KLOGD(8) Linux System Administration KLOGD(8)
NAME
klogd - Kernel Log Daemon
SYNOPSIS
klogd [ -c n ] [ -d ] [ -f fname ] [ -iI ] [ -n ] [ -o ] [ -p ] [ -s ] [ -k fname ] [ -v ]
[ -x ] [ -2 ]
DESCRIPTION
klogd is a system daemon which intercepts and logs Linux kernel messages.
OPTIONS
-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.
OVERVIEW
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
buffers. One shot mode is selected by specifying the -o switch on the command line. Out-
put 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 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:
/boot/System.map
/System.map
/usr/src/linux/System.map
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
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 sym-
bol information maintained in klogd is always consistent with the current kernel state.
SIGNAL HANDLING
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.
FILES
/proc/kmsg
One Source for kernel messages klogd
/var/run/klogd.pid
The file containing the process id of klogd
/boot/System.map, /System.map, /usr/src/linux/System.map
Default locations for kernel system maps.
BUGS
Probably numerous. Well formed context diffs appreciated.
AUTHOR
The klogd was originally written by Steve Lord (), Greg Wettstein made major
improvements.
Dr. Greg Wettstein ()
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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