Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!brutus.cs.uiuc.edu!apple!apple.com!chewy
From: chewy@apple.com (Paul Snively)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Tail patches
Message-ID: <5056@internal.Apple.COM>
Date: 6 Nov 89 17:01:05 GMT
Sender: usenet@Apple.COM
Organization: Apple Computer, Inc.
Lines: 68
References:<1459@sequent.cs.qmc.ac.uk> <36250@apple.Apple.COM>

In article <36250@apple.Apple.COM> marc@Apple.COM (Mark Dawson) writes:
> In article <1459@sequent.cs.qmc.ac.uk> jeremyr@cs.qmc.ac.uk (Jeremy 
Roussak) writes:
> >
> >I hope the net will pardon my ignorance, but I have seen a few comments
> >recently about "tail patching" of traps and have a couple of questions:
> >
> >1. What exactly are "tail patches"?
> >2. If they are what I suspect they are, why are they bad?
> >
> 
> 1.  There are times that you may wish to replace or modify a Mac trap 
call
>    (Say for example, _Write).  Suppose you have this horribly gross 
program
>    that passes the volume name, instead of the drive # to the _Write 
call.
>    You could write an intercept routine that would decode this name into 
the
>    correct drive # and drive refnum for the "real" system _Write call.  
It
>    would then call the system _Write (This would be a "head" patch).  You
>    could direct the system _Write to return to another piece of code (the
>    "tail" patch) to do some special cleanup if there was some error.
> 
>    The problem with the "tail" patch is that there is some possibility 
that
>    an Apple patch to that routine (to fix some bug/enhance it)  might be
>    checking the return address to see if it should do something.  Since 
your
>    return address would be on the stack first, this check would fail.  
Before
>    I worked at Apple I did use both the "head" and "tail" patches; I 
didn't see
>    anyway around them.  I also never ran into this "return address 
check".  I
>    am always crossing my fingers, hoping this case will never happen (it 
would
>    REALLY break my code).   
> 
>    Someone else in Apple will have to comment on if they know of any of 
patches
> that DO check the return addresses.  I don't know of any, but I don't 
claim to 
> know the whole system.    Below is a snippet of code to do head & tail 
patches.

[Evil code deleted...]

ARRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRGH!!!

PLEASE PLEASE PLEASE don't do tail patches!  Mark gives a reasonably 
cogent explanation as to WHY you shouldn't tail patch, and then explains 
how to do it!?  [sigh...]

Listen now, and listen hard: you CAN'T KNOW AHEAD OF TIME WHETHER ANY TRAP 
WILL EVER HAVE "COME-FROM" RETURN-ADDRESS CHECKING OR NOT.  Just because 
it doesn't under System 6.0.3.7.2.1.8 on the Macintosh IIsex doesn't mean 
that it NEVER has or NEVER will!

DON'T TAIL PATCH.

__________________________________________________________________________
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.
__________________________________________________________________________