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RedHat 9 (Linux i386) - man page for klogd (redhat section 8)

KLOGD(8)			   Linux System Administration				 KLOGD(8)

       klogd - Kernel Log Daemon

       klogd [ -c n ] [ -d ] [ -f fname ] [ -iI ] [ -n ] [ -o ] [ -p ] [ -s ] [ -k fname ] [ -v ]
       [ -x ] [ -2 ]

       klogd is a system daemon which intercepts and logs Linux kernel messages.

       -c n   Sets the default log level of console messages to n.

       -d     Enable debugging mode.  This will generate LOTS of output to stderr.

       -f file
	      Log messages to the specified filename rather than to the syslog facility.

       -i -I  Signal the currently executing klogd daemon.  Both of these  switches  control  the
	      loading/reloading  of  symbol  information.   The  -i  switch signals the daemon to
	      reload the kernel module symbols.  The -I switch signals for a reload of	both  the
	      static kernel symbols and the kernel module symbols.

       -n     Avoid  auto-backgrounding.   This  is needed especially if the klogd is started and
	      controlled by init(8).

       -o     Execute in 'one-shot' mode.  This causes klogd to read and  log  all  the  messages
	      that  are  found	in the kernel message buffers.	After a single read and log cycle
	      the daemon exits.

       -p     Enable paranoia.	This option controls when klogd loads kernel module symbol infor-
	      mation.  Setting this switch causes klogd to load the kernel module symbol informa-
	      tion whenever an Oops string is detected in the kernel message stream.

       -s     Force klogd to use the system call interface to the kernel message buffers.

       -k file
	      Use the specified file as the source of kernel symbol information.

       -v     Print version and exit.

       -x     Omits EIP translation and therefore doesn't read the System.map file.

       -2     When symbols are expanded, print the line twice.	Once with addresses converted  to
	      symbols, once with the raw text.	This allows external programs such as ksymoops do
	      their own processing on the original data.

       The functionality of klogd has been typically incorporated into other versions of  syslogd
       but  this  seems to be a poor place for it.  In the modern Linux kernel a number of kernel
       messaging issues such as sourcing, prioritization and resolution of kernel addresses  must
       be addressed.  Incorporating kernel logging into a separate process offers a cleaner sepa-
       ration of services.

       In Linux there are two potential sources of kernel log information: the /proc file  system
       and  the  syscall  (sys_syslog)	interface, although ultimately they are one and the same.
       Klogd is designed to choose whichever source of information is the most	appropriate.   It
       does  this  by first checking for the presence of a mounted /proc file system.  If this is
       found the /proc/kmsg file is used as the source of kernel log information.   If	the  proc
       file  system  is not mounted klogd uses a system call to obtain kernel messages.  The com-
       mand line switch (-s) can be used to force klogd to use the system call interface  as  its
       messaging source.

       If kernel messages are directed through the syslogd daemon the klogd daemon, as of version
       1.1, has the ability to properly prioritize kernel messages.  Prioritization of the kernel
       messages  was added to it at approximately version 0.99pl13 of the kernel.  The raw kernel
       messages are of the form:

	      <[0-7]>Something said by the kernel.

       The priority of the kernel message is encoded as a single numeric  digit  enclosed  inside
       the  <>	pair.	The  definitions of these values is given in the kernel include file ker-
       nel.h.  When a message is received from the kernel the klogd daemon  reads  this  priority
       level  and  assigns  the appropriate priority level to the syslog message.  If file output
       (-f) is used the prioritization sequence is left pre-pended to the kernel message.

       The klogd daemon also allows the ability to alter the presentation of kernel  messages  to
       the  system console.  Consequent with the prioritization of kernel messages was the inclu-
       sion of default messaging levels for the kernel.  In a stock kernel the the  default  con-
       sole  log  level is set to 7.  Any messages with a priority level numerically lower than 7
       (higher priority) appear on the console.

       Messages of priority level 7 are considered to be  'debug'  messages  and  will	thus  not
       appear  on  the	console.   Many administrators, particularly in a multi-user environment,
       prefer that all kernel messages be handled by klogd and either directed to a  file  or  to
       the  syslogd  daemon.  This prevents 'nuisance' messages such as line printer out of paper
       or disk change detected from cluttering the console.

       When -c is given on the commandline the klogd daemon will execute a system call to inhibit
       all  kernel  messages  from being displayed on the console.  Former versions always issued
       this system call and defaulted to all kernel messages except for panics.  This is  handled
       differently nowardays so klogd doesn't need to set this value anymore.  The argument given
       to the -c switch specifies the priority level of messages which will be	directed  to  the
       console.   Note	that messages of a priority value LOWER than the indicated number will be
       directed to the console.

