Difference between revisions of "Tutorial:CodeInjection Integers"

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== Fractional Multiplier ==
 
== Fractional Multiplier ==
But what if we wanted to be able to multiply by a fractional number (i.e.: "0.5"). Well this can take a bit more, but we can use [[Assembler:Commands:CVTSI2SS|CVTSI2SS]] and [[Assembler:Commands:CVTSS2SI|CVTSS2SI]] to convert the value form an integer to a float and back a gain. Then we can just use [[Assembler:Commands:MULSS|MULSS]] to do the multiplying, but we will need an [[Assembler#Structure|XMM]] [[Assembler#Registers|registry]] to work with. So we will need some extra memory and use [[Assembler:Commands:MOVUPS|MOVUPS]] to save and restore the XMM registry.
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But what if we wanted to be able to multiply by a fractional number (i.e.: "0.5"). Well this can take a bit more, but we can use [[Assembler:Commands:CVTSI2SS|CVTSI2SS]] and [[Assembler:Commands:CVTSS2SI|CVTSS2SI]] to convert the value from an integer to a float and back a gain. Then we can just use [[Assembler:Commands:MULSS|MULSS]] to do the multiplying, but we will need an [[Assembler#Structure|XMM]] [[Assembler#Registers|registry]] to work with. So we will need some extra memory and use [[Assembler:Commands:MOVUPS|MOVUPS]] to save and restore the XMM registry.
 
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Latest revision as of 09:15, 8 July 2020


This tutorial builds on the topic of Code Injection:


Let's say you have an integer and some code that increases the value.

add [eax+10],ecx

What if what writes to the value is only a MOV. Try to find a spot above the write instruction that has an ADD (or a SUB depending on what you want to do).

add ecx,ebx
//...
mov [eax+10],ecx


Hardcoded value[edit]

We could just hardcode a value for this.

add dword ptr [eax+10],(int)100 // #100 //// "#" is a short hand for integer


Editable value[edit]

We could use a label, giving it some memory. And optionally register it so the label can be used on the table as an address.

//...
alloc(someMem, 0x400)
//...
label(someSymbol)
registerSymbol(someSymbol)
//...
someMem:
  //...
  mov ecx,[someSymbol]
  add [eax+10],ecx
  //...
  jmp return
  //...
  someSymbol:
    dd (int)100
//...


Adding a Multiplier[edit]

We could add an editable value like above but use IMUL to add a multiplier to the script.

//...
alloc(someMem, 0x400)
//...
label(someSymbol)
registerSymbol(someSymbol)
//...
someMem:
  //...
  imul ecx,[someSymbol]
  add [eax+10],ecx
  //...
  jmp return
  //...
  someSymbol:
    dd (int)10
//...


Fractional Multiplier[edit]

But what if we wanted to be able to multiply by a fractional number (i.e.: "0.5"). Well this can take a bit more, but we can use CVTSI2SS and CVTSS2SI to convert the value from an integer to a float and back a gain. Then we can just use MULSS to do the multiplying, but we will need an XMM registry to work with. So we will need some extra memory and use MOVUPS to save and restore the XMM registry.

//...
alloc(someMem, 0x400)
//...
label(someSymbol)
registerSymbol(someSymbol)
label(extraStuff)
//...
someMem:
  //...
  movups [extraStuff],xmm0 //// save
  cvtsi2ss xmm0,ecx
  mulss xmm0,[someSymbol]
  cvtss2si ecx,xmm0
  movups xmm0,[extraStuff] //// restore
  //...
  jmp return
  //...
  someSymbol:
    dd (int)10
  extraStuff:
    dd 0 //// Data double-word (4 bytes)
    dd 0
    dq 0 //// Data quad-word (8 bytes)
//...


Calculate a value for a Multiplier[edit]

Let's say we just can't find an ADD or a SUB, and all we have is a MOV.

mov [eax+10],ecx

We can just do some math in the script, to calculate a value for a multiplier.

//...
alloc(someMem, 0x400)
//...
label(someSymbol)
registerSymbol(someSymbol)
//...
someMem:
  //...
  sub ecx,[eax+10]
  imul ecx,[someSymbol]
  add ecx,[eax+10]
  mov [eax+10],ecx
  //...
  jmp return
  //...
  someSymbol:
    dd (int)10
//...


See Also[edit]