How to prevent VI from getting a shell?

[Dan Bernstein]

In article <1990Sep17.210110.26060 at robobar.co.uk> ronald at robobar.co.uk (Ronald S H Khoo) writes:
> I got around the problem by binary patching out the call to fork() in
> vi.  Ugh.  Also, I never found a way to defeat letting the user read and
> write files other than the current file.  Any offers ?

A slightly brutish but correct solution is illustrated in the pty
package. You simply convert exclamation points, colons, and other
dangerous characters into something dull like a question mark.

  % pty -0 tr \!: \?\? | pty vi

The only problem is that the user can't even type ! or : any more!
To kludge around this, convert ! into <esc>a! and : into <esc>a:.
You should also provide macros (not using ! or :) to allow whatever
restricted : commands the user might reasonably want.

This does, however, solve the stated problem.

(Be sure to disable keyboard interrupts, by the way.)

---Dan