Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!usc!apple!oracle!news
From: omullarn@oracle.oracle.com (Oliver Mullarney)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Tail patches
Summary: Tail patching - if enough of us do it...
Keywords: Patches, Tails, "The Right Thing To Do"
Message-ID: <1989Nov7.212837.5146@oracle.com>
Date: 7 Nov 89 21:28:37 GMT
References: <1459@sequent.cs.qmc.ac.uk> <36250@apple.Apple.COM> <5056@internal.Apple.COM>
Reply-To: omullarn@oracle.com (Oliver Mullarney)
Followup-To: comp.sys.mac.programmer
Organization: Oracle Corporation, Belmont, CA
Lines: 25


OK - I understand why we shouldn't use tail patches, but sometimes
you just gotta. How else can you catch a function result from a
trap call?
Now why doesn't Apple keep everyone happy by being a little bit
more inventive in their scheme for hacking patches called by traps
called by patches called by ...
Instead of maintaining hard coded return addresses (a bad move anyway,
considering TN 110, "Processor Compatibility", final para.) it would
be possible to examine the trap table for the address of the trap of
interest (in case it has been patched), scan forward from this address 
to find the RTS, and check to see if the return address on the stack 
lies between the trap/patch start and the RTS. Not lightning fast, but...
Maybe in the 50Hz machine we wouldn't even notice.

While on the subject, anyone out there got assembly to do stack crawling?
I need it for some of my tail patches... :-)
(No, really - I would be grateful if someone's done it already, and
wants to share their experience).

 Oliver
| Oliver Mullarney     |  "I have knowledge of Digital Watches,  and soon I   |
| Oracle Corporation   |   shall have knowledge of Video Cassette Recorders"  |
| omullarn@oracle.com  |                                      Time Bandits    |
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