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ESIL

condret edited this page · 34 revisions

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.

movb $0, 0x80480     ->   0,0x80480,=[1]

Conditionals are expressed with the '?' char at the begining 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.

cmp eax, 123  ->   123,eax,==,%z,zf,=
jz eax        ->   zf,?{,eax,eip,=,}

So.. if you want to run more than one expression under a conditional, you'll have to write it

zf,?{,eip,esp,=[],eax,eip,=,%r,esp,-=,}

The whitespace, newlines and other chars are ignored in esil, so the first thing to do is:

esil = r_str_replace (esil, " ", "", R_TRUE);

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-=?)
indexOf("=")       ->    retrieve src and dst
indexOf(":")       ->    unknown esil, raw opcode ahead
indexOf("%")       ->    accesses internal esil vm flags
indexOf("$")       ->    syscall
indexOf("$$")      ->    can trap
indexOf('++')      ->    has iterator
indexOf('--')      ->    count to zero
indexOf("?{")      ->    conditional
indexOf("LOOP")    ->    is a loop (rep?)
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.

Those internal flags are prefixed by the '%' character.

z - zero flag, only set if the result of an operation is 0
b - borrow, this requires to specify from which bit (example: %b4 - checks if borrow from bit 4)
c - carry, same like above (example: %c7 - checks if carry from bit 7)
p - parity
r - regsize ( asm.bits/8 )

Variables

  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
3,GOTO   - goto instruction 3
LOOP     - alias for 0,GOTO
BREAK    - stop evaluating the expression
STACK    - dump stack contents to screen
CLEAR    - clear stack

Usage example:

rep cmpsb

cx,!,?{,BREAK,},esi,[1],edi,[1],==,?{,BREAK,},esi,++,edi,++,cx,--,LOOP

Unimplemented/unhandled instructions

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.

For example:

fmulp ST(1), ST(0)      =>      TODO,fmulp ST(1),ST(0)

Disassembly example:

[0x1000010f8]> e asm.esil=true
[0x1000010f8]> pd $r @ entry0
   ;      [0] va=0x1000010f8 pa=0x000010f8 sz=13299 vsz=13299 rwx=-r-x 0.__text
            ;-- section.0.__text:
            0x1000010f8    55           8,rsp,-=,rbp,rsp,=[8]
            0x1000010f9    4889e5       rsp,rbp,=
            0x1000010fc    4883c768     104,rdi,+=
            0x100001100    4883c668     104,rsi,+=
            0x100001104    5d           rsp,[8],rbp,=,8,rsp,+=                                          ┌─< 0x100001105    e950350000   0x465a,rip,= ;[1]
        │   0x10000110a    55           8,rsp,-=,rbp,rsp,=[8]
        │   0x10000110b    4889e5       rsp,rbp,=                                                       │   0x10000110e    488d4668     rsi,104,+,rax,=
        │   0x100001112    488d7768     rdi,104,+,rsi,=
        │   0x100001116    4889c7       rax,rdi,=
        │   0x100001119    5d           rsp,[8],rbp,=,8,rsp,+=                                         ┌──< 0x10000111a    e93b350000   0x465a,rip,= ;[1]
       ││   0x10000111f    55           8,rsp,-=,rbp,rsp,=[8]
       ││   0x100001120    4889e5       rsp,rbp,=
       ││   0x100001123    488b4f60     rdi,96,+,[8],rcx,=
       ││   0x100001127    4c8b4130     rcx,48,+,[8],r8,=                                              ││   0x10000112b    488b5660     rsi,96,+,[8],rdx,=
       ││   0x10000112f    b801000000   1,eax,= ;  0x00000001
