Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!cs.utexas.edu!usc!apple!apple.com!chewy
From: chewy@apple.com (Paul Snively)
Newsgroups: comp.sys.mac.programmer
Subject: Re: converting tail patches to head patches...
Message-ID: <5437@internal.Apple.COM>
Date: 27 Nov 89 23:07:08 GMT
References: <63359@tiger.oxy.edu>
Sender: usenet@Apple.COM
Distribution: usa
Organization: Apple Computer, Inc.
Lines: 60

First of all, thanks for making the effort to avoid tail-patching! :-)

Now, to work:

In article <63359@tiger.oxy.edu> sonenbli@oxy.edu (Andrew D. Sonenblick) 
writes:
>       Now, how I changed things for to make it a head patchy-watchy...
> 
>       myPatch()
>       {
>       long  rtsAddr;
>       asm{
>            Move.L    (SP)+,rtsAddr         /* save return address in 
rtsAddr */
>          }
>       doMyStuff();
>       asm{
>            Move.L    rtsAddr,-(SP)
>            Lea       @1,A0
>            Move.L    (A0),A0
>            Jmp       (A0)
>          }
> 
>       }
> 
>       Ok, so, what's the ups with this?  (I have tried all sorts of 
things
>       similar to this--most just make my mac hang, some crash, none 
works.)
> 
>       Needless to say, if anyone has insights into this, I--perhaps 
others
>       on the net would too--would appreciate the help.

A couple of comments:

Since the problem with tail-patching is that the ORIGINAL code that you're 
calling may check the return address, your approach of removing the return 
address at the beginning of the patch and replacing it immediately before 
jumping to the original code doesn't help.  In fact, one reason that 
you're probably crashing is because you've done the same thing 
(semantically) as if you had simply changed your original JSR to a JMP!  
Not a good idea.

Now for the good news: if you're trying to patch _SystemTask, you can 
probably get away with it, because I don't believe that _SystemTask is 
called from within the ROM, which means that it won't be patched by us 
with this nasty come-from code that's causing all of the headaches.  As 
Larry Rosenstein pointed out, you have a higher probability of being 
"patch safe" with traps that have a high probability of not being called 
from ROM.

Hope this helps.

__________________________________________________________________________
Just because I work for Apple Computer, Inc. doesn't mean that they 
believe what I believe or vice-versa.
__________________________________________________________________________
C++ -- The language in which only friends can access your private members.
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