Adding upstream version 5.10.209.upstream/5.10.209upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+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 :kbd:`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 :kbd:`Alt`, press :kbd:`SysRq`,
+	   release :kbd:`SysRq`, press :kbd:`<command key>`, release everything.
+
+On SPARC
+	You press :kbd:`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 :kbd:`ALT - Print Screen` (or :kbd:`F13`) - :kbd:`<command key>`.
+        :kbd:`Print Screen` (or :kbd:`F13`) - :kbd:`<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
+
+The :kbd:`<command key>` is case sensitive.
+
+What are the 'command' keys?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+=========== ===================================================================
+Command	    Function
+=========== ===================================================================
+``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 the oom killer to kill a memory hog process, but do not
+	    panic if nothing can be killed.
+
+``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.
+            Dump all TLB entries on MIPS.
+
+``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
+   c2 compliant system, and it should not be mistaken as
+   such.
+
+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, it is an equivalent
+of pressing the "reset" button.
+
+``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 handy before yanking removable medium or after using a rescue
+shell that provides no graceful shutdown -- it will ensure your data is
+safely written to the disk. Note that the sync hasn't taken place until you see
+the "OK" and "Done" appear on the screen.
+
+``umount(u)`` can be used to mark filesystems as properly unmounted. From the
+running system's point of view, they will be remounted read-only. The remount
+isn't complete 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 :kbd:`alt-sysrq-z`). Switching to
+another virtual console (:kbd:`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/uapi/linux/input-event-codes.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, const 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, const 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/tty/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 :kbd:`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>