# Core dump

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 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[1][2]:

    /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.

### Using ulimit

The maximum core dump size is enforced by ulimit. Setting it to zero disables core dumps entirely. [3]

    /etc/security/limits.conf

    * hard core 0

### Using sysctl

sysctl can be used to modify the `fs.suid_dumpable` kernel parameter. This only applies to suid processes. [4]

    /etc/sysctl.conf

    fs.suid_dumpable = 0

## 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?

The factual accuracy of this article or section is disputed.

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 `man coredumpctl`

## 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

- american fuzzy lop - An tool for automated tests of the kernel and programs
- Filesystem fuzzing - LWN article about testing filesystems for bugs

Categories:

- Development
- Security

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