LTTng core dump snapshot handler
Christian Babeux, June 2013

This is a custom core dump program that will be called when a core dump
occurs. The program will save the core data in CORE_PATH and also, if a
root session daemon is running, will record a snapshot of tracing data
using the lttng command line utility.

The core dump snapshot handler can be installed by using the provided
install.sh script or by adding the appropriate program pipe line to
/proc/sys/kernel/core_pattern. Refer to core(5) for more information
about the Linux kernel core dump handling and custom handler mechanism.

Installation:

# ./install.sh
Backup current core_pattern in core_pattern.bkp.
Successfully installed core_pattern.

How to use:

You can use the provided test.sh script to test that the core dump snapshot
handler is working properly:

# ./test.sh
Setup...
Spawning a session daemon
Session auto-20130626-224838 created.
Kernel channel chan enabled for session auto-20130626-224838
kernel event sched_switch created in channel chan
Tracing started for session auto-20130626-224838
Sleeping...
Crashing...
./test.sh: line 35: 16980 Segmentation fault      (core dumped)
$(dirname $0)/crash
Waiting for data availability
Tracing stopped for session auto-20130626-224838
Session auto-20130626-224838 destroyed
Core dump and snapshot will be available in /tmp/lttng/{core,snapshot}.

# tree /tmp/lttng
/tmp/lttng
|-- core
|   `-- core.16980
`-- snapshot
    `-- snapshot
        |-- chan_0
        |-- chan_1
        |-- chan_2
        |-- chan_3
        |-- chan_4
        |-- chan_5
        |-- chan_6
        |-- chan_7
        `-- metadata

Chaining with other core dump handler:

Some Linux distributions already use their own core dump handler
(such as systemd 'systemd-coredump' utility). It is possible to chain these
core dump utility with the core dump snapshot handler. In order to achieve
this, the core dump snapshot handler must be first in the chain (e.g.
installed in /proc/sys/kernel/core_pattern) and the other core dump
handler must be called from within the core dump snapshot handler script.

Example (chaining with systemd systemd-coredump):

# cat /proc/sys/kernel/core_pattern
|/path/to/lttng/handler.sh %p %u %g %s %t %h %e %E %c

In LTTng handler.sh script:

[...]
# Save core dump from stdin.
$MKDIR_BIN -p "${CORE_PATH}"

# Optional, chain core dump handler with original systemd script.
$CAT_BIN - | /usr/lib/systemd/systemd-coredump $p $u $g $s $t $e
[...]