forked from luck/tmp_suning_uos_patched
8eaede49df
Turn the initial value of sysctl kernel.sysrq (SYSRQ_DEFAULT_ENABLE) into a Kconfig variable. Original version by Bastian Blank <waldi@debian.org>. Signed-off-by: Ben Hutchings <ben@decadent.org.uk> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
256 lines
12 KiB
Plaintext
256 lines
12 KiB
Plaintext
Linux Magic System Request Key Hacks
|
|
Documentation for sysrq.c
|
|
|
|
* What is the magic SysRq key?
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
It is a 'magical' key combo you can hit which the kernel will respond to
|
|
regardless of whatever else it is doing, unless it is completely locked up.
|
|
|
|
* How do I enable the magic SysRq key?
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
|
|
configuring the kernel. When running a kernel with SysRq compiled in,
|
|
/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
|
|
the SysRq key. The default value in this file is set by the
|
|
CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults
|
|
to 1. Here is the list of possible values in /proc/sys/kernel/sysrq:
|
|
0 - disable sysrq completely
|
|
1 - enable all functions of sysrq
|
|
>1 - bitmask of allowed sysrq functions (see below for detailed function
|
|
description):
|
|
2 = 0x2 - enable control of console logging level
|
|
4 = 0x4 - enable control of keyboard (SAK, unraw)
|
|
8 = 0x8 - enable debugging dumps of processes etc.
|
|
16 = 0x10 - enable sync command
|
|
32 = 0x20 - enable remount read-only
|
|
64 = 0x40 - enable signalling of processes (term, kill, oom-kill)
|
|
128 = 0x80 - allow reboot/poweroff
|
|
256 = 0x100 - allow nicing of all RT tasks
|
|
|
|
You can set the value in the file by the following command:
|
|
echo "number" >/proc/sys/kernel/sysrq
|
|
|
|
The number may be written here either as decimal or as hexadecimal
|
|
with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE must always be
|
|
written in hexadecimal.
|
|
|
|
Note that the value of /proc/sys/kernel/sysrq influences only the invocation
|
|
via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always
|
|
allowed (by a user with admin privileges).
|
|
|
|
* How do I use the magic SysRq key?
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
On x86 - You press the key combo 'ALT-SysRq-<command key>'. Note - Some
|
|
keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
|
|
also known as the 'Print Screen' key. Also some keyboards cannot
|
|
handle so many keys being pressed at the same time, so you might
|
|
have better luck with "press Alt", "press SysRq", "release SysRq",
|
|
"press <command key>", release everything.
|
|
|
|
On SPARC - You press 'ALT-STOP-<command key>', I believe.
|
|
|
|
On the serial console (PC style standard serial ports only) -
|
|
You send a BREAK, then within 5 seconds a command key. Sending
|
|
BREAK twice is interpreted as a normal BREAK.
|
|
|
|
On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,
|
|
Print Screen (or F13) - <command key> may suffice.
|
|
|
|
On other - If you know of the key combos for other architectures, please
|
|
let me know so I can add them to this section.
|
|
|
|
On all - write a character to /proc/sysrq-trigger. e.g.:
|
|
|
|
echo t > /proc/sysrq-trigger
|
|
|
|
* What are the 'command' keys?
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
'b' - Will immediately reboot the system without syncing or unmounting
|
|
your disks.
|
|
|
|
'c' - Will perform a system crash by a NULL pointer dereference.
|
|
A crashdump will be taken if configured.
|
|
|
|
'd' - Shows all locks that are held.
|
|
|
|
'e' - Send a SIGTERM to all processes, except for init.
|
|
|
|
'f' - Will call oom_kill to kill a memory hog process.
|
|
|
|
'g' - Used by kgdb (kernel debugger)
|
|
|
|
'h' - Will display help (actually any other key than those listed
|
|
here will display help. but 'h' is easy to remember :-)
|
|
|
|
'i' - Send a SIGKILL to all processes, except for init.
