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From: ed@mtxinu.UUCP
Newsgroups: net.unix-wizards,net.bugs.usg
Subject: Re: sdb poser solved -- 5.0 sdb users read this
Message-ID: <115@mtxinu.UUCP>
Date: Mon, 21-May-84 17:15:43 EDT
Article-I.D.: mtxinu.115
Posted: Mon May 21 17:15:43 1984
Date-Received: Wed, 23-May-84 08:29:31 EDT
References: <467@burl.UUCP>
Organization: mt Xinu, Berkeley
Lines: 21

The kernel (this was true in V7, and I presume is still true in
5.0) does write-protect the text segment of shared-text
programs.  There is a hook in the kernel to prevent two people
from using the same copy of a shared program when one of them
wants to debug it.

If someone executes a shared program and then tries to write
on the text with a debugger, the kernel un-write protects the
text segment and "loses" the memory that this program is
sharable.  If someone else tries to execute it, they'll get
their own copy.  If, however, there are two people using the
program when one of them tries to write on the text, the
kernel won't let it happen because it can't break the
interdependancy on the same text any more.

I haven't looked at the 4.2 kernel for this yet, but I suspect that
it does the same thing as well.

-- 
Ed Gould
ucbvax!mtxinu!ed