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Message-ID: <1336495850.1179.41.camel@mop> Date: Tue, 08 May 2012 18:50:50 +0200 From: Kay Sievers <kay@...y.org> To: Greg Kroah-Hartmann <greg@...ah.com> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>, Karel Zak <kzak@...hat.com> Subject: Re: [PATCH RESEND 2/3] kmsg: export printk records to the /dev/kmsg interface On Tue, May 8, 2012 at 2:28 AM, Greg Kroah-Hartmann <greg@...ah.com> wrote: > Can you add a file somwhere in Documentation (Documentatin/ABI?) that > documents the file format for this file? Something like that? Please let me know, what else might be useful here. Thanks, Kay From: Kay Sievers <kay@...y.org> Subject: kmsg - add Documentation/ABI/testing/dev-kmsg Signed-off-by: Kay Sievers <kay@...y.org> --- Documentation/ABI/testing/dev-kmsg | 90 +++++++++++++++++++++++++++++++++++++ Documentation/devices.txt | 3 - 2 files changed, 92 insertions(+), 1 deletion(-) --- /dev/null +++ b/Documentation/ABI/testing/dev-kmsg @@ -0,0 +1,90 @@ +What: /dev/kmsg +Date: Mai 2012 +KernelVersion: 3.4 +Contact: Kay Sievers <kay@...y.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 higher 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. + The values are separated by a ','. Future extensions might + add more comma separated values before the terminating ';'. + Unknown 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 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 + +Users: dmesg(1), userspace kernel log consumers --- a/Documentation/devices.txt +++ b/Documentation/devices.txt @@ -98,7 +98,8 @@ Your cooperation is appreciated. 8 = /dev/random Nondeterministic random number gen. 9 = /dev/urandom Faster, less secure random number gen. 10 = /dev/aio Asynchronous I/O notification interface - 11 = /dev/kmsg Writes to this come out as printk's + 11 = /dev/kmsg Writes to this come out as printk's, reads + export the buffered printk records. 12 = /dev/oldmem Used by crashdump kernels to access the memory of the kernel that crashed. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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