	      For example, to have the kernel display all messages with a  priority  level  of	3
	      (KERN_ERR) or more severe the following command would be executed:

		   klogd -c 4

       The  definitions  of the numeric values for kernel messages are given in the file kernel.h
       which can be  found  in	the  /usr/include/linux  directory  if	the  kernel  sources  are
       installed.  These values parallel the syslog priority values which are defined in the file
       syslog.h found in the /usr/include/sys sub-directory.

       The klogd daemon can also be used in a 'one-shot' mode for reading the kernel message buf-
       fers.   One shot mode is selected by specifying the -o switch on the command line.  Output
       will be directed to either the syslogd daemon or to an alternate file specified by the  -f

	      For example, to read all the kernel messages after a system boot and record them in
	      a file called krnl.msg the following command would be given.

		   klogd -o -f ./krnl.msg

       If the kernel detects an internal error condition a general protection fault will be trig-
       gered.	As part of the GPF handling procedure the kernel prints out a status report indi-
       cating the state of the processor at the time of the fault.  Included in this display  are
       the  contents  of  the  microprocessor's registers, the contents of the kernel stack and a
       tracing of what functions were being executed at the time of the fault.

       This information is EXTREMELY IMPORTANT in determining what caused the internal error con-
       dition.	 The  difficulty  comes when a kernel developer attempts to analyze this informa-
       tion.  The raw numeric information present in the protection fault  printout  is  of  very
       little  use to the developers.  This is due to the fact that kernels are not identical and
       the addresses of variable locations or functions will not be the same in all kernels.   In
       order  to  correctly  diagnose  the cause of failure a kernel developer needs to know what
       specific kernel functions or variable locations were involved in the error.

       As part of the kernel compilation process a listing is created which specified the address
       locations  of important variables and function in the kernel being compiled.  This listing
       is saved in a file called System.map in the top	of  the  kernel  directory  source  tree.
       Using this listing a kernel developer can determine exactly what the kernel was doing when
       the error condition occurred.

       The process of resolving the numeric addresses from the protection fault printout  can  be
       done manually or by using the ksymoops program which is included in the kernel sources.

       As  a convenience klogd will attempt to resolve kernel numeric addresses to their symbolic
       forms if a kernel symbol table is available at execution time.  If you require the  origi-
       nal  address  of  the symbol, use the -2 switch to preserve the numeric address.  A symbol
       table may be specified by using the -k switch on the command line.  If a  symbol  file  is
       not explicitly specified the following filenames will be tried:


       Version	information  is  supplied  in  the system maps as of kernel 1.3.43.  This version
       information is used to direct an intelligent search of the list of  symbol  tables.   This
       feature is useful since it provides support for both production and experimental kernels.

       For  example  a production kernel may have its map file stored in /boot/System.map.  If an
       experimental or test kernel is compiled with the sources in  the  'standard'  location  of
       /usr/src/linux  the  system  map  will  be found in /usr/src/linux/System.map.  When klogd
       starts under the experimental kernel the map in /boot/System.map will be bypassed in favor
       of the map in /usr/src/linux/System.map.

       Modern  kernels	as of 1.3.43 properly format important kernel addresses so that they will
       be recognized and translated by klogd.  Earlier kernels require a  source  code	patch  be
       applied to the kernel sources.  This patch is supplied with the sysklogd sources.

       The  process  of analyzing kernel protections faults works very well with a static kernel.
       Additional difficulties are encountered when attempting to diagnose errors which occur  in
       loadable kernel modules.  Loadable kernel modules are used to implement kernel functional-
       ity in a form which can be loaded or unloaded at will.  The use	of  loadable  modules  is
       useful from a debugging standpoint and can also be useful in decreasing the amount of mem-
       ory required by a kernel.

       The difficulty with diagnosing errors in loadable modules is due to the dynamic nature  of
       the  kernel  modules.  When a module is loaded the kernel will allocate memory to hold the
       module, when the module is unloaded this memory will be returned back to the kernel.  This
       dynamic	memory	allocation  makes  it  impossible to produce a map file which details the
       addresses of the variable and functions in a kernel loadable module.  Without  this  loca-
       tion  map it is not possible for a kernel developer to determine what went wrong if a pro-
       tection fault involves a kernel module.

       klogd has support for dealing with the problem of diagnosing protection faults  in  kernel
       loadable modules.  At program start time or in response to a signal the daemon will inter-
       rogate the kernel for a listing of all modules loaded and the addresses in memory they are
       loaded at.  Individual modules can also register the locations of important functions when
       the module is loaded.  The addresses of these exported symbols are also determined  during
       this interrogation process.

