mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-26 10:49:33 +00:00
![James Byrne](/assets/img/avatar_default.png)
Commit 5aa068ea40
("printk: remove games with previous record flags")
abolished the practice of setting the log flag to 'c' for the first
continuation line and '+' for subsequent lines. Now all continuation
lines are flagged with 'c' and '+' is never used.
Update the 'dev-kmsg' documentation to remove the reference to the
obsolete '+' flag. In addition, state explicitly that only 8 bits of the
<N> syslog prefix are used for the facility number when writing to
/dev/kmsg.
Link: http://lkml.kernel.org/r/0102016cf1b26630-8e9b337b-da49-43c6-b028-4250c2fac3ef-000000@eu-west-1.amazonses.com
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: James Byrne <james.byrne@origamienergy.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
109 lines
4.5 KiB
Text
109 lines
4.5 KiB
Text
What: /dev/kmsg
|
|
Date: Mai 2012
|
|
KernelVersion: 3.5
|
|
Contact: Kay Sievers <kay@vrfy.org>
|
|
Description: The /dev/kmsg character device node provides userspace access
|
|
to the kernel's printk buffer.
|
|
|
|
Injecting messages:
|
|
Every write() to the opened device node places a log entry in
|
|
the kernel's printk buffer.
|
|
|
|
The logged line can be prefixed with a <N> syslog prefix, which
|
|
carries the syslog priority and facility. The single decimal
|
|
prefix number is composed of the 3 lowest bits being the syslog
|
|
priority and the next 8 bits the syslog facility number.
|
|
|
|
If no prefix is given, the priority number is the default kernel
|
|
log priority and the facility number is set to LOG_USER (1). It
|
|
is not possible to inject messages from userspace with the
|
|
facility number LOG_KERN (0), to make sure that the origin of
|
|
the messages can always be reliably determined.
|
|
|
|
Accessing the buffer:
|
|
Every read() from the opened device node receives one record
|
|
of the kernel's printk buffer.
|
|
|
|
The first read() directly following an open() always returns
|
|
first message in the buffer; there is no kernel-internal
|
|
persistent state; many readers can concurrently open the device
|
|
and read from it, without affecting other readers.
|
|
|
|
Every read() will receive the next available record. If no more
|
|
records are available read() will block, or if O_NONBLOCK is
|
|
used -EAGAIN returned.
|
|
|
|
Messages in the record ring buffer get overwritten as whole,
|
|
there are never partial messages received by read().
|
|
|
|
In case messages get overwritten in the circular buffer while
|
|
the device is kept open, the next read() will return -EPIPE,
|
|
and the seek position be updated to the next available record.
|
|
Subsequent reads() will return available records again.
|
|
|
|
Unlike the classic syslog() interface, the 64 bit record
|
|
sequence numbers allow to calculate the amount of lost
|
|
messages, in case the buffer gets overwritten. And they allow
|
|
to reconnect to the buffer and reconstruct the read position
|
|
if needed, without limiting the interface to a single reader.
|
|
|
|
The device supports seek with the following parameters:
|
|
SEEK_SET, 0
|
|
seek to the first entry in the buffer
|
|
SEEK_END, 0
|
|
seek after the last entry in the buffer
|
|
SEEK_DATA, 0
|
|
seek after the last record available at the time
|
|
the last SYSLOG_ACTION_CLEAR was issued.
|
|
|
|
The output format consists of a prefix carrying the syslog
|
|
prefix including priority and facility, the 64 bit message
|
|
sequence number and the monotonic timestamp in microseconds,
|
|
and a flag field. All fields are separated by a ','.
|
|
|
|
Future extensions might add more comma separated values before
|
|
the terminating ';'. Unknown fields and values should be
|
|
gracefully ignored.
|
|
|
|
The human readable text string starts directly after the ';'
|
|
and is terminated by a '\n'. Untrusted values derived from
|
|
hardware or other facilities are printed, therefore
|
|
all non-printable characters and '\' itself in the log message
|
|
are escaped by "\x00" C-style hex encoding.
|
|
|
|
A line starting with ' ', is a continuation line, adding
|
|
key/value pairs to the log message, which provide the machine
|
|
readable context of the message, for reliable processing in
|
|
userspace.
|
|
|
|
Example:
|
|
7,160,424069,-;pci_root PNP0A03:00: host bridge window [io 0x0000-0x0cf7] (ignored)
|
|
SUBSYSTEM=acpi
|
|
DEVICE=+acpi:PNP0A03:00
|
|
6,339,5140900,-;NET: Registered protocol family 10
|
|
30,340,5690716,-;udevd[80]: starting version 181
|
|
|
|
The DEVICE= key uniquely identifies devices the following way:
|
|
b12:8 - block dev_t
|
|
c127:3 - char dev_t
|
|
n8 - netdev ifindex
|
|
+sound:card0 - subsystem:devname
|
|
|
|
The flags field carries '-' by default. A 'c' indicates a
|
|
fragment of a line. Note, that these hints about continuation
|
|
lines are not necessarily correct, and the stream could be
|
|
interleaved with unrelated messages, but merging the lines in
|
|
the output usually produces better human readable results. A
|
|
similar logic is used internally when messages are printed to
|
|
the console, /proc/kmsg or the syslog() syscall.
|
|
|
|
By default, kernel tries to avoid fragments by concatenating
|
|
when it can and fragments are rare; however, when extended
|
|
console support is enabled, the in-kernel concatenation is
|
|
disabled and /dev/kmsg output will contain more fragments. If
|
|
the log consumer performs concatenation, the end result
|
|
should be the same. In the future, the in-kernel concatenation
|
|
may be removed entirely and /dev/kmsg users are recommended to
|
|
implement fragment handling.
|
|
|
|
Users: dmesg(1), userspace kernel log consumers
|