Linux Magic System Request Key Hacks
-Documentation for sysrq.c version 1.15
-Last update: $Date: 2001/01/28 10:15:59 $
+Documentation for sysrq.c
+Last update: 2007-JAN-06
* What is the magic SysRq key?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* 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 on a kernel with SysRq compiled in, it
-may be DISABLED at run-time using following command:
-
- echo "0" > /proc/sys/kernel/sysrq
-
-Note that previous versions disabled sysrq by default, and you were required
-to specifically enable it at run-time. That is not the case any longer.
+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. By default the file contains 1 which means that every
+possible SysRq request is allowed (in older versions SysRq was disabled
+by default, and you were required to specifically enable it at run-time
+but this is not the case any more). 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 - enable control of console logging level
+ 4 - enable control of keyboard (SAK, unraw)
+ 8 - enable debugging dumps of processes etc.
+ 16 - enable sync command
+ 32 - enable remount read-only
+ 64 - enable signalling of processes (term, kill, oom-kill)
+ 128 - allow reboot/poweroff
+ 256 - allow nicing of all RT tasks
+
+You can set the value in the file by the following command:
+ echo "number" >/proc/sys/kernel/sysrq
+
+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 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. eg:
+On all - write a character to /proc/sysrq-trigger. e.g.:
echo t > /proc/sysrq-trigger
'b' - Will immediately reboot the system without syncing or unmounting
your disks.
+'c' - Will perform a kexec reboot in order to take a crashdump.
+
+'d' - Shows all locks that are held.
+
'o' - Will shut your system off (if configured and supported).
's' - Will attempt to sync all mounted filesystems.
'm' - Will dump current memory info to your console.
+'n' - Used to make RT tasks nice-able
+
'v' - Dumps Voyager SMP processor info to your console.
+'w' - Dumps tasks that are in uninterruptable (blocked) state.
+
+'x' - Used by xmon interface on ppc/powerpc platforms.
+
'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.)
+'f' - Will call oom_kill to kill a memory hog process.
+
'e' - Send a SIGTERM to all processes, except for init.
-'i' - Send a SIGKILL to all processes, except for init.
+'g' - Used by kgdb on ppc platforms.
-'l' - Send a SIGKILL to all processes, INCLUDING init. (Your system
- will be non-functional after this.)
+'i' - Send a SIGKILL to all processes, except for init.
-'h' - Will display help ( actually any other key than those listed
+'h' - Will display help (actually any other key than those listed
above will display help. but 'h' is easy to remember :-)
* Okay, so what can I use them for?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Well, un'R'aw is very handy when your X server or a svgalib program crashes.
-sa'K' (Secure Access Key) is useful when you want to be sure there are no
-trojan program is running at console and which could grab your password
-when you would try to login. It will kill all programs on given console
-and thus letting you make sure that the login prompt you see is actually
+sa'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 :IMPORTANT
-IMPORTANT:c2 compliant systems, and it should be mistook as such. :IMPORTANT
- It seems other find it useful as (System Attention Key) which is
+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.)
re'B'oot is good when you're unable to shut down. But you should also 'S'ync
and 'U'mount first.
+'C'rashdump can be used to manually trigger a crashdump when the system is hung.
+The kernel needs to have been built with CONFIG_KEXEC enabled.
+
'S'ync 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
Again, the unmount (remount read-only) hasn't taken place until you see the
"OK" and "Done" message appear on the screen.
-The loglevel'0'-'9' is useful when your console is being flooded with
-kernel messages you do not want to see. Setting '0' will prevent all but
+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.)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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. (ie, something like alt-sysrq-z). Switching to another
+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?
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 protoype in 'sysrq.h'.
+handler is called. Your handler must conform to the prototype in 'sysrq.h'.
-After the sysrq_key_op is created, you can call the macro
-register_sysrq_key(int key, struct sysrq_key_op *op_p) that is defined in
-sysrq.h, 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 macro unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
+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 4 functions are exported for interface to it: __sysrq_lock_table,
-__sysrq_unlock_table, __sysrq_get_key_op, and __sysrq_put_key_op. The
-functions __sysrq_swap_key_ops and __sysrq_swap_key_ops_nolock are defined
-in the header itself, and the REGISTER and UNREGISTER macros are built from
-these. More complex (and dangerous!) manipulations of the table are possible
-using these functions, but you must be careful to always lock the table before
-you read or write from it, and to unlock it again when you are done. (And of
-course, to never ever leave an invalid pointer in the table). Null pointers in
-the table are always safe :)
+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
* I have more questions, who can I ask?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-You may feel free to send email to myrdraal@deathsdoor.com, and I will
-respond as soon as possible.
- -Myrdraal
-
And I'll answer any questions about the registration system you got, also
responding as soon as possible.
-Crutcher
* Credits
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Written by Mydraal <myrdraal@deathsdoor.com>
+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>
git://git.onelab.eu / linux-2.6.git / blobdiff