|
|
|
|
'j' - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
|
|
|
|
'k' - Secure Access Key (SAK) Kills all programs on the current virtual
|
|
console. NOTE: See important comments below in SAK section.
|
|
|
|
'l' - Shows a stack backtrace for all active CPUs.
|
|
|
|
'm' - Will dump current memory info to your console.
|
|
|
|
'n' - Used to make RT tasks nice-able
|
|
|
|
'o' - Will shut your system off (if configured and supported).
|
|
|
|
'p' - Will dump the current registers and flags to your console.
|
|
|
|
'q' - Will dump per CPU lists of all armed hrtimers (but NOT regular
|
|
timer_list timers) and detailed information about all
|
|
clockevent devices.
|
|
|
|
'r' - Turns off keyboard raw mode and sets it to XLATE.
|
|
|
|
's' - Will attempt to sync all mounted filesystems.
|
|
|
|
't' - Will dump a list of current tasks and their information to your
|
|
console.
|
|
|
|
'u' - Will attempt to remount all mounted filesystems read-only.
|
|
|
|
'v' - Forcefully restores framebuffer console
|
|
'v' - Causes ETM buffer dump [ARM-specific]
|
|
|
|
'w' - Dumps tasks that are in uninterruptable (blocked) state.
|
|
|
|
'x' - Used by xmon interface on ppc/powerpc platforms.
|
|
Show global PMU Registers on sparc64.
|
|
|
|
'y' - Show global CPU Registers [SPARC-64 specific]
|
|
|
|
'z' - Dump the ftrace buffer
|
|
|
|
'0'-'9' - Sets the console log level, controlling which kernel messages
|
|
will be printed to your console. ('0', for example would make
|
|
it so that only emergency messages like PANICs or OOPSes would
|
|
make it to your console.)
|
|
|
|
* Okay, so what can I use them for?
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
Well, unraw(r) is very handy when your X server or a svgalib program crashes.
|
|
|
|
sak(k) (Secure Access Key) is useful when you want to be sure there is no
|
|
trojan program running at console which could grab your password
|
|
when you would try to login. It will kill all programs on given console,
|
|
thus letting you make sure that the login prompt you see is actually
|
|
the one from init, not some trojan program.
|
|
IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
|
|
IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT
|
|
IMPORTANT: such. :IMPORTANT
|
|
It seems others find it useful as (System Attention Key) which is
|
|
useful when you want to exit a program that will not let you switch consoles.
|
|
(For example, X or a svgalib program.)
|
|
|
|
reboot(b) is good when you're unable to shut down. But you should also
|
|
sync(s) and umount(u) first.
|
|
|
|
crash(c) can be used to manually trigger a crashdump when the system is hung.
|
|
Note that this just triggers a crash if there is no dump mechanism available.
|
|
|
|
sync(s) is great when your system is locked up, it allows you to sync your
|
|
disks and will certainly lessen the chance of data loss and fscking. Note
|
|
that the sync hasn't taken place until you see the "OK" and "Done" appear
|
|
on the screen. (If the kernel is really in strife, you may not ever get the
|
|
OK or Done message...)
|
|
|
|
umount(u) is basically useful in the same ways as sync(s). I generally sync(s),
|
|
umount(u), then reboot(b) when my system locks. It's saved me many a fsck.
|
|
Again, the unmount (remount read-only) hasn't taken place until you see the
|
|
"OK" and "Done" message appear on the screen.
|
|
|
|
The loglevels '0'-'9' are useful when your console is being flooded with
|
|
kernel messages you do not want to see. Selecting '0' will prevent all but
|
|
the most urgent kernel messages from reaching your console. (They will
|
|
still be logged if syslogd/klogd are alive, though.)
|
|
|
|
term(e) and kill(i) are useful if you have some sort of runaway process you
|
|
are unable to kill any other way, especially if it's spawning other
|
|
processes.
|
|
|
|
"just thaw it(j)" is useful if your system becomes unresponsive due to a frozen
|
|
(probably root) filesystem via the FIFREEZE ioctl.
|
|
|
|
* Sometimes SysRq seems to get 'stuck' after using it, what can I do?
