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COMP3301/COMP7308 2019 Assignment 2 (DRAFT)
• Revision : 232
1 Process Accounting Pseudo Device Driver
This assignment asks you to write a device driver for the OpenBSD kernel that implements a replacement
for the current process accounting facility.
Process accounting is a facility that allows administrators to audit the system resource utilisation of
commands run by users.
Process accounting in OpenBSD is currently implemented via the acct(2) system call that enables or
disables of logging of commands to a file, the accton(8) utility for calling the syscall, and the sa(8) and
lastcom(8) utilities for processing said file. The format of the file is described in the acct(5) manual
A pseudo device driver is an actual driver, but not one that implements support for physical hardware. It
provides a virtual, software only, service for user programs to use.
Device special file are entries in a filesystem that refer to a set of functions in the kernel that implement
file behaviour such as open, read, write, and close.
Examples of pseudo device drivers that provide device special files are zero(4), null(4), tun(4), and
This is an individual assignment. You should feel free to discuss aspects of C programming and the
assignment specification with fellow students. You should not actively help (or seek help from) other
students with the actual coding of your assignment solution. It is cheating to look at another student’s
code and it is cheating to allow your code to be seen or shared in printed or electronic form. You should
note that all submitted code may be subject to automated checks for plagiarism and collusion. If we detect
plagiarism or collusion, formal misconduct proceedings will be initiated against you. If you're having
trouble, seek help from a member of the teaching staff. Don't be tempted to copy another student's code.
You should read and understand the statements on student misconduct in the course profile and on the
school web-site: https://www.itee.uq.edu.au/itee-student-misconduct-including-plagiarism
This assigment specifies the replacement of the acct(2) system call with a pseudo device driver that
provides a device special file that produces messages analogous to the entries written to the accounting
The acct(4) driver will provide a superset of the functionality that is provided by the current system
call. The system call only records information about the process when it exits, but the driver will also
report information about process forks and execs.
2.1 Code Style
Your code is to be written according to OpenBSD's style guide, as per the style(9) man page.
Your code is to be built as part of the kernel on an amd64 OpenBSD 6.5 or -current system.
The changes to the kernel necessary to configure an acct(4) driver so it can be compiled will be supplied
as a diff available from Blackboard. The diff can be applied by running the following:
dlg@r630 ~$ cd /usr/src/ sys
dlg@r630 sys$ patch < /path/to/assignment2-boilerplate.diff
Hmm... Looks like a unified diff to me...
The driver must be implemented in a single file, and placed in sys/dev/acct .c next to the sys/dev/acct.h
provided by the diff described above.
The messages that a program reads from the device driver are represented as a set of structs. The kernel
driver populates the structs when the relevant events occur in the kernel, and makes them available for a
program to read.
The structure of the messages the driver should produce is provided in sys/dev/acct.h.
2.3.1 Common Fields
All messages from the driver start with a common set of fields that are contained in struct acct_common.
The other messages all contain struct acct_common as their first field.
The first three fields of the common structure refer to the message itself, rather than the process the
message is about. The ac_type field contains a number representing the type of the current message, eg, a
value of 0 or ACCT_MSG_FORK states that the message is about a process forking and should be interpreted
as the associated message structure.
The ac_len field contains the number of bytes used for this message, including the ac_type and ac_len
ac_seq is a simple wrapping counter that increments for every message that the driver generates. If the
driver receives notification from the rest of the kernel that an event has occurred (eg, acct_fork() is
called when a process forks), but is unable to generate a message about it, the sequence number should
still be incremented so that the userland consumer of the messages will know that an event has been lost.
The counter should be reset to 0 when the acct(4) device is opened.
The remaining common fields should be set for the process the message is about.
2.3.2 exit message
The exit message corresponds with struct acct_exit. The information in this message corresponds with
the information described in acct(5). acct(2) may be used as a reference when filling in the information
in this message.
2.3.3 fork event
The fork message corresponds with struct acct_fork, and is generated when a process forks a new child.
The information in the message should be based on the parent of the new process, apart from ac_cpid
which contains the process ID of the new child. Note that acct_fork is given a pointer to the child, not
2.3.4 exec event
The exec message corresponds with struct acct_exec, and is geneated when a process calls exec(). It
exists to record the new name of the binary the program is executing.
2.4 Driver entry points
acct.c must provide the following functions to support the integration into the kernel, and to provide
the required interface for userland to access the driver.
2.4.1 Kernel entry points
The kernel is patched to call 3 functions when a process forks, execs, or exits. Those functions are
acct_fork(), acct_exec(), and acct_exit() respectively. All these functions take a struct process
* as their only argument, and do not return anything to the caller.
2.4.2 Userland entry points
acctattach( ) is called when the kernel starts running for the driver to configure any state needed for it
acctopen() is called when a program attempts to open a device file with the corresponding major number
to this driver. It should allow only the 0th minor to be opened, opened exclusively, and only opened for
reading. Read/write or write only opens of the device should fail with EPERM. The sequence number for
generated messages should be reset to 0 on every open.
acctclose() should clean up any state associated with the open driver.
acctioctl() should support FIONREAD, and FIONBIO as per the ioctl(2) manpage. FIONASYNC should
not be implemented.
acctread() dequeues aa single message, and copies as much of that one message as possible to userland.
It should support non-blocking reads.
acctwrite() should return EOPNOTSUPP as the driver does not support being written to by a userland
The driver should support non-blocking I/O (well, just O) by implementing acctpoll() and
You are required to implement the acct(4) driver by writing your code in a single file, sys/ dev/acct.c.
This file in the OpenBSD source tree will be committed to your SVN repo as ass2/acct.c.
Submission must be made electronically by committing to your Subversion repository on
source.eait.uq.edu.au. In order to mark your assignment the markers will check out ass2/acct.c
from your repository. Code checked in to any other part of your repository will not be marked.
As per the source.eait.uq.edu.au usage guidelines, you should only commit source code and Makefiles.
The due date for the code submission is the beggining of your prac session, either 10am on Wednesday
the 19th, or 2pm on Thursday the 20th of September 2018. Note that no submissions can be made more
than 120 hours past the deadline under any circumstances.
The following kernel functionality may or may not be useful in the implementation of this assigment:
• malloc(9) - kernel memory allocator
• pool(9) - resource pool manager
• TAILQ_INIT(3) - doubly-linked list macros
• KASSERT(9) - kernel assert routines
• uiomove(9) - move (copy) data described by a struct uio
• mutex(9) - kernel mutex implementation
• rwlock(9) - kernel read/write lock implementation
• tsleep(9), and wakeup(9) - process context sleep and wakeup
The majority of the OpenBSD kernel still runs under a Big Giant lock, known as the kernel lock, which
can be used to provide implicit serialisation of code in this driver . The kernel lock is taken and released
with KERNEL_LOCK() and KERNEL_UNLOCK() respectively, or if it is assumed to be held, may be asserted
A tool will be provided that reads from the special file and parses the messages as per sys/dev/acct.h.