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The Meaning of 'Hack' - The Jargon File

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"The word {hack} doesn't really have 69 different meanings", according to Phil Agre, an MIT hacker. "In fact, {hack} has only one meaning, an extremely subtle and profound one which defies articulation. Which connotation is implied by a given use of the word depends in similarly profound ways on the context. Similar remarks apply to a couple of other hacker words, most notably {random}."

Hacking might be characterized as "an appropriate application of ingenuity". Whether the result is a quick-and-dirty patchwork job or a carefully crafted work of art, you have to admire the cleverness that went into it.

An important secondary meaning of {hack} is `a creative practical joke'. This kind of hack is often easier to explain to non-hackers than the programming kind. Accordingly, here are some examples of practical joke hacks:

In 1961, students from Caltech (California Institute of Technology in Pasadena) hacked the Rose Bowl football game. One student posed as a reporter and `interviewed' the director of the University of Washington card stunts (such stunts involve people in the stands who hold up colored cards to make pictures). The reporter learned exactly how the stunts were operated, and also that the director would be out to dinner later.

While the director was eating, the students (who called themselves the `Fiendish Fourteen') picked a lock and stole one of the direction sheets for the card stunts. They then had a printer run off 2300 copies of the sheet. The next day they picked the lock again and stole the master plans for the stunts, large sheets of graph paper colored in with the stunt pictures. Using these as a guide, they carefully made `corrections' for three of the stunts on the duplicate instruction sheets. Finally, they broke in once more, replacing the stolen master plans and substituting the stack of altered instruction sheets for the original set.

The result was that three of the pictures were totally different. Instead of spelling "WASHINGTON", the word "CALTECH" was flashed. Another stunt showed the word "HUSKIES", the Washington nickname, but spelled it backwards. And what was supposed to have been a picture of a husky instead showed a beaver. (Both Caltech and MIT use the beaver as a mascot. Beavers are nature's engineers.)

After the game, the Washington faculty athletic representative said, "Some thought it ingenious; others were indignant." The Washington student body president remarked, "No hard feelings, but at the time it was unbelievable. We were amazed."

This is now considered a classic hack, particularly because revising the direction sheets constituted a form of programming not unlike computer programming.

Another classic hack:

Some MIT students once illicitly used a quantity of thermite to weld a trolley car to its tracks. The hack was actually not dangerous, as they did this at night to a parked trolley. It took the transit people quite a while to figure out what was wrong with the trolley, and even longer to figure out how to fix it. They ended up putting jacks under the trolley and cutting the section of track on either side of the wheel with oxyacetylene torches. Then they unbolted the wheel, welded in a new piece of track, bolted on a new wheel, and removed the jacks. The hackers sneaked in the next night and stole the piece of track and wheel!

The piece of trolley track with the wheel still welded to it was later used as the trophy at the First Annual All-Tech Sing. They carted it in on a very heavy duty dolly up the freight elevator of the Student Center. Six feet of rail and a trolley wheel is a *lot* of steel.

A rather similar hack, perpetrated by a fraternity at CMU, cost their campus its trolley service.

Though these displayed some cleverness, the side-effect of expensive property damage was definitely an esthetic minus. The best hacks are harmless ones.

And another:

One winter, late at night, an MIT fraternity hosed down an underpass that is part of a commuter expressway near MIT. This produced an ice slick that `trapped' a couple of small cars: they didn't have the momentum or traction to climb out of the underpass. While it was clever to apply some simple science to trap a car, it was also very dangerous as it could have caused a collision. Therefore this was a very poor hack overall.

And yet another:

On November 20, 1982, MIT hacked the Harvard-Yale football game. Just after Harvard's second touchdown against Yale in the second quarter, a small black ball popped up out of the ground at the 40-yard line, and grew bigger, and bigger, and bigger. The letters "MIT" appeared all over the ball. As the players and officials stood around gawking, the ball grew to six feet in diameter and then burst with a bang and a cloud of white smoke.

As the Boston Globe later reported, "If you want to know the truth, M.I.T. won The Game."

The prank had taken weeks of careful planning by members of MIT's Delta Kappa Epsilon fraternity. The device consisted of a weather balloon, a hydraulic ram powered by Freon gas to lift it out of the ground, and a vacuum-cleaner motor to inflate it. They made eight separate expeditions to Harvard Stadium between 1 and 5 AM, in which they located an unused 110-volt circuit in the stadium, and ran buried wiring from the stadium circuit to the 40-yard line, where they buried the balloon device. When the time came to activate the device, two fraternity members had merely to flip a circuit breaker and push a plug into an outlet.

