proc.5: Add description of /proc/sys/kernel/sysrq

Reported by: Goerghe Cosorea <gheorghe.coserea@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man5/proc.5

@@ -58,7 +58,7 @@
 .\" to see what information could be imported from that file
 .\" into this file.
 .\"
-.TH PROC 5 2009-02-03 "Linux" "Linux Programmer's Manual"
+.TH PROC 5 2009-02-12 "Linux" "Linux Programmer's Manual"
 .SH NAME
 proc \- process information pseudo-file system
 .SH DESCRIPTION
@@ -2291,6 +2291,33 @@ This file
 specifies the system-wide maximum number of System V shared memory
 segments that can be created.
 .TP
+.I /proc/sys/kernel/sysrq
+This file controls the functions allowed to be invoked by the SysRq key.
+By default,
+the file contains 1 meaning that every possible SysRq request is allowed
+(in older kernel versionsm, SysRq was disabled by default,
+and you were required to specifically enable it at run-time,
+but this is not the case any more).
+Possible values in this file are:
+
+   0 - disable sysrq completely
+   1 - enable all functions of sysrq
+  >1 - bitmask of allowed sysrq functions, as follows:
+          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 real-time tasks
+
+This file is only present if the
+.B CONFIG_MAGIG_SYSRQ
+kernel configuration option is enabled.
+For further details see the kernel source file
+.IR Documentation/sysrq.txt .
+.TP
 .I /proc/sys/kernel/version
 This file contains a string like: