Date:	Tue, 3 Mar 2009 21:06:46 -0600
From:	Christopher Brannon <cmbrannon@....net>
To:	linux-kernel@...r.kernel.org
Subject: [PATCH] /dev/time for Linux, inspired by Plan 9

Under Plan 9 from Bell Labs, one queries or sets the system clock by
reading or writing text strings to a special file named /dev/time.
I implemented such a facility for Linux.  A read of /dev/time produces
four decimal numbers: epoch seconds, nanoseconds since start of epoch,
jiffies since boot, and jiffies per second.  Writing a decimal number to
/dev/time sets the system clock to the given number of epoch seconds.
Anyone who is permitted to write to /dev/time may set the clock.
Granting this privilege becomes as easy as modifying groups and file
permissions.

Signed-off-by: Christopher Brannon <cmbrannon@....net>
---
 Documentation/time-device.txt |   49 ++++++++++++++++++
 arch/um/Kconfig.char          |   11 ++++
 drivers/char/Kconfig          |   11 ++++
 drivers/char/Makefile         |    2 +
 drivers/char/devtime.c        |  112 +++++++++++++++++++++++++++++++++++++++++
 5 files changed, 185 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/time-device.txt
 create mode 100644 drivers/char/devtime.c

diff --git a/Documentation/time-device.txt b/Documentation/time-device.txt
new file mode 100644
index 0000000..6294732
--- /dev/null
+++ b/Documentation/time-device.txt
@@ -0,0 +1,49 @@
+/dev/time for Linux
+===================
+
+Christopher M. Brannon <cmbrannon@....net>
+
+Inspiration
+-----------
+This document describes an implementation for the Linux kernel of a facility
+that is provided by the Plan 9 operating system from Bell Laboratories [1].
+
+Functionality
+-------------
+/dev/time provides a file-based interface to the system clock.
+Using this interface, one can query or set the system clock by reading or
+writing text strings.  A read of /dev/time yields four decimal integers:
+seconds since start of epoch, nanoseconds since start of epoch,
+jiffies since boot, and jiffies per second.
+Writing a decimal number n to /dev/time sets the system clock to the
+date and time which is n seconds greater than January 1, 1970, 00:00:00 GMT.
+
+Examples
+--------
+cat /dev/time
+# Produces: 1236121128 123612012877063000 495497 300,
+# at time of writing on a test machine.
+echo 287604960 > /dev/time
+# sets the clock to the approximate time of my birth.
+
+Use Case
+--------
+Traditionally, super-user privileges were required in order to set the
+system clock under Linux.
+Since the advent of capabilities, any process having the CAP_SYS_TIME
+capability may perform this operation.
+The /dev/time device ties privileges to file permissions.  If a user
+can write to /dev/time, then he can set the system clock.
+
+Difference from Plan 9
+-----------------------
+This implementation differs from Plan 9 in one respect.  the Plan 9
+Programmer's Manual exactly specifies the format of data read from the
+device, including the widths of the four numeric fields [1].  This Linux
+implementation does not pad values with whitespace, so the four fields
+are of variable width.
+
+References
+----------
+[1] Plan 9 Programmers Manual, section 3.
+http://plan9.bell-labs.com/magic/man2html/3/cons
diff --git a/arch/um/Kconfig.char b/arch/um/Kconfig.char
index 70dabd1..7d0da78 100644
--- a/arch/um/Kconfig.char
+++ b/arch/um/Kconfig.char
@@ -236,4 +236,15 @@ config MMAPPER
 	  This driver allows a host file to be used as emulated IO memory inside
 	  UML.
 
+config DEVTIME
+	tristate "/dev/time virtual device support."
+	depends on EXPERIMENTAL
+	help
+	  This device provides a file-based interface to the system clock.
+	  The interface is based on the /dev/time device used in the
+	  Plan 9 operating system from Bell Labs.
+	  See <file:Documentation/time-device.txt> for a full description.
+	  To compile this driver as a module, choose M here: the
+	  module will be called devtime.
+
 endmenu
diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index 735bbe2..5bad918 100644
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -97,6 +97,17 @@ config DEVKMEM
 	  kind of kernel debugging operations.
 	  When in doubt, say "N".
 
