The acct structure for collecting process accounting details is documented in the header files /usr/include/linux/acct.h and /usr/include/sys/acct.h. Table 2 displays the members available in the acct struct and a brief description of each member.
As you can see from the table, a lot of information is packed into the 64-byte accounting record. If you feel you need more information than is available with standard process accounting, consult the book by Mann and Mitchell listed in Resources at the end of this article.
Listing 1 is a simple demonstration of the use of the acct system call. The acct call takes one argument, the name of the file to which process accounting information is appended. If the argument is NULL, process accounting will be turned off. In addition, the file already must exist when the system call is made, or the call will fail and an error will be returned.
If a program running with the ID of an ordinary user makes a call to acct, the call also will fail and return an error. Programs that attempt to switch process accounting on and off must have root privileges to succeed.
The code in Listing 1 is similar to a typical implementation of the accton command, but there are two main differences. The first is that this code will report its actions in messages to standard output. The second is that if the file specified on the command line does not exist, it will be created.
The file includes the <unistd.h> header file. All programs that make use of the acct call should include this file. The program checks to see if argc is equal to one, meaning no arguments were passed on the command line. If this is so, the program attempts to turn off process accounting by calling acct with a NULL argument.
If the command is run with an argument, the program will assume that the first argument is the filename. If the file does not exist, the program will attempt to create it with the creat system call. Then, the program will call acct with the filename as an argument to turn on process accounting. If an error code is returned from a system call, a message will be printed and the program will exit.
Listing 2 demonstrates how to read records from the log file into an acct structure in memory so that the information can be printed out or operated upon. This program includes the <sys/acct.h> header file. All programs that need to work with the acct structure should include this file. Local variables in the main function include a file descriptor, a variable to hold the number of bytes read from the file and an acct struct.
The user of the program must specify a filename on the command line. The program attempts to open this file for read-only access. If the open was successful, the program will read() a record from the file directly into the local acct structure, a. Due to space constraints for the article, I've made the assumption that a read() always will return exactly the number of bytes requested, until the end of file is reached. The program continues to read and print the command name from the records until a zero is returned from the read() call, signalling the end of file condition.
The Listings in this article are intended to be simple introductions to the system accounting structures. Robust programs would create a buffer to read multiple accounting records at once, and they would check for issues such as fewer bytes read from the file than were requested. To see examples of robust programs, look at the source for the process accounting utilities that you've installed.
You now have enough information to enable process accounting and use the standard commands to retrieve information about programs executed on a Linux system. If you're so inclined, you also can learn to make custom tools that parse the process accounting log files.
If you're using process accounting for system security, keep in mind that it is not by any means a comprehensive solution, but only one small tool. In fact, as Mann and Mitchell point out, you should be careful about trusting the information in the process accounting log files; the logs may have been modified by a technically savvy attacker.
With a basic understanding of the process accounting tools in Linux and some experimentation, you can set up these utilities on your own computer. If you're fortunate enough to have root access to the systems at work, you'll also be prepared to remove all traces of the Sokoban game from the accounting log files—in case that evil corporate auditor really does show up in your department one day.
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