TweetFollow Us on Twitter

Self Modifying Code
Volume Number:8
Issue Number:4
Column Tag: Article Rebuttal

Self Modifying code is a No-No!

A better way to do an event patch without self-modifying code or Assembly

By Scott T. Boyd, Apple Computer, Inc. and Mike Scanlin, MacTutor Regular Contributing Author

Note: Source code files accompanying article are located on MacTech CD-ROM or source code disks.

Mike Scanlin’s article “Rotten Apple INIT for April Fool’s” brings up a minor but essential point. The GetNextEvent patch looks like:

;1

@first
 Lea    @exitAddress, A0
 Move.L (SP)+,(A0)
 Lea    @eventRecPtr,A0
 Move.L (SP),(A0)
 Pea    @tailPatch

 DCJmpInstruction
@origTrap
 NOP
 NOP

The NOPs get replaced with the original GetNextEvent trap address by the installation code:

;2

 Lea    @origTrap,A1
 Move.L A0,(A1)

Now, consider the processor instruction cache. It’s a piece of the processor which remembers what’s at a set of memory locations. It does this so the CPU won’t have to do a memory access for recently referenced instructions. This is designed to save time. It’s a neat hardware feature.

However, the Macintosh system software doesn’t make a distinction between code and data. That’s different from OSs like Unix, which keep code and data in separate address spaces. When Mike’s code installs the original trap address with the Move.L A0,(A1), it’s putting an address into the middle of a piece of code. Unfortunately, the cache which records the new value is the data cache.

The instruction cache has no clue that instructions just changed. This is one way of doing what’s commonly called “self-modifying code”.

Self-modifying code is, in general, a bad thing to write. Apple has long discouraged, and continues to discourage self-modifying code.

This is bad in this case because the processor, if it were to execute this code right away, might believe (for some unspecified reason) that those memory locations were cached in the instruction cache. If it did, it would pull whatever had been in that location the last time code was executed from that spot, then try to execute whatever was there. The odds that the instruction cache held the value you just put into the data cache are not in your favor.

Contrast that approach with this approach:

;3

@first
 ...

 Move.L @origTrap,A0
 JMP.L  (A0)
 ...

/* branch around this, or put it somewhere else, but don’t let the PC run through 
here */

@origTrap
 NOP
 NOP

While this approach still stores the old address into a piece of code, it’s never referenced as code by the processor. It’s treated specifically as data. The instruction cache never comes into play since the original address is moved as data.

Yet another approach, which also saves a register:

;4

@first
                        ...
                        Move.L@origTrap,-(SP)
                        RTS
                        ...

TN #261: “Cache As Cache Can” discusses this topic in more detail, especially with regard to moving whole chunks of code around.

As it happens, the caches are almost certainly flushed before this particular eight bytes ever get loaded for execution, but that’s a happy coincidence, and not something you should rely on. What’s happening is that we have made several traps flush the caches (guaranteeing that there won’t be any misunderstanding about something being in the instruction cache when it’s not), but we may change our minds about which traps should flush, and when. You shouldn’t count on any given trap’s current cache-flushing behavior.

One final consideration. Putting data into code does not work if code is ever write-protected, and that may happen one day. So where can you put something when you can’t allocate any global storage (e.g., PC-relative data or low-memory globals with a fixed address)? You can use NewGestalt to register a new selector. When you call Gestalt, it can return a value which is actually a pointer (or handle) to your global data. This technique won’t work well if you can’t afford to make the trap call (like from some time-sensitive routine you’ve patched), but it works nicely if you have the time and you want to avoid putting data into your patch code.

Scott T Boyd, Apple Computer, Inc.

Mike Scanlin Says

Scott's point about stale code in the instruction cache is well taken and I deserve a thumping for having written it. I made the poor judgement call that it wouldn't matter in this case because I expected the instruction cache to be flushed between the time the patch installation code finished and the first time the patch code was executed. I hang my head in shame.

