Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!necntc!linus!mbunix!jcmorris
From: jcmorris@mitre-bedford.ARPA (Joseph C. Morris)
Newsgroups: comp.sys.ibm.pc
Subject: Re: skyplot planetarium simulator posted to binaries
Message-ID: <28115@linus.UUCP>
Date: 30 Mar 88 23:15:26 GMT
References: <37175GMS@PSUVM> <3288@drivax.UUCP>
Sender: news@linus.UUCP
Reply-To: jcmorris@mbunix (Morris)
Distribution: na
Organization: The MITRE Corporation, McLean, VA.
Lines: 27
Keywords: self-extracting archives
Summary: It *can* be checked!

In article <3288@drivax.UUCP> frotz@drivax.UUCP (Frotz) writes:
>                                         [SKYPLOT] looks like a very
>nice package and I want to risk it, but I will have to wait until I
>find an isolated machine.

The SKY.EXE file was constructed by creating a normal PKXARC archive,
then prepending the PKSFX.PGM code which provides the self-unpacking 
function.  The easiest way to test this is to use an intellegent file-
compare utility like SUPERC to match the suspect file against a known 
good copy of PKSFX.PGM.  If the delta report says that the first mismatch
is at decimal 9758 (the current size of PKXFX.PGM), you should be safe.
I did this, and got the expected (and safe) report that it matched.

Of course, this assumes that you got PKSFX.PGM from a trusted source.

An alternative way would be to have a program which strips off the known
number of bytes in PKSFX.PGM (9758) from the suspect file and writes 
out the remainder to disk.  This remainder is a standard ARC file
which can be investigated with PKXARC -t etc.

Finally, I could have (but didn't) tried the thing safely by turning off
the expansion chassis which houses both of my hard disks and rebooting.
A less drastic measure which I used to use was to modify the drive
controller card to add a switch which disabled the on-board PROM.  The
card could still be accessed by using raw IN/OUT instructions, but none
of the BIOS interrupts would be recognized.  Modifying one of the
original IBM HD controller cards to do this was trivial, but the newer
cards aren't as easy.