently
of their location, to display information, and to process them e.g. by
passing to the GNU debugger (gdb).

Data stored in the journal can be also viewed with journalctl(1) as
usual (or from any other process, using the sd-journal(3) API). The
relevant messages have MESSAGE_ID=fc2e22bc6ee647b6b90729ab34a250b1:

$ journalctl MESSAGE_ID=fc2e22bc6ee647b6b90729ab34a250b1 -o verbose
...
MESSAGE_ID=fc2e22bc6ee647b6b90729ab34a250b1
COREDUMP_PID=552351
COREDUMP_UID=1000
COREDUMP_GID=1000
COREDUMP_SIGNAL_NAME=SIGSEGV
COREDUMP_SIGNAL=11
COREDUMP_TIMESTAMP=1614342930000000
COREDUMP_COMM=Web Content
COREDUMP_EXE=/usr/lib64/firefox/firefox
COREDUMP_USER_UNIT=app-gnome-firefox-552136.scope
COREDUMP_CMDLINE=/usr/lib64/firefox/firefox -contentproc -childID 5 -isForBrowser ...
COREDUMP_CGROUP=/user.slice/user-1000.slice/user@1000.service/app.slice/app-....scope
COREDUMP_FILENAME=/var/lib/systemd/coredump/core.Web....552351.....zst
...

The following fields are saved (if known) with the journal entry

COREDUMP_UID=, COREDUMP_PID=, COREDUMP_GID=
The process number (PID), owner user number (UID), and group number
(GID) of the crashed process.

When the crashed process was part of a container (or in a process
or user namespace in general), those are the values as seen
outside, in the namespace where systemd-coredump is running.

COREDUMP_TIMESTAMP=
The time of the crash as reported by the kernel (in <micro>s since
the epoch).

COREDUMP_RLIMIT=
The core file size soft resource limit, see getrlimit(2).

COREDUMP_UNIT=, COREDUMP_SLICE=
The system unit and slice names.

When the crashed process was in container, those are the units
names outside, in the main system manager.

COREDUMP_CGROUP=
Control group information in the format used in /proc/self/cgroup.
On systems with the unified cgroup hierarchy, this is a single path
prefixed with "0::", and multiple paths prefixed with controller
numbers on legacy systems.

When the crashed process was in a container, this is the full path,
as seen outside of the container.

COREDUMP_OWNER_UID=, COREDUMP_USER_UNIT=
The numerical UID of the user owning the login session or systemd
user unit of the crashed process, and the user manager unit. Both
fields are only present for user processes.

When the crashed process was in container, those are the values
outside, in the main system.

COREDUMP_SIGNAL_NAME=, COREDUMP_SIGNAL=
The terminating signal name (with the "SIG" prefix [3]) and
numerical value. (Both are included because signal numbers vary by
architecture.)

COREDUMP_CWD=, COREDUMP_ROOT=
The current working directory and root directory of the crashed
process.

When the crashed process is in a container, those paths are
relative to the root of the container's mount namespace.

COREDUMP_OPEN_FDS=
Information about open file descriptors, in the following format:

fd:/path/to/file
pos: ...
flags: ...
...

The first line contains the file descriptor number fd and the path,
while subsequent lines show the contents of /proc/pid/fdinfo/fd.

COREDUMP_EXE=
The destination of the /proc/pid/exe symlink.

When the crashed process is in a container, that path is relative
to the root of the container's mount namespace.

COREDUMP_COMM=, COREDUMP_PROC_STATUS=, COREDUMP_PROC_MAPS=,
COREDUMP_PROC_LIMITS=, COREDUMP_PROC_MOUNTINFO=, COREDUMP_ENVIRON=
Fields that map the per-process entries in the /proc/ filesystem:
/proc/pid/comm (the command name associated with the process),
/proc/pid/exe (the filename of the executed command),
/proc/pid/status (various metadata about the process),
/proc/pid/maps (memory regions visible to the process and their
access permissions), /proc/pid/limits (the soft and hard resource
limits), /proc/pid/mountinfo (mount points in the process's mount
namespace), /proc/pid/environ (the environment block of the crashed
process).

See proc(5) for more information.

COREDUMP_HOSTNAME=
The system hostname.

When the crashed process was in container, this is the container
hostname.

COREDUMP_CONTAINER_CMDLINE=
For processes running in a container, the commandline of the
process spawning the container (the first parent process with a
different mount namespace).

COREDUMP=
When the core is stored in the journal, the core image itself.

COREDUMP_FILENAME=
When the core is stored externally, the path to the core file.

COREDUMP_TRUNCATED=
Set to "1" when the saved coredump was truncated. (A partial core
image may still be processed by some tools, though obviously not
all information is available.)

COREDUMP_PACKAGE_NAME=, COREDUMP_PACKAGE_VERSION=,
COREDUMP_PACKAGE_JSON=
If the executable contained .package metadata ELF notes, they will
be parsed and attached. The package and packageVersion of the
'main' ELF module (ie: the executable) will be appended
individually. The JSON-formatted content of all modules will be
appended as a single JSON object, each with the module name as the
key. For more information about this metadata format and content,
see the coredump metadata spec[4].

MESSAGE=
The message generated by systemd-coredump that includes the
backtrace if it was successfully generated. When systemd-coredump
is invoked with --backtrace, this field is provided by the caller.

Various other fields exist in the journal entry, but pertain to the
logging process, i.e. systemd-coredump, not the crashed process. See
systemd.journal-fields(7).

The following fields are saved (if known) with the external file listed
in COREDUMP_FILENAME= as extended attributes:

user.coredump.pid, user.coredump.uid, user.coredump.gid,
user.coredump.signal, user.coredump.timestamp, user.coredump.rlimit,
user.coredump.hostname, user.coredump.comm, user.coredump.exe
Those are the same as COREDUMP_PID=, COREDUMP_UID=, COREDUMP_GID=,
COREDUMP_SIGNAL=, COREDUMP_TIMESTAMP=, COREDUMP_RLIMIT=,
COREDUMP_HOSTNAME=, COREDUMP_COMM=, and COREDUMP_EXE=, described
above.

Those can be viewed using getfattr(1). For the core file described in
the journal entry shown above:

$ getfattr --absolute-names -d /var/lib/systemd/coredump/core.Web....552351.....zst
# file: /var/lib/systemd/coredump/core.Web....552351.....zst
user.coredump.pid="552351"
user.coredump.uid="1000"
user.coredump.gid="1000"
user.coredump.signal="11"
user.coredump.timestamp="1614342930000000"
user.coredump.comm="Web Content"
user.coredump.exe="/usr/lib64/firefox/firefox"
...

SEE ALSO
coredump.conf(5), coredumpctl(1), systemd-journald.service(8), systemd-
tmpfiles(8), core(5), sysctl.d(5), systemd-sysctl.service(8).

NOTES
1. Journal Export Format
https://systemd.io/JOURNAL_EXPORT_FORMATS#journal-export-format

2. systemd-coredump-python
https://github.com/systemd/systemd-coredump-python

3. kill(1) expects signal names without the prefix; kill(2) uses the
prefix; all systemd tools accept signal names both with and without
the prefix.

4. the coredump metadata spec
https://systemd.io/COREDUMP_PACKAGE_METADATA/

systemd 252 SYSTEMD-COREDUMP(8)

man 8 systemd-coredump

man `8 systemd-coredump`