As partial retribution (and to satisfy a few requests for a non-assembly version) I have written a trap patching shell in C that doesn't use any self-modifying code (see listing below). It obeys all of the rules except for the one about storing data into a code segment (Scott's solutions have this problem, too, as he mentions). Until we have write-protected code segments, this will not be a problem.

Mike Scanlin

/*********************************************************
 * PatchGNE.c:
 * This INIT installs a patch on GetNextEvent and 
 * SystemEvent that intercepts keyDown and autoKey events. 
 * For this example, the intercepted key events are 
 * converted to lower case if both the capsLock key and the 
 * shiftKey are down (thus making the Mac keyboard behave 
 * like an IBM keyboard). However, you can use this shell to 
 * do generalized event intercepting as well as generalized 
 * trap patching (with no asm and no self-modifying code). 
 * If your patches need globals, put them in the 
 * PatchGlobals struct and initialize them in main.
 * In Think C, set the Project Type to Code Resource, the 
 * File Type to INIT, the Creator to anything, the Type to 
 * INIT, the ID to something like 55 (55 will work but it 
 * doesn't have to be 55), turn Custom Header ON and Attrs 
 * to 20 (purgeable) and Multi Segment OFF.
 *
 * Mike Scanlin. 16 May 1992.
 *********************************************************/

#include "Traps.h"

/**********************************************************
 * typedefs
 *********************************************************/
typedef pascal short (*GNEProcPtr)(short eventMask,
 EventRecord *theEvent);
typedef pascal short (*SEProcPtr)(EventRecord *theEvent);

typedef struct PatchGlobals {
 GNEProcPtr pgOldGNE;
 SEProcPtrpgOldSE;
} PatchGlobals, *PatchGlobalsPtr;

/**********************************************************
 * prototypes
 *********************************************************/
void main(void);
void StartPatchCode(void);
pascal short MyGetNextEvent(short eventMask, EventRecord
 *theEvent); 
pascal short MySystemEvent(EventRecord *theEvent);
void CheckKeyCase(EventRecord *theEvent);
void EndPatchCode(void);

/**********************************************************
 * main:
 * Gets some memory in the system heap and installs the GNE 
 * and SE patches (as well as allocating and initializing 
 * the patc 8.4  Self Modifying Codeutine that gets 
 * executed at startup time (by the INIT mechanism).
 *
 * The block of memory that main allocates will look like 
 * this when main has finished:
 *
 *                   +--------------------+
 *                   |    PatchGlobals    |
 *                   +--------------------+
 *                   |  StartPatchCode()  |
 *  GNE trap addr -> +--------------------+
 *                   |  MyGetNextEvent()  |
 *   SE trap addr -> +--------------------+
 *                   |  MySystemEvent()   |
 *                   +--------------------+
 *                   |   CheckKeyCase()   |
 *                   +--------------------+
 *                   |   EndPatchCode()   |
 *                   +--------------------+
 *
 *********************************************************/
void main()
{
    Ptr             patchPtr;
    PatchGlobalsPtr pgPtr;
    long            codeSize, offset;

    /* try and get some memory in the system heap for code
       and globals */
    codeSize = (long) EndPatchCode - (long) StartPatchCode;
    patchPtr = NewPtrSys(codeSize + sizeof(PatchGlobals));
    if (!patchPtr)
        return; /* out of memory -- abort patching */

    /* initialize the patch globals at the beginning 
       of the block */
    pgPtr = (PatchGlobalsPtr) patchPtr;
    pgPtr->pgOldGNE = (GNEProcPtr)
      GetTrapAddress(_GetNextEvent);
    pgPtr->pgOldSE = (SEProcPtr)
      GetTrapAddress(_SystemEvent);

    /* move the code into place after the globals */
    BlockMove(StartPatchCode, patchPtr +
      sizeof(PatchGlobals), codeSize);

    /* set the patches */
    patchPtr += sizeof(PatchGlobals);
    offset = (long) MyGetNextEvent - (long) StartPatchCode;
    SetTrapAddress((long) patchPtr + offset, _GetNextEvent);
    offset = (long) MySystemEvent - (long) StartPatchCode;
    SetTrapAddress((long) patchPtr + offset, _SystemEvent);
}