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
That happens to me, also. I've found that tapping shift, alt, and control
|
|
on both sides of the keyboard, and hitting an invalid sysrq sequence again
|
|
will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
|
|
virtual console (ALT+Fn) and then back again should also help.
|
|
|
|
* I hit SysRq, but nothing seems to happen, what's wrong?
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
There are some keyboards that produce a different keycode for SysRq than the
|
|
pre-defined value of 99 (see KEY_SYSRQ in include/linux/input.h), or which
|
|
don't have a SysRq key at all. In these cases, run 'showkey -s' to find an
|
|
appropriate scancode sequence, and use 'setkeycodes <sequence> 99' to map
|
|
this sequence to the usual SysRq code (e.g., 'setkeycodes e05b 99'). It's
|
|
probably best to put this command in a boot script. Oh, and by the way, you
|
|
exit 'showkey' by not typing anything for ten seconds.
|
|
|
|
* I want to add SysRQ key events to a module, how does it work?
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
In order to register a basic function with the table, you must first include
|
|
the header 'include/linux/sysrq.h', this will define everything else you need.
|
|
Next, you must create a sysrq_key_op struct, and populate it with A) the key
|
|
handler function you will use, B) a help_msg string, that will print when SysRQ
|
|
prints help, and C) an action_msg string, that will print right before your
|
|
handler is called. Your handler must conform to the prototype in 'sysrq.h'.
|
|
|
|
After the sysrq_key_op is created, you can call the kernel function
|
|
register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
|
|
register the operation pointed to by 'op_p' at table key 'key',
|
|
if that slot in the table is blank. At module unload time, you must call
|
|
the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
|
|
will remove the key op pointed to by 'op_p' from the key 'key', if and only if
|
|
it is currently registered in that slot. This is in case the slot has been
|
|
overwritten since you registered it.
|
|
|
|
The Magic SysRQ system works by registering key operations against a key op
|
|
lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
|
|
a number of operations registered into it at compile time, but is mutable,
|
|
and 2 functions are exported for interface to it:
|
|
register_sysrq_key and unregister_sysrq_key.
|
|
Of course, never ever leave an invalid pointer in the table. I.e., when
|
|
your module that called register_sysrq_key() exits, it must call
|
|
unregister_sysrq_key() to clean up the sysrq key table entry that it used.
|
|
Null pointers in the table are always safe. :)
|
|
|
|
If for some reason you feel the need to call the handle_sysrq function from
|
|
within a function called by handle_sysrq, you must be aware that you are in
|
|
a lock (you are also in an interrupt handler, which means don't sleep!), so
|
|
you must call __handle_sysrq_nolock instead.
|
|
|
|
* When I hit a SysRq key combination only the header appears on the console?
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
Sysrq output is subject to the same console loglevel control as all
|
|
other console output. This means that if the kernel was booted 'quiet'
|
|
as is common on distro kernels the output may not appear on the actual
|
|
console, even though it will appear in the dmesg buffer, and be accessible
|
|
via the dmesg command and to the consumers of /proc/kmsg. As a specific
|
|
exception the header line from the sysrq command is passed to all console
|
|
consumers as if the current loglevel was maximum. If only the header
|
|
is emitted it is almost certain that the kernel loglevel is too low.
|
|
Should you require the output on the console channel then you will need
|
|
to temporarily up the console loglevel using alt-sysrq-8 or:
|
|
|
|
echo 8 > /proc/sysrq-trigger
|
|
|
|
Remember to return the loglevel to normal after triggering the sysrq
|
|
command you are interested in.
|
|
|
|
* I have more questions, who can I ask?
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
Just ask them on the linux-kernel mailing list:
|
|
linux-kernel@vger.kernel.org
|
|
|
|
* Credits
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
Written by Mydraal <vulpyne@vulpyne.net>
|
|
Updated by Adam Sulmicki <adam@cfar.umd.edu>
|
|
Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
|
|
Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>
|