mcelog is the user space backend for logging machine check errors
reported by the hardware to the kernel. The kernel does the immediate
actions (like killing processes etc.) and mcelog decodes the errors
and manages various other advanced error responses like
offlining memory, CPUs or triggering events.

It primarily handles machine checks and thermal events, which
are reported for errors detected by the CPU.

It is recommended that mcelog runs on all x86 machines, both
64bit (since early 2.6) and 32bit (since 2.6.32)

mcelog can run in several modi: cronjob, trigger, daemon

cronjob is the old method. mcelog runs every 5 minutes from cron and checks
for errors. Disadvantage of this is that it can delay error reporting 
significantly (upto 10 minutes) and does not allow mcelog to keep extended state.

trigger is a newer method where the kernel runs mcelog on a error.
This is configured with 
echo /usr/sbin/mcelog > /sys/devices/system/machinecheck/machinecheck0/trigger
This is faster, but still doesn't allow mcelog to keep state,
and has relatively high overhead for each error because a program has
to be initialized from scratch.

In daemon mode mcelog runs continuously as a daemon in the background
and wait for errors. It is enabled by running mcelog --daemon & 
from a init script. This is the fastest and most feature-ful.

The recommended mode is daemon, because several new functions (like page error
predictive failure analysis) require a continuously running daemon.

Documentation:

The primary reference documentation are the man pages.
lk10-mcelog.pdf has a overview over the errors mcelog handles
(originally from Linux Kongress 2010)
mce.pdf is a very old paper describing the first releases of mcelog
(some parts are obsolete)

For distributors:

Please install a init script by default that runs mcelog in daemon mode.
The mcelog.init script is a good starting point.

Also install a logrotated file (mcelog.logrotate) or equivalent 
when mcelog is running in daemon mode. 

These two are not in make install.

The installation also requires a config file (/etc/mcelog.conf) and
the default triggers. These are all installed by "make install"

/dev/mcelog is needed for mcelog operation
If it's not there it can be created with mknod /dev/mcelog c 10 227
Normally it should be created automatically in udev.

Security:

mcelog needs to run as root because it might trigger actions like
page-offlining, which require CAP_SYS_ADMIN. Also it opens /dev/mcelog
and a unix socket for client support.

It also opens /dev/mem to parse the BIOS DMI tables. It is careful
to close the file descriptor and unmap any mappings after using them.

There is support for changing the user in daemon mode after opening
the device and the sockets, but that would stop triggers from
doing corrective action that require root.

In principle it would be possible to only keep CAP_SYS_ADMIN
for page-offling, but that would prevent triggers from doing root
only actions not covered by it (and CAP_SYS_ADMIN is not that different 
from full root)

In daemon mode mcelog listens to a unix socket and processes
requests from mcelog --client. This can be disabled in the configuration file.
The uid/gid of the requestor is checked on access and is configurable
(default 0/0 only). The command parsing code is very straight forward
(server.c) The client parsing/reply is currently done with full privileges
of the daemon.

Testing:

There is a simple test suite in tests/. The test suite requires root to 
run and access to mce-inject and a kernel with MCE injection support 
(CONFIG_X86_MCE_INJECT).  It will kill any running mcelog daemon.

Run it with "make test"

The test suite requires the mce-inject tool, available from
git:https://git.kernel.org/pub/utils/cpu/mce/mce-inject.git
The mce-inject executable must be either in $PATH or in the
../mce-inject directory.

You can also test under valgrind with "make valgrind-test". For 
this valgrind needs to be installed of course.  Advanced
valgrind options can be specified with 
make VALGRIND="valgrind --option" valgrind-test

Other checks:

make iccverify and make clangverify run the static verifiers
in clang and icc respectively.

License:

This program is licensed under the subject of the GNU Public General
License, v.2