/**********************************************************
 * StartPatchCode:
 * Dummy proc to mark the beginning of the code for the 
 * patches.  Make sure all of your patch code is between 
 * here and EndPatchCode.
*********************************************************/
void StartPatchCode()
{
}

/*********************************************************
 * MyGetNextEvent:
 * Tail patch on GetNextEvent.
 *
 * The reason this returns a short instead of a Boolean is 
 * because we need to make sure the low byte of the top word 
 * on the stack is zero because some programs do a Tst.W 
 * (SP)+ when this returns instead of Tst.B (SP)+ like they 
 * should (which is technically their bug but, we might as 
 * well work around it since it's not hard).
 *
 * If you want to eat the event and not pass it on to the 
 * caller then set returnValue to zero.
 *********************************************************/
pascal short MyGetNextEvent(short eventMask,
  EventRecord *theEvent)
{
    PatchGlobalsPtr pgPtr;
    short           returnValue;

    /* find our globals */
    pgPtr = (PatchGlobalsPtr) ((long) StartPatchCode -
      sizeof(PatchGlobals));

    /* call original GNE first */
    returnValue = (*pgPtr->pgOldGNE)(eventMask, theEvent);

    /* do some post-processing */
    CheckKeyCase(theEvent);

    /* return to original caller */
    return (returnValue);
}

/**********************************************************
 * MySystemEvent:
 * Tail patch on SystemEvent.
 *
 * The reason this returns a short instead of a Boolean is 
 * because we need to make sure the low byte of the top word 
 * on the stack is zero because some programs do a Tst.W 
 * (SP)+ when this returns instead of Tst.B (SP)+ like they 
 * should (which is technically their bug but, we might as 
 * well work around it since it's not hard).
 * 
 * We need this patch as well as the one on GetNextEvent 
 * because of desk accessories. If you don't patch 
 * SystemEvent then the patch will not apply to events that 
 * are sent to DAs.
 * 
 * If you want to eat the event and not pass it on to the 
 * caller then set returnValue to zero.
 *********************************************************/
pascal short MySystemEvent(EventRecord *theEvent)
{
    PatchGlobalsPtr pgPtr;
    short           returnValue;

    /* find our globals */
    pgPtr = (PatchGlobalsPtr) ((long) StartPatchCode -
      sizeof(PatchGlobals));

    /* call original GNE first */
    returnValue = (*pgPtr->pgOldSE)(theEvent);

    /* do some post-processing */
    CheckKeyCase(theEvent);

    /* return to original caller */
    return (returnValue);
}

/*********************************************************
 * CheckKeyCase:
 * If theEvent was a keyDown or autoKey event, this checks 
 * if both the shiftKey and the capsLock key were down. If 
 * so, it changes theEvent to be a lowercase letter. If not, 
 * nothing is changed.  Also, if either the optionKey or 
 * cmdKey is down then nothing is changed.
 ********************************************************/
void CheckKeyCase(EventRecord *theEvent)
{
    register long   theMods, theMessage;
    register char   theChar;

