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 mes
-c n Sets the default log level of console messages to n.
-d Enable debugging mode. This will generate LOTS of output to
Log messages to the specified filename rather than to the syslog
-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 information. Setting this switch causes klogd to
load the kernel module symbol information whenever an Oops
string is detected in the kernel message stream.
-s Force klogd to use the system call interface to the kernel mes
Use the specified file as the source of kernel symbol informa
-v Print version and exit.
-x Omits EIP translation and therefore doesnt read the System.map
-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 process
ing 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 sourc
ing, prioritization and resolution of kernel addresses must be
addressed. Incorporating kernel logging into a separate process offers
a cleaner separation 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 sys
tem call to obtain kernel messages. The command line switch (-s) can
be used to force klogd to use the system call interface as its messag
If kernel messages are directed through the syslogd daemon the klogd
daemon, as of version 1.1, has the ability to properly prioritize ker
nel 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 kernel.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
The klogd daemon also allows the ability to alter the presentation of
kernel messages to the system console. Consequent with the prioritiza
tion of kernel messages was the inclusion of default messaging levels
for the kernel. In a stock kernel the the default console 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 doesnt need to set this value anymore. The argu
ment given to the -c switch specifies the priority level of messages
which will be directed to the console. Note that messages of a prior
ity value LOWER than the indicated number will be directed to the con
For example, to have the kernel display all messages with a pri
ority 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 direc
tory 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 buffers. One shot mode is selected by specifying the -o
switch on the command line. Output will be directed to either the sys
logd daemon or to an alternate file specified by the -f switch.
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
KERNEL ADDRESS RESOLUTION
If the kernel detects an internal error condition a general protection
fault will be triggered. As part of the GPF handling procedure the
kernel prints out a status report indicating the state of the processor
at the time of the fault. Included in this display are the contents of
the microprocessors 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 condition. The difficulty comes when a kernel developer
attempts to analyze this information. 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 vari
able 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 Sys
tem.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 execu
tion time. If you require the original address of the symbol, use the
-2 switch to preserve the numeric address. A symbol table may be spec
ified 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 sup
port 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 attempt
ing to diagnose errors which occur in loadable kernel modules. Load
able kernel modules are used to implement kernel functionality in a
form which can be loaded or unloaded at will. The use of loadable mod
ules is useful from a debugging standpoint and can also be useful in
decreasing the amount of memory 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 ker
nel 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 load
able module. Without this location map it is not possible for a kernel
developer to determine what went wrong if a protection fault involves a
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 interrogate the kernel for a list
ing 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 ker
nel 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.
Additional information may be available if the module developer chose
to export symbol information 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 importance to proper res
olution of module symbols is the -i switch. Each time a kernel module
is loaded or removed from the kernel the following command should be
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 pos
sibility that the system may be too unstable to capture useful informa
tion. 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 occur.
Included in the sysklogd source distribution is a patch to the mod
ules-2.0.0 package which allows the insmod, rmmod and modprobe utili
ties to automatically signal klogd whenever a module is inserted or
removed from the kernel. Using this patch will insure that the symbol
information maintained in klogd is always consistent with the current
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 log
ging. Upon receipt of a SIGTSTP signal the daemon will close its log
sources and spin in an idle loop. Subsequent receipt of a SIGCONT sig
nal will cause the daemon to go through its initialization sequence and
re-choose an input source. Using SIGSTOP and SIGCONT in combination
the kernel 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 docu
menting 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 generally 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 sym
bols will occur if a protection fault occurs in the address space occu
pied 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 (email@example.com), Greg
Wettstein made major improvements.
Dr. Greg Wettstein (firstname.lastname@example.org)
Enjellic Systems Development
Oncology Research Divsion Computing Facility
Roger Maris Cancer Center
Fargo, ND 58122
Version 1.4 21 August, 1999 KLOGD(8)