- Objective: turn a x86 binary executable back into C source code.
- Understand how the compiler turns C into assembly code.
- Low-level OS structures and executable file format.
##Assembly 101
mov eax,2 ; eax = 2
mov ebx,3 ; ebx = 3
add eax,ebx ; eax = eax + ebx
sub ebx, 2 ; ebx = ebx - 2
mox eax, [1234] ; eax = *(int*)1234
mov ebx, 1234 ; ebx = 1234
mov eax, [ebx] ; eax = *ebx
mov [ebx], eax ; *ebx = eax
cmp eax, 2 ; compare eax with 2
je label1 ; if(eax==2) goto label1
ja label2 ; if(eax>2) goto label2
jb label3 ; if(eax<2) goto label3
jbe label4 ; if(eax<=2) goto label4
jne label5 ; if(eax!=2) goto label5
jmp label6 ; unconditional goto label6
First calling a function:
call func ; store return address on the stack and jump to func
The first operations is to save the return pointer:
pop esi ; save esi
Right before leaving the function:
pop esi ; restore esi
ret ; read return address from the stack and jump to it
C code --> Parsing --> Intermediate representation --> optimization --> Low-level intermediate representation --> register allocation --> x86 assembly
For example, the function c:
int foo(int a, int b){
return a+b
}
c = foo(a, b+1)
translates to c = a+b+1
The loop:
for(i=0; i<2; i++){
a[i]=0;
}
becomes
a[0]=0;
a[1]=0;
The loop:
for (i = 0; i < 2; i++) {
a[i] = p + q;
}
becomes:
temp = p + q;
for (i = 0; i < 2; i++) {
a[i] = temp;
}
The variable attributions:
a = b + (z + 1)
p = q + (z + 1)
becomes
temp = z + 1
a = b + z
p = q + z
```
#### Constant folding and propagation
The assignments:
```
a = 3 + 5
b = a + 1
func(b)
```
Becomes:
```
func(9)
```
#### Dead code elimination
Delete unnecessary code:
```
a = 1
if (a < 0) {
printf(“ERROR!”)
}
```
to
```
a = 1
```
### Low-Level Optimizations
#### Strength reduction
Codes such as:
```
y = x * 2
y = x * 15
```
Becomes:
```
y = x + x
y = (x << 4) - x
```
#### Code block reordering
Codes such as :
```
if (a < 10) goto l1
printf(“ERROR”)
goto label2
l1:
printf(“OK”)
l2:
return;
```
Becomes:
```
if (a > 10) goto l1
printf(“OK”)
l2:
return
l1:
printf(“ERROR”)
goto l2
```
#### Register allocation
* Memory access is slower than registers.
* Try to fit as many as local variables as possible in registers.
* The mapping of local variables to stack location and registers is not constant.
#### Instruction scheduling
Assembly code like:
```
mov eax, [esi]
add eax, 1
mov ebx, [edi]
add ebx, 1
```
Becomes:
```
mov eax, [esi]
mov ebx, [edi]
add eax, 1
add ebx, 1
```
---
## Tools Folder
- X86 Win32 Cheat sheet
- Intro X86
- base conversion
- Command line tricks
----
## Other Tools
- gdb
- IDA Pro
- Immunity Debugger
- OllyDbg
- Radare2
- nm
- objdump
- strace
- ILSpy (.NET)
- JD-GUI (Java)
- FFDec (Flash)
- dex2jar (Android)
- uncompyle2 (Python)
- unpackers, hex editors, compilers
---
## Encondings/ Binaries
```
file f1
ltrace bin
strings f1
base64 -d
xxd -r
nm
objcopy
binutils
```
---
## Online References
[Reverse Engineering, the Book]: http:https://beginners.re/
[Intel x86 Assembler Instruction Set Opcode Table]: http:https://sparksandflames.com/files/x86InstructionChart.html
---
## IDA
- Cheat sheet
- [IDA PRO](https://www.hex-rays.com/products/ida/support/download_freeware.shtml)
---
## gdb
- Commands and cheat sheet
```sh
$ gcc -ggdb -o <filename> <filename>.c
```
Starting with some commands:
```sh
$ gdb <program name> -x <command file>
```
For example:
```sh
$ cat command.txt
set disassembly-flavor intel
disas main
```
---
## objdump
Display information from object files: Where object file can be an intermediate file
created during compilation but before linking, or a fully linked executable
```sh
$ objdump -d <bin>
```
----
## hexdump & xxd
For canonical hex & ASCII view:
```
$hexdump -C
```
----
## xxd
Make a hexdump or do the reverse:
```
xxd hello > hello.dump
xxd -r hello.dump > hello
```