    if (theEvent->what == keyDown ||
      theEvent->what == autoKey) {
        theMods = theEvent->modifiers;
        theMods &= shiftKey | alphaLock |
          optionKey | cmdKey;
        theMods ^= shiftKey | alphaLock;
        if (!theMods) {
            theMessage = theEvent->message;
            theChar = theMessage & charCodeMask;
            if (theChar >= 'A' && theChar <= 'Z') {
                theMessage &= ~charCodeMask;
                theMessage |= theChar + 'a' - 'A';
                theEvent->message = theMessage;
            }
        }
    }
}
/*********************************************************
 * EndPatchCode:
 * Dummy proc to mark the end of the code for the patches.
 * Make sure all of your patch code is between here and 
 * StartPatchCode.
 *********************************************************/
void EndPatchCode()
{
}
 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Luminar 1.0.2 - Powerful, adaptive, conf...
Luminar is the new full-featured image editor that adapts to the way you edit photos. Over 300 essential tools to fix, edit, and enhance your photos with comfort. The future of photo editing is here... Read more
Slack 2.3.3 - Collaborative communicatio...
Slack is a collaborative communication app that simplifies real-time messaging, archiving, and search for modern working teams. Version 2.3.3: Fixed window zoom jumping back-and-forth OS X 10.9... Read more
WhatRoute 2.0.10 - Geographically trace...
WhatRoute is designed to find the names of all the routers an IP packet passes through on its way from your Mac to a destination host. It also measures the round-trip time from your Mac to the router... Read more
Luminar 1.0.2 - Powerful, adaptive, conf...
Luminar is the new full-featured image editor that adapts to the way you edit photos. Over 300 essential tools to fix, edit, and enhance your photos with comfort. The future of photo editing is here... Read more
WhatRoute 2.0.10 - Geographically trace...
WhatRoute is designed to find the names of all the routers an IP packet passes through on its way from your Mac to a destination host. It also measures the round-trip time from your Mac to the router... Read more
Slack 2.3.3 - Collaborative communicatio...
Slack is a collaborative communication app that simplifies real-time messaging, archiving, and search for modern working teams. Version 2.3.3: Fixed window zoom jumping back-and-forth OS X 10.9... Read more
Lens Blur 1.4.3 - True out-of-focus boke...
Lens Blur transforms your existing photo into true SLR-quality out-of-focus bokeh effect! Everyone needs a gorgeous personalized background for a social profile, blog, Web/UI design, presentation, or... Read more
CleanMyMac 3.6.0 - $39.95
CleanMyMac makes space for the things you love. Sporting a range of ingenious new features, CleanMyMac lets you safely and intelligently scan and clean your entire system, delete large, unused files... Read more
DEVONthink Pro 2.9.8 - Knowledge base, i...
DEVONthink Pro is your essential assistant for today's world, where almost everything is digital. From shopping receipts to important research papers, your life often fills your hard drive in the... Read more
MacFamilyTree 8.1 - Create and explore y...
MacFamilyTree gives genealogy a facelift: modern, interactive, convenient and fast. Explore your family tree and your family history in a way generations of chroniclers before you would have loved.... Read more

Latest Forum Discussions

See All

Amateur Surgeon 4 Guide: Become the worl...
It's time to wield your trusty pizza cutter again, as Amateur Surgeon has returned with a whole fresh set of challenges (and some old, familiar ones, too). Starting anew isn't easy, especially when all you have at your disposal is a lighter, the... | Read more »
Le Parker: Sous Chef Extraordinaire (Ga...
Le Parker: Sous Chef Extraordinaire 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »
Telltale Games really is working on a Gu...
Telltale Games' next episodic adventure is indeed Guardians of the Galaxy. A document tied to the voice actors strike suggested that the project was in the work, but now we have direct confirmation following an announcement at the Game Awards that... | Read more »
Amateur Surgeon returns to iOS and Andro...
Amateur Surgeon and its two sequels disappeared from the App Store some time and it was sad days for all. But now, just in time for the holidays, the Adult Swim favorite makes its joyous return in the shape of Amateur Surgeon 4, a remake with... | Read more »
The best board games on mobile
Sometimes you need to ditch all of the high speed, high action games in favor of something a little more traditional. If you don't feel like parting ways from your mobile device, though, there are still plenty of ways to get that old-school fix.... | Read more »
The best Facebook Messenger Instant Game...
Facebook's new Instant Games is now here, meaning you can play games with your friends directly via Facebook. It's a fun new way to connect with friends, of course, but it's also proving to be a solid gaming experience in its own right, with a... | Read more »
You can now play game's on Facebook...
Facebook launched its new Instant Games platform in an exciting new attempt to engage its user base. As a result, you can now play a number of different games directly through Facebook Messenger. All of these games run with HTML5, meaning you play... | Read more »
Apollo Justice Ace Attorney (Games)
Apollo Justice Ace Attorney 1.00.00 Device: iOS Universal Category: Games Price: $.99, Version: 1.00.00 (iTunes) Description: Court Is Back In Session Star as rookie defense attorney, Apollo Justice, as he visits crime scenes,... | Read more »
KORG iWAVESTATION (Music)
KORG iWAVESTATION 1.0 Device: iOS Universal Category: Music Price: $19.99, Version: 1.0 (iTunes) Description: A revolutionary new world of sound.The Wave Sequence Synthesizer for iPad - KORG iWAVESTATION | Read more »
Don't Grind Guide: Tips for becomin...
Don’t Grind is a surprising, derpy little one touch game with fun hand-drawn graphics. The goal is simple -- get the high score without being chopped to bits. That can be tough when you’re not used to the game, and that’s compounded by the fact... | Read more »

