625d344978
This reverts commit 8ece3b3eb5.
This commit broke userspace. Bash uses ESPIPE to determine whether or
not the file should be read using "unbuffered I/O", which means reading
1 byte at a time instead of 128 bytes at a time. I used to use bash to
read through kmsg in a really quite nasty way:
while read -t 0.1 -r line 2>/dev/null || [[ $? -ne 142 ]]; do
echo "SARU $line"
done < /dev/kmsg
This will show all lines that can fit into the 128 byte buffer, and skip
lines that don't. That's pretty awful, but at least it worked.
With this change, bash now tries to do 1-byte reads, which means it
skips all the lines, which is worse than before.
Now, I don't really care very much about this, and I'm already look for
a workaround. But I did just spend an hour trying to figure out why my
scripts were broken. Either way, it makes no difference to me personally
whether this is reverted, but it might be something to consider. If you
declare that "trying to read /dev/kmsg with bash is terminally stupid
anyway," I might be inclined to agree with you. But do note that bash
uses lseek(fd, 0, SEEK_CUR)==>ESPIPE to determine whether or not it's
reading from a pipe.
Cc: Bruno Meneguele <bmeneg@redhat.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Petr Mladek <pmladek@suse.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
110 lines
4.5 KiB
Plaintext
110 lines
4.5 KiB
Plaintext
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
|