klogd.8

linux下记录系统日志代码以及记录内核日志代码

.\" Copyright 1994 Dr. Greg Wettstein, Enjellic Systems Development.
.\" May be distributed under the GNU General Public License
.\" Sun Jul 30 01:35:55 MET: Martin Schulze: Updates
.\" Sun Nov 19 23:22:21 MET: Martin Schulze: Updates
.\" Mon Aug 19 09:42:08 CDT 1996: Dr. G.W. Wettstein: Updates
.\"
.TH KLOGD 8 "21 August, 1999" "Version 1.4" "Linux System Administration"
.SH NAME
klogd \- Kernel Log Daemon
.LP
.SH SYNOPSIS
.B klogd
.RB [ " \-c "
.I n
]
.RB [ " \-d " ]
.RB [ " \-f "
.I fname
]
.RB [ " \-iI " ]
.RB [ " \-n " ]
.RB [ " \-o " ]
.RB [ " \-p " ]
.RB [ " \-s " ]
.RB [ " \-k "
.I fname
]
.RB [ " \-v " ]
.RB [ " \-x " ]
.RB [ " \-2 " ]
.LP
.SH DESCRIPTION
.B klogd
is a system daemon which intercepts and logs Linux kernel
messages.
.LP
.SH OPTIONS
.TP
.BI "\-c " n
Sets the default log level of console messages to \fIn\fR.
.TP
.B "\-d"
Enable debugging mode.  This will generate \fBLOTS\fR of output to
stderr.
.TP
.BI "\-f " file
Log messages to the specified filename rather than to the syslog facility.
.TP
.BI "\-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.
.TP
.B "\-n"
Avoid auto-backgrounding.  This is needed especially if the
.B klogd
is started and controlled by 
.BR init (8).
.TP
.B "-o"
Execute in 'one\-shot' mode.  This causes \fBklogd\fP 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.
.TP
.B "-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.
.TP
.B "-s"
Force \fBklogd\fP to use the system call interface to the kernel message
buffers.
.TP
.BI "\-k " file
Use the specified file as the source of kernel symbol information.
.TP
.B "\-v"
Print version and exit.
.TP
.B "\-x"
Omits EIP translation and therefore doesn't read the System.map file.
.TP
.B "\-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.
.LP
.SH 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 separation of services.

In Linux there are two potential sources of kernel log information: the 
.I /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 
.I /proc
file system.  If this is found the 
.I /proc/kmsg
file is used as the source of kernel log
information.  If the proc file system is not mounted 
.B klogd
uses a
system call to obtain kernel messages.  The command line switch
.RB ( "\-s" )
can be used to force klogd to use the system call interface as its
messaging source.

If kernel messages are directed through the 
.BR 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:
.IP
\<[0\-7]\>Something said by the kernel.
.PP
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
(\fB-f\fR) is used the prioritization sequence is left pre\-pended to the
kernel message.

The
.B 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
console.

When
.B \-c
is given on the commandline the
.B 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
.B klogd
doesn't need to set this value anymore.  The
argument given to the \fB\-c\fR 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.
.IP
For example, to have the kernel display all messages with a
priority level of 3
.BR "" ( KERN_ERR )
or more severe the following
command would be executed:
.IP
.nf
	klogd \-c 4
.fi
.PP
The definitions of the numeric values for kernel messages are given in
the file 
.IR 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 
.IR 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
\fB\-o\fR switch on the command line.  Output will be directed to either the
syslogd daemon or to an alternate file specified by the \fB-f\fR switch.
.IP
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.
.IP
.nf
	klogd -o -f ./krnl.msg
.fi
.PP
.SH 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
.B 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