Price Scanner via MacPrices.net

Parallels Toolbox 1.3 for Mac Offers 25 Singl...
Parallels has launched Parallels Toolbox 1.3 for Mac, an upgrade that adds five new utilities to the stand-alone application which was released in August and is available exclusively online at http... Read more
OWC Mercury Elite Pro Dual mini Ultra-Portabl...
OWC has introduced the new OWC Mercury Elite Pro Dual mini, a powerful yet ultra-portable dual-drive RAID solution. The new Mercury Elite Pro Dual mini packs phenomenal performance into a small... Read more
Clearance 13-inch Retina MacBook Pros availab...
B&H Photo has clearance 2015 13″ Retina Apple MacBook Pros available for up to $200 off original MSRP. Shipping is free, and B&H charges NY tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro: $... Read more
Roundup of 2016 13-inch 2.0GHz MacBook Pro sa...
B&H has the non-Touch Bar 13″ MacBook Pros in stock today for $50-$100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.0GHz MacBook Pro Space Gray (MLL42LL/A): $1449 $... Read more
New 13-inch 2.0GHz Space Gray MacBook Pro in...
Adorama has the new 13″ 2.0GHz Space Gray MacBook Pro (non-Touch Bar, MLL42LL/A) in stock for $1499 including a free 3-year AppleCare Protection Plan. Shipping is free, and Adorama charges sales tax... Read more
Finnair Adopts iOS Enterprise iPad Apps from...
Finnair and IBM have announced a first-of-its-kind agreement to utilize iOS enterprise apps from IBM to support the airline’s overall digital transformation. Finnair is focused on Asia-Europe traffic... Read more
Tech21 Launches Evo Go iPhone 7 Case Availabl...
Tech21 has announced the launch of the Evo Go case for Apple iPhone 7 and iPhone 7 Plus, exclusively at T-Mobile. Available online and at participating T-Mobile stores nationwide, Evo Go cases start... Read more
Apple Turns (RED) with More Ways to Join the...
In recognition of World AIDS Day, Apple is offering more ways than ever for customers to join (RED) in its mission to create an AIDS-free generation. Apple is the worlds largest corporate contributor... Read more
Deals on new 15-inch Touch Bar MacBook Pros,...
B&H Photo has new 2016 Apple 15″ Touch Bar MacBook Pro models in stock today with some available for $50 off MSRP, each including free shipping plus NY sales tax only: - 15″ 2.7GHz Touch Bar... Read more
12-inch Retina MacBooks on sale for up to $10...
12-inch Retina MacBooks remain on sale at B&H Photo with models available for up to $100 off MSRP. Shipping is free, and B&H charges NY sales tax only. B&H will also include a free copy... Read more

Jobs Board

*Apple* Retail - Multiple Positions- White P...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Automotive Detailer - *Apple* Used Autos -...
We are currently conductinginterviews and will be accepting applications for a part-time detailer. Apple Used Autos is a great place to work andstart a career. We Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
US- *Apple* Store Leader Program - Apple (Un...
…Summary Learn and grow as you explore the art of leadership at the Apple Store. You'll master our retail business inside and out through training, hands-on Read more
*Apple* & PC Desktop Support Technician...
Apple & PC Desktop Support Technician job in Dallas TX Introduction: We have immediate job openings for several Desktop Support Technicians with one of our most Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.