klogd - Kernel Log Daemon
klogd [ -c n ] [ -d ] [ -f fname ] [ -iI ] [ -n ] [ -o ] [ -p ] [ -P
path ] [ -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.
-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.
Use path instead of /proc/kmsg as the source of the kernel
message. Specify "-" to read from standard input. This allows
klogd to run entirely without root privileges.
-s Force klogd to use the system call interface to the kernel
Use the specified file as the source of kernel symbol
-v Print version and exit.
-x Omits EIP translation and therefore doesn’t 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
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 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. If the -P
switch is used, klogd opens the specified path as the source of kernel
log information. Otherwise klogd checks for the presence of a mounted
/proc file system and 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 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 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 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.
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 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 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 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. 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
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:
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 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.
CONSOLE LOG LEVEL
The klogd daemon 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 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
currently 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
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 console log level is usually configured with the sysctl(8) program,
directly or via its configuration file /etc/sysctl.conf. In this file
the following line
kernel.printk = 4 4 1 7
corresponds to the sampe setting above.
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. 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:
# 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 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 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 kernel log daemon klogd was originally written by Steve Lord
<firstname.lastname@example.org>, Greg Wettstein made major improvements. Martin
Schulze <email@example.com> fixed some bugs and took over maintenance.