Difference between revisions of "Tutorial:Stack"

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[[Category:Tutorial]]
 
[[Category:Auto Assembler]]
 
{{DISPLAYTITLE:Tutorial - The Stack}}
 
If you're unfamiliar with registries please start here: [[Assembler#Registers|Registers]]
 
 
 
So what is ''the stack''?
 
 
 
Well I hate to use the word in the definition, but it's just a metaphorical ''stack of bytes''; or an abstract data type that serves as a collection of elements, with two principal operations.
 
;PUSH
 
:Adds an element to the collection.
 
;POP
 
:Removes the most recently added element that was not yet removed.
 
 
 
The stack is primarily used to store values for a process. It uses a '''LIFO''' (last in, first out) behavior. So if we [[Assembler:Commands:PUSH|PUSH]] value A onto the stack and then [[Assembler:Commands:PUSH|PUSH]] value B, when we [[Assembler:Commands:POP|POP]] it will be value B first then value A with the second [[Assembler:Commands:POP|POP]].[https://wikipedia.org/wiki/Stack_(abstract_data_type)]
 
 
 
[[File:Lifo_stack.png||LIFO|border]]
 
 
 
<br>
 
== Working with the Stack ==
 
So let's just dig in, if we have some code like this.
 
<pre>
 
push 123ABC
 
push 00DEAD
 
push 00BEEF
 
 
 
pop dword ptr [TestVals]
 
pop dword ptr [TestVals+4]
 
pop dword ptr [TestVals+8]
 
</pre>
 
 
 
And if we assemble this in some memory.<br>
 
[[File:TutorialStack.01.png||Assembled opcode|border]]
 
 
 
So let's set a breakpoint and watch the stack as we step though the opcode.
 
:Note: You can select a line of opcode in the memory view form and press ''F5'' to set a breakpoint.
 
:Note: Step though opcode in the memory view form by pressing ''F7'', after a breakpoint has been hit.
 
 
 
:Note: to view the stack you may need to select it for viewing.
 
[[File:StackView.01.png||stack view|border]]
 
 
 
<br>
 
=== Code step 1 ===
 
[[File:TutorialStack.02.png||step 1|border]]
 
 
 
=== Code step 2 ===
 
So it's here that we start to see the values on the stack.
 
[[File:TutorialStack.03.png||step 2|border]]
 
 
 
=== Code step 3 ===
 
[[File:TutorialStack.04.png||step 3|border]]
 
 
 
=== Code step 4 ===
 
[[File:TutorialStack.05.png||step 4|border]]
 
 
 
=== Code step 5 ===
 
And here we can start to see the values being popped in the reverse order that they were pushed.
 
[[File:TutorialStack.06.png||step 5|border]]
 
 
 
=== Code step 6 ===
 
[[File:TutorialStack.07.png||step 6|border]]
 
 
 
=== Code step 7 ===
 
[[File:TutorialStack.08.png||step 7|border]]
 
 
 
 
 
And that's really all there is to the basics of the stack.
 
 
 
The thing to remember is that if you push in some injected code then you will need to pop in order to clean (or sanitize) the stack.
 
 
 
<br>
 
== Working with the registries ==
 
We can [[Assembler:Commands:PUSH|PUSH]] and [[Assembler:Commands:POP|POP]] most registries, this is useful for saving and restoring stuff in injected code. The x86 architecture has 8 General-Purpose Registers (GPR), 6 Segment Registers, 1 Flags Register and an Instruction Pointer. 64-bit x86 has additional registers.[https://wikibooks.org/wiki/X86_Assembly/X86_Architecture]
 
<pre>
 
push eax
 
push ebx
 
push ecx
 
push edx
 
push esi
 
push edi
 
push ebp
 
push esp
 
 
 
pop esp
 
pop ebp
 
pop edi
 
pop esi
 
pop edx
 
pop ecx
 
pop ebx
 
pop eax
 
</pre>
 
 
 
There is the [[Assembler:Commands:PUSHAD|PUSHAD]] and [[Assembler:Commands:POPAD|POPAD]] to save and restore all the generic registries. So the above code could just be written like this.
 
<pre>
 
pushad
 
 
 
popad
 
</pre>
 
:Note: [[Assembler:Commands:PUSHAD|PUSHAD]] and [[Assembler:Commands:POPAD|POPAD]] are not available in 64 bit mode.
 
 
 
<br>
 
== Working with the Flags registry ==
 
We can even [[Assembler:Commands:PUSH|PUSH]] and [[Assembler:Commands:POP|POP]] the Flags registry, using [[Assembler:Commands:PUSHF|PUSHF]] and [[Assembler:Commands:POPF|POPF]] (for 16 bit registry). This allows us to save and restore the Flags when doing compares (conditional opcodes). The FLAGS register is the status register in Intel x86 microprocessors that contains the current state of the processor. This register is 16 bits wide. Its successors, the EFLAGS and RFLAGS registers, are 32 bits and 64 bits wide, respectively. The wider registers retain compatibility with their smaller predecessors.[https://wikipedia.org/wiki/FLAGS_register]
 
:Note: when in 32 bit mode you should use [[Assembler:Commands:PUSHFD|PUSHFD]] and [[Assembler:Commands:POPFD|POPFD]] to manipulate the full 32 bits of the EFlags registry.
 
:Note: when in 64 bit mode you should use [[Assembler:Commands:PUSHFQ|PUSHFQ]] and [[Assembler:Commands:POPFQ|POPFQ]] to manipulate the full 32 bits of the EFlags registry.
 
<pre>
 
pushfd
 
cmp eax,ebx
 
movcc eax,ecx
 
popfd
 
</pre>
 
 
 
<br>
 
== See Also ==
 
{{TutorialsCE}}
 
 
 
<br>
 
== Sources ==
 
# [https://wikipedia.org/wiki/Stack_(abstract_data_type) wikipedia.org/wiki/Stack_(abstract_data_type)]
 
# [https://wikibooks.org/wiki/X86_Assembly/X86_Architecture wikibooks.org/wiki/X86_Assembly/X86_Architecture]
 
# [https://wikipedia.org/wiki/FLAGS_register wikipedia.org/wiki/FLAGS_register]
 

Revision as of 11:26, 16 March 2019

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