klogd - Unix, Linux Command
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
-c n ||
Sets the default log level of console messages to n.
Enable debugging mode. This will generate LOTS of output to
-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.
Avoid auto-backgrounding. This is needed especially if the
klogd is started and controlled by
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.
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
Force klogd to use the system call interface to the kernel message
-k file ||
Use the specified file as the source of kernel symbol information.
Print version and exit.
Omits EIP translation and therefore doesnt read the System.map file.
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
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 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
system call to obtain kernel messages. The command line switch
(-s) can be used to force klogd to use the system call interface as its
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:
lt;[0-7]gt;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
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 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
-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
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
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:
The definitions of the numeric values for kernel messages are given in
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 buffers. 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 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.
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 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 original 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 functionality 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 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
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 location map it is not possible for a
kernel developer to determine what went wrong if a protection 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 interrogate 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
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. 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
klogd be informed whenever the kernel module status changes. The
-I switches can be used to signal the currently executing daemon that
symbol information be reloaded. Of most importance 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:
-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 occur.
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 symbol information maintained in klogd is
always consistent with the current kernel state.
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
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. Subsequent 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 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:
Notations will be made in the system logs with
documenting the start/stop of logging.
# kill -TSTP pid
# umount /proc
# kill -CONT pid
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
SIGUSR2 will cause both the static kernel symbols and the kernel module symbols to
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 symbols will
occur if a protection fault occurs in the address space occupied by a kernel
One Source for kernel messages
The file containing the process id of
/boot/System.map, /System.map, /usr/src/linux/System.map |
Default locations for kernel system maps.
Probably numerous. Well formed context diffs appreciated.
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