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From: oster@lapis.berkeley.edu.UUCP
Newsgroups: comp.lang.fortran
Subject: Re: Changing constants (was Re: Collecting favorite bug stories)
Message-ID: <2334@jade.BERKELEY.EDU>
Date: Sat, 24-Jan-87 05:48:28 EST
Article-I.D.: jade.2334
Posted: Sat Jan 24 05:48:28 1987
Date-Received: Sun, 25-Jan-87 14:44:25 EST
References: <3669@curly.ucla-cs.UCLA.EDU> <2722@udenva.UUCP> <494@gec-mi-at.co.uk> <11754@sun.uucp> <32add81d.71d0@apollo.uucp>
Sender: usenet@jade.BERKELEY.EDU
Reply-To: oster@lapis.berkeley.edu.UUCP (David Phillip Oster)
Organization: University of California, Berkeley
Lines: 34

My favorite "job security" hack in Fortran is done as follows:
1.) Take a routine that is called from only one or two places in the code,
compile it separately, at make a file of the pure machine bits that
result.
2.) Express that bit pattern as the result of a series of floating point
operations. Ideally, you'd like a matrix of floating point numbers that
when inverted has the same bit pattern as the code routine in <1.)>.
3.) use DATA //... to make that an array of initialized data in your
program.
4.) At some inconspicuous point in the running of the program, apply the
transform, so the array now holds the correct bit pattern for machine
code. (A really clever programmer will do the transform in widely
scattered pieces.)
5.) pass the array as an argument to a procedure. (Say the third argument
of FOO.)
6.) Declare the third argument of FOO as a procedural parameter. That way
step 5 will run it.

To anyone who comes after you, this will be completely opaque. Note:
Although many machines have non-writable code space, almost all machines
will execute out of data space. If your machine won't, a refinement of
this technique is to write a p-code interpreter, (where say, each byte
does a different operation) and pass your inverse of an initialized
floating point matrix to your p-code interpreter.

Anyone see the Apple ][ game based on the reference in my signature?
According to a review I read, there was a sequence of operations you could
do to make it crash back to Apple's ROM Basic. But, you couldn't REALLY
crash it, you were in an illusion of ROM Basic created by the program to
make you think you had gotten out.
--- David Phillip Oster		-- "We live in a Global Village."
Arpa: oster@lapis.berkeley.edu  --
Uucp: ucbvax!ucblapis!oster     -- "You are Number Six."
pro