ESIL stands for 'Evaluable Strings Intermedate Language'. It aims to describe a Forth-like representation for every opcode. Those representations can be evaluated in order to emulate code. Each element of an esil expression is separated by a comma. The VM can be described as this:
```
while ((word=haveCommand())) {
if (word.isKeyword()) {
esilCommands[word](esil);
} else {
esil.push (evaluateToNumber(word));
}
nextCommand();
}
```
The esil commands are operations that pop values from the stack, performs some calculations and pushes the result in the stack (if any). They aim to cover all common operations done by CPUs, permitting to do binary operations, memory peeks and pokes, spawning a syscall, etc.
#### Use ESIL
```
[0x00000000]> e asm.esil = true
```
Syntax
======
An opcode is translated into a comma separated list of ESIL expressions.
```
xor eax, eax -> 0,eax,=,1,zf,=
```
Memory access is defined by brackets.
```
mov eax, [0x80480] -> 0x80480,[],eax,=
```
Default size is the destination of the operation. In this case 8bits, aka 1 byte.
Conditionals are expressed with the '?' char at the beginning of the expression. This checks if the rest of the expression is 0 or not and skips the next expression if doesn't matches. '$' is the prefix for internal vars.
Syscalls are specially handled by '$' at the beginning of the expression. After that char you have an optional numeric value that specifies the number of syscall. The emulator must handle those expressions and 'simulate' the syscalls. (r_esil_syscall)
Order of arguments
==================
As discussed on irc, current implementation works like this:
```
a,b,- b - a
a,b,/= b /= a
```
This approach is more readable, but it's less stack-friendly
Special instructions
====================
NOPs are represented as empty strings. Unknown or invalid instructions
Syscalls are implemented with the '0x80,$' command. It delegates the execution of the esil vm into a callback that implements the syscall for a specific kernel.
Traps are implemented with the `<trap>,<code>,$$` command. They are used to throw exceptions like invalid instructions, division by zero, memory read error, etc.
Quick analysis
==============
Here's a list of some quick checks to retrieve information from an esil string. Relevant information will be probably found in the first expression of the list.
```
indexOf('[') -> have memory references
indexOf("=[") -> write in memory
indexOf("pc,=") -> modifies program counter (branch, jump, call)
indexOf("sp,=") -> modifies the stack (what if we found sp+= or sp-=?)
equalsTo("") -> empty string, means: nop (wrong, if we append pc+=x)
```
Common operations:
* Check dstreg
* Check srcreg
* Get destinaion
* Is jump
* Is conditional
* Evulate
* Is syscall
CPU Flags
=========
CPU flags are usually defined as 1 bit registers in the RReg profile. and sometimes under the 'flg' register type.
ESIL Flags
==========
ESIL VM have an internal state flags that can are read only and can be used to export those values to the underlaying CPU flags. This is because the ESIL vm defines all the flag changes, while the CPUs only update the flags under certain conditions or specific instructions.
1. No predefined bitness (should be easy to extend them to 128,256 and 512bits, e.g. for MMX, SSE, AVX, Neon)
2. Infinite number (for SSA-form compatibility)
3. Register names have no specific syntax. They are just strings
4. Numbers can be specified in any base supported by RNum (dec, hex, oct, binary ...)
5. Each ESIL backend should have an associated RReg profile to describe the esil register specs
Bitarrays
=========
What to do with them? What about bit arithmetics if use variables instead of registers?
Arithmetics
===========
1. ADD ("+")
2. MUL ("*")
3. SUB ("-")
4. DIV ("/")
5. MOD ("%")
Bit arithmetics
===============
1. AND "&"
2. OR "|"
3. XOR "^"
4. SHL "<<"
5. SHR ">>"
6. ROL "<<<"
7. ROR ">>>"
8. NEG "!"
Floating point
==============
_TODO_
The x86 REP prefix in ESIL
==========================
ESIL specifies that the parsing control-flow commands are in uppercase. Bear in mind that some archs have uppercase register names. The register profile should take care to not reuse any of the following:
```
3,SKIP - skip N instructions. used to make relative forward GOTOs
Those are expressed with the 'TODO' command. which acts as a 'BREAK', but displaying a warning message describing which instruction is not implemented and will not be emulated.
To ease esil parsing we should have a way to express introspection expressions to extract the data we want. For example. We want to get the target address of a jmp.
The parser for the esil expressions should be implemented in an API to make it possible to extract information by analyzing the expressions easily.
```
> ao~esil,opcode
opcode: jmp 0x10000465a
esil: 0x10000465a,rip,=
```
We need a way to retrieve the numeric value of 'rip'. This is a very simple example, but there will be more complex, like conditional ones and we need expressions to get:
- opcode type
- destination of jump
- condition depends on
- all regs modified (write)
- all regs accessed (read)
API HOOKS
=========
It is important for emulation to be able to setup hooks in the parser, so we can extend the parser to implement the analysis without having to write the parser again and again. This is, every time an operation is going to be executed we call a user hook which can be used to determine if rip is changing or if the instruction updates the stack.
Later, at this level we can split that callback into several ones to have an event based analysis api that may be extended in js like this:
esil.on('regset', function(){..
esil.on('syscall', function(){esil.regset('rip'
we have already them. see hook_flag_read() hook_execute() hook_mem_read() ...
return true if you want to override the action taken for a callback. for example. avoid mem reads in a region or mem writes to make all memory read only.
return false or 0 if you want to trace esil expression parsing. aka emulation
..
Other operations that require bindings to external functionalities to work. In this case r_ref and r_io. This must be defined when initializing the esil vm.