This stunt had all the earmarks of a perfect hack: surprise, publicity, the ingenious use of technology, safety, and harmlessness. The use of manual control allowed the prank to be timed so as not to disrupt the game (it was set off between plays, so the outcome of the game would not be unduly affected). The perpetrators had even thoughtfully attached a note to the balloon explaining that the device was not dangerous and contained no explosives.

Harvard president Derek Bok commented: "They have an awful lot of clever people down there at MIT, and they did it again." President Paul E. Gray of MIT said, "There is absolutely no truth to the rumor that I had anything to do with it, but I wish there were."

Finally, here is a great story about one of the classic computer hacks.

Back in the mid-1970s, several of the system support staff at Motorola discovered a relatively simple way to crack system security on the Xerox CP-V timesharing system. Through a simple programming strategy, it was possible for a user program to trick the system into running a portion of the program in `master mode' (supervisor state), in which memory protection does not apply. The program could then poke a large value into its `privilege level' byte (normally write-protected) and could then proceed to bypass all levels of security within the file-management system, patch the system monitor, and do numerous other interesting things. In short, the barn door was wide open.

Motorola quite properly reported this problem to XEROX via an official `level 1 SIDR' (a bug report with a perceived urgency of `needs to be fixed yesterday'). Because the text of each SIDR was entered into a database that could be viewed by quite a number of people, Motorola followed the approved procedure: they simply reported the problem as `Security SIDR', and attached all of the necessary documentation, ways-to-reproduce, etc. separately.

Xerox sat on their thumbs...they either didn't realize the severity of the problem, or didn't assign the necessary operating-system-staff resources to develop and distribute an official patch.

Months passed. The Motorola guys pestered their Xerox field-support rep, to no avail. Finally they decided to take Direct Action, to demonstrate to Xerox management just how easily the system could be cracked and just how thoroughly the system security systems could be subverted.

They dug around in the operating-system listings and devised a thoroughly devilish set of patches. These patches were then incorporated into a pair of programs called Robin Hood and Friar Tuck. Robin Hood and Friar Tuck were designed to run as `ghost jobs' (daemons, in UNIX terminology); they would use the existing loophole to subvert system security, install the necessary patches, and then keep an eye on one another's statuses in order to keep the system operator (in effect, the superuser) from aborting them.

So... one day, the system operator on the main CP-V software development system in El Segundo was surprised by a number of unusual phenomena. These included the following:

  • Tape drives would rewind and dismount their tapes in the middle of a job.
  • Disk drives would seek back and forth so rapidly that they'd attempt to walk across the floor (see {walking drives}).
  • The card-punch output device would occasionally start up of itself and punch a {lace card} (every hole punched). These would usually jam in the punch.
  • The console would print snide and insulting messages from Robin Hood to Friar Tuck, or vice versa.
  • The Xerox card reader had two output stackers; it could be instructed to stack into A, stack into B, or stack into A unless a card was unreadable, in which case the bad card was placed into stacker B. One of the patches installed by the ghosts added some code to the card-reader driver... after reading a card, it would flip over to the opposite stacker. As a result, card decks would divide themselves in half when they were read, leaving the operator to recollate them manually.

There were some other effects produced, as well.

Naturally, the operator called in the operating-system developers. They found the bandit ghost jobs running, and X'ed them... and were once again surprised. When Robin Hood was X'ed, the following sequence of events took place:

     !X id1

     id1: Friar Tuck... I am under attack!  Pray save me!
     id1: Off (aborted)

     id2: Fear not, friend Robin!  I shall rout the Sheriff of 
          Nottingham's men!

     id1: Thank you, my good fellow!

Each ghost-job would detect the fact that the other had been killed, and would start a new copy of the recently-slain program within a few milliseconds. The only way to kill both ghosts was to kill them simultaneously (very difficult) or to deliberately crash the system.

Finally, the system programmers did the latter... only to find that the bandits appeared once again when the system rebooted! It turned out that these two programs had patched the boot-time image (the /vmunix file, in UNIX terms) and had added themselves to the list of programs that were to be started at boot time...

The Robin Hood and Friar Tuck ghosts were finally eradicated when the system staff rebooted the system from a clean boot-tape and reinstalled the monitor. Not long thereafter, Xerox released a patch for this problem.

It is alleged that Xerox filed a complaint with Motorola's management about the merry-prankster actions of the two employees in question. It is not recorded that any serious disciplinary action was taken against either of them.