+config DEVTIME
+	tristate "/dev/time virtual device support."
+	depends on EXPERIMENTAL
+	help
+	  This device provides a file-based interface to the system clock.
+	  The interface is based on the /dev/time device used in the
+	  Plan 9 operating system from Bell Labs.
+	  See <file:Documentation/time-device.txt> for a full description.
+	  To compile this driver as a module, choose M here: the
+	  module will be called devtime.
+
 config SERIAL_NONSTANDARD
 	bool "Non-standard serial port support"
 	depends on HAS_IOMEM
diff --git a/drivers/char/Makefile b/drivers/char/Makefile
index 9caf5b5..aba7403 100644
--- a/drivers/char/Makefile
+++ b/drivers/char/Makefile
@@ -111,6 +111,8 @@ obj-$(CONFIG_PS3_FLASH)		+= ps3flash.o
 obj-$(CONFIG_JS_RTC)		+= js-rtc.o
 js-rtc-y = rtc.o
 
+obj-$(CONFIG_DEVTIME)		+= devtime.o
+
 # Files generated that shall be removed upon make clean
 clean-files := consolemap_deftbl.c defkeymap.c
 
diff --git a/drivers/char/devtime.c b/drivers/char/devtime.c
new file mode 100644
index 0000000..5e4ca66
--- /dev/null
+++ b/drivers/char/devtime.c
@@ -0,0 +1,112 @@
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/fs.h>
+#include <linux/miscdevice.h>
+#include <linux/time.h>
+#include <linux/jiffies.h>
+#include <linux/uaccess.h>
+
+MODULE_AUTHOR("Christopher Brannon <cmbrannon79@...il.com>");
+MODULE_LICENSE("GPL");
+
+const long long NS_PER_SEC = 1000000000;
+const long long NS_PER_USEC = 1000;
+const size_t time_bufsize = 256;
+
+static ssize_t time_read(struct file *, char __user *, size_t, loff_t *);
+static ssize_t time_write(struct file *, const char __user *, size_t, loff_t *);
+
+static const struct file_operations time_fops = {
+	.owner = THIS_MODULE,
+	.read = time_read,
+	.write = time_write,
+};
+
+static struct miscdevice timedev = {
+	.minor = MISC_DYNAMIC_MINOR,
+	.name = "time",
+	.fops = &time_fops
+};
+
+static int time2text(char *buffer, size_t bufsize)
+{
+	int count = 0;
+	struct timeval tv;
+	long long nanos;
+
+/* jiffies isn't 0 at boot time; its value is INITIAL_JIFFIES. */
+	u64 ticks = get_jiffies_64() - INITIAL_JIFFIES;
+
+	do_gettimeofday(&tv);
+	nanos = tv.tv_sec * NS_PER_SEC + tv.tv_usec * NS_PER_USEC;
+	count =
+	    scnprintf(buffer, bufsize, "%ld %lld %llu %d\n", tv.tv_sec, nanos,
+		      ticks, HZ);
+	return count;
+}
+
+static int text2time(char *buffer)
+{
+	struct timespec tv;
+	int result = strict_strtol(buffer, 10, &tv.tv_sec);
+	if ((result == 0) && (tv.tv_sec > 0)) {
+		tv.tv_nsec = 0;
+		do_settimeofday(&tv);
+	} else
+		result = -EINVAL;	/* only positive longs are valid. */
+	return result;
+}
+
+static ssize_t
+time_read(struct file *f, char __user *buffer, size_t count, loff_t * offset)
+{
+	int result = 0;
+	if (*offset != 0)
+		result = 0;
+	else {
+		char tmpbuf[time_bufsize];
+		int timetextlen = time2text(tmpbuf, time_bufsize);
+		unsigned long readcount = min(count, (size_t) timetextlen);
+		if (timetextlen <= 0)
+			return -EAGAIN;
+		if (!copy_to_user(buffer, tmpbuf, readcount)) {
+			*offset += readcount;
+			result = readcount;
+		} else
+			result = -EFAULT;
+	}
+	return result;
+}
+
+static ssize_t
+time_write(struct file *f, const char __user * buffer, size_t count,
+	   loff_t *offset)
+{
+	unsigned int result = 0;
+	char tmpbuf[time_bufsize];
+
+	if (*offset != 0)
+		return -EINVAL;
+	if (count > ((size_t) time_bufsize - 1))
+		return -EINVAL;	/* Likely trying to feed bogus data anyway. */
+	result = copy_from_user(tmpbuf, buffer, count);
+	if (result)
+		return -EFAULT;
+	tmpbuf[count] = '\0';
+	if (text2time(tmpbuf))
+		return -EINVAL;
+	return count;
+}
+
+static int __init time_init(void)
+{
+	return misc_register(&timedev);
+}
+
+static void __exit time_exit(void)
+{
+	misc_deregister(&timedev);
+}
+
+module_init(time_init);
+module_exit(time_exit);
-- 
1.6.1.3

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