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From: david@linc.cis.upenn.edu (David Feldman)
Newsgroups: comp.unix.wizards
Subject: Re: Aliasing text and data segments of a process
Message-ID: <3106@super.upenn.edu>
Date: 21 Jan 88 20:01:27 GMT
References: <202@sdti.UUCP>
Sender: news@super.upenn.edu
Reply-To: david@linc.cis.upenn.edu.UUCP (David Feldman)
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Organization: University of Pennsylvania
Lines: 17
Keywords: Ultrix
Summary: yes, but...

You can execute out of the data segment, at least on SOME Unix systems.  In
Ultrix, you can tell the loader to make the code "IMPURE", although with cc
you usually get demand paged pure executables unless you specify the right
option for ld.  You can also execute code out of the stack, of course, and if
you catch signals you are forced to do this.  On receiving a signal, Ultrix
inserts a segment of code above the stack in the stack space - on a VAX at
least.  This code is the infamous 'sigtramp'.  So, yes, a program can be
modified while it is running.

As an aside, I had planned on writing a machine simulator which executed code
out of a malloc'ed memory space.  I never started the project, but I was able
to get some assembly running that jumped into a malloc space and then out
again.  I would assume that any Unix running on a machine that does not enforce
separate I & D could do this.  Check the manual page for ld.

					Dave F.
					david@linc.cis.upenn.edu