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From: guy%gorodish@Sun.COM (Guy Harris)
Newsgroups: comp.lang.c,comp.unix.questions
Subject: Re: Destroying arguments
Message-ID: <19131@sun.uucp>
Date: Mon, 18-May-87 16:18:57 EDT
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Posted: Mon May 18 16:18:57 1987
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This really isn't a C question at all, it's a UNIX question, so I'll
redirect it where it should have been sent in the first place.

> Newsgroups: comp.lang.c
> Distribution: comp.unix.questions

In fact, it looks like the orignal poster may have tried directing it
there, but goofed.

> Alas, this wonderfully simple technique simply does not work with
> the 4BSD `ps', which reads the strings themselves, not the pointers
> thereto.

As do most other versions of "ps".  The 3B2 version of "ps" does it
differently; the kernel stashes a string away in the U area that
contains the original arguments used to invoke the program.  (I have
no idea whether this was done because somebody thought this was the
right way to do things, or because they couldn't figure out how to
snarf the arguments up on a machine whose stack grows upward in
memory....)  On this machine, you *can't* smash your argument list.
Period.

The answer is "forget it".  In general, it can't be done.  Some
versions of "crypt" write '\0' all over the password it is handed,
which means it'll smash the argument if that was where the key
appeared, but 1) this won't work on a system that works the way 3B2
UNIX does and 2) still leaves a window in which the argument can be
seen.

We won't mention that it also leaves the encryption key on your
screen when you type the command, or that using "crypt" in this
fashion in a *script* is a very bad mistake as it leaves the
encryption key in a file.  In short, the answer is "always get
encryption keys from the user, and turn off echoing when you do so".