Core dump

From ArchWiki
Jump to: navigation, search

A core dump is a file containing a process's address space (memory) when the process terminates unexpectedly. Core dumps may be produced on-demand (such as by a debugger), or automatically upon termination. Core dumps are triggered by the kernel in response to program crashes, and may be passed to a helper program (such as systemd-coredump) for further processing. A core dump is not typically used by an average user, but may be passed on to developers upon request where it can be invaluable as a post-mortem snapshot of the program's state at the time of the crash, especially if the fault is hard to reliably reproduce.

Disabling automatic core dumps

Users may wish to disable automatic core dumps for a number of reasons:

  • Performance: generating core dumps for memory-heavy processes can waste system resources and delay the cleanup of memory.
  • Disk space: core dumps of memory-heavy processes may consume disk space equal to, if not greater, than the process's memory footprint if not compressed.
  • Security: core dumps, although typically readable only by root, may contain sensitive data (such as passwords or cryptographic keys), which are written to disk following a crash.

Using sysctl

sysctl can be used to set the kernel.core_pattern to nothing to disable core dump handling. Create this file[1]:

/etc/sysctl.d/50-coredump.conf
kernel.core_pattern=|/bin/false

To apply the setting immediately, use sysctl:

# sysctl -p /etc/sysctl.d/50-coredump.conf

Using ulimit

The maximum core dump size is enforced by ulimit. Setting it to zero disables core dumps entirely. [2]

/etc/security/limits.conf
* hard core 0

Using systemd

systemd's default behavior is to generate core dumps for all processes in /var/lib/systemd/coredump. This behavior can be overridden by creating a configuration snippet in the /etc/systemd/coredump.conf.d/ directory with the following content[3][4]:

/etc/systemd/coredump.conf.d/custom.conf
[Coredump]
Storage=none
Note: Don't forget to include the [Coredump] section name, otherwise this option will be ignored: systemd-coredump[1728]: [/etc/systemd/coredump.conf.d/custom.conf:1] Assignment outside of section. Ignoring.

Then reload systemd's configuration.

# systemctl daemon-reload

This method alone is usually sufficient to disable userspace core dumps, so long as no other programs enable automatic core dumps on the system, but the coredump is still generated in memory and systemd-coredump run.

Making a core dump

To generate a core dump of an arbitrary process, first install the gdb package. Then find the PID of the running process, for example with pgrep:

$ pgrep -f firefox
2071 firefox

Attach to the process:

$ gdb -p 2071

Then at the (gdb) prompt:

(gdb) generate-core-file
Saved corefile core.2071
(gdb) quit

Now you have a coredump file called core.2071.

Where do they go?

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: systemd modifies the kernel.core_pattern kernel parameter at runtime, overriding that of /usr/lib/sysctl.d/50-coredump.conf. See #Using systemd for a better solution. (Discuss in Talk:Core dump#)

The kernel.core_pattern sysctl decides where automatic core dumps go:

$ cat /proc/sys/kernel/core_pattern 
|/usr/lib/systemd/systemd-coredump %p %u %g %s %t %e

The default set in /usr/lib/sysctl.d/50-coredump.conf sends all core dumps to journald as part of the system logs.

Note: If you do not have full-disk encryption, this means your program's memory will be written to raw disk! This is a potential information leak even if you have encrypted swap.

To retrieve a core dump from the journal, see coredumpctl(1)

Examining a core dump

Use coredumpctl to find the corresponding dump:

# coredumpctl list

You need to uniquely identify the relevant dump. This is possible by specifying a PID, name of the executable, path to the executable or a journalctl predicate (see coredumpctl(1) and journalctl(1) for details). To see details of the core dumps:

# coredumpctl info match

Pay attention to "Signal" row, that helps to identify crash cause. For deeper analysis you can examine the backtrace using gdb:

# coredumpctl gdb match

When gdb is started, use the bt command to print the backtrace:

(gdb) bt

See also