       When  a	protection  fault occurs an attempt will be made to resolve kernel addresses from
       the static symbol table.  If this fails the symbols from the currently loaded modules  are
       examined in an attempt to resolve the addresses.  At the very minimum this allows klogd to
       indicate which loadable module was responsible for generating the protection fault.  Addi-
       tional  information may be available if the module developer chose to export symbol infor-
       mation from the module.

       Proper and accurate resolution of addresses in  kernel  modules	requires  that	klogd  be
       informed whenever the kernel module status changes.  The -i and -I switches can be used to
       signal the currently executing daemon that symbol information be reloaded.  Of most impor-
       tance  to proper resolution of module symbols is the -i switch.	Each time a kernel module
       is loaded or removed from the kernel the following command should be executed:

       klogd -i

       The -p switch can also be used to insure that module symbol information	is  up	to  date.
       This  switch instructs klogd to reload the module symbol information whenever a protection
       fault is detected.  Caution should be used before invoking the program in 'paranoid' mode.
       The  stability of the kernel and the operating environment is always under question when a
       protection fault occurs.  Since the klogd daemon must execute system  calls  in	order  to
       read  the  module  symbol  information there is the possibility that the system may be too
       unstable to capture useful information.	A much better policy is to insure that	klogd  is
       updated	whenever  a  module  is  loaded  or unloaded.  Having uptodate symbol information
       loaded increases the probability of properly resolving a protection  fault  if  it  should

       Included in the sysklogd source distribution is a patch to the modules-2.0.0 package which
       allows the insmod, rmmod and modprobe utilities to automatically signal klogd  whenever	a
       module is inserted or removed from the kernel.  Using this patch will insure that the sym-
       bol information maintained in klogd is always consistent with the current kernel state.

       The klogd will respond to  eight  signals:  SIGHUP,  SIGINT,  SIGKILL,  SIGTERM,  SIGTSTP,
       SIGUSR1,  SIGUSR2 and SIGCONT.  The SIGINT, SIGKILL, SIGTERM and SIGHUP signals will cause
       the daemon to close its kernel log sources and terminate gracefully.

       The SIGTSTP and SIGCONT signals are used to start and stop kernel logging.   Upon  receipt
       of  a SIGTSTP signal the daemon will close its log sources and spin in an idle loop.  Sub-
       sequent receipt of a SIGCONT signal will cause the daemon to go through its initialization
       sequence and re-choose an input source.	Using SIGSTOP and SIGCONT in combination the ker-
       nel log input can be re-chosen without stopping and restarting the daemon.  For example if
       the /proc file system is to be un-mounted the following command sequence should be used:

	    # kill -TSTP pid
	    # umount /proc
	    # kill -CONT pid

       Notations  will	be  made  in  the  system  logs  with  LOG_INFO  priority documenting the
       start/stop of logging.

       The SIGUSR1 and SIGUSR2 signals are used to initiate loading/reloading  of  kernel  symbol
       information.   Receipt  of  the	SIGUSR1 signal will cause the kernel module symbols to be
       reloaded.  Signaling the daemon with SIGUSR2 will cause both the static kernel symbols and
       the kernel module symbols to be reloaded.

       Provided  that the System.map file is placed in an appropriate location the signal of gen-
       erally greatest usefulness is the SIGUSR1 signal.  This signal is designed to be  used  to
       signal  the  daemon  when  kernel modules are loaded/unloaded.  Sending this signal to the
       daemon after a kernel module state change will insure that proper  resolution  of  symbols
       will occur if a protection fault occurs in the address space occupied by a kernel module.

	      One Source for kernel messages klogd
	      The file containing the process id of klogd
       /boot/System.map, /System.map, /usr/src/linux/System.map
	      Default locations for kernel system maps.

       Probably numerous.  Well formed context diffs appreciated.

       The  klogd was originally written by Steve Lord (lord@cray.com), Greg Wettstein made major

       Dr. Greg Wettstein (greg@wind.enjellic.com)
       Enjellic Systems Development

       Oncology Research Divsion Computing Facility
       Roger Maris Cancer Center
       Fargo, ND 58122

Version 1.4				 21 August, 1999				 KLOGD(8)

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