Assignment 2 – Process Accounting Pseudo Device Driver


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COMP3301/COMP7308  Assignment 2 – Process Accounting
Pseudo Device Driver

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 page.
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 tap(4).
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:
1.1 Virtual Machine Hard Drive Image Backup
It is recommended that before you begin this assignment, first back up your virtual machine hard drive image
file. You can do this buy copying your virtual machine’s hard drive image file (.vdmk or .vhi) to another
locaiton. If an error occurs and you are unable to reboot your vitural machine, then you can create a new
virtual machine, using your backed up virtual machine hard drive image file. To find your virtual machine
Hard drive image, using VirtualBox, right click on your virtual machine icon, in the VirtualBox Manager.
1.2 Subversion
It is highly recommended that you use subversion to regularly backup your assignment code. Beaware that
mishaps with the Virtual Machine kernel do happen and can take a significant amount to recover from.
Regular backups with subversion will pervent this.
2 Specifications
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 file.
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.
2.2 Compilation
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:
[email protected] ~$ cd /usr/src/sys
[email protected] 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.
2.3 Messages
The messages that a program reads from the device driver are represented as a set of structs. The kernel
driver populates the structs when it is open and the relevant events occur in the kernel, and then 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 fields.
ac_seq is a simple wrapping counter that increments by one bit shift (of 0x01) for every message that the
driver generates. E.g. sequence number 3 is 0x04. 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 by one bit shift, so that the userland
consumer of the messages will know that an event has been lost. The bit shift should be reset to 0 when the
acct(4) device is opened (e.g. 0x01).
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 the parent.
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 to
operate. It is used for initialisation only of the driver only.
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 0x01 (bit shift of 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 a a single message, and copies as much of that one message as possible to userland.
acctwrite() should return EOPNOTSUPP as the driver does not support being written to by a userland process.
Only blocking I/O must be supported. Non blocking I/O functions must not be implemented.
3 Submission
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 a2/acct.c.
Submission must be made electronically by committing to your Subversion repository on
In order to mark your assignment the markers will check out a2/acct.c from your repository. Code checked
in to any other part of your repository will not be marked.
As per the usage guidelines, you should only commit source code and Makefiles.
The due date for the code submission is the 6th of October, 2020, at 4pm. Once submitted, no further changes
to the code is permitted.
3.1 Demo on Zoom
You are required to demo your assignment via a Zoom desktop screen sharing session, during your assigned
Zoom Practical Session in week 9. If you do not demo via the Zoom session, then your assignment will not be
marked. You are only permitted to demo the code that was submitted by the due date.
3.2 Recommendations
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 with
4 Testing
A tool will be provided that reads from the special file and parses the messages as per sys/dev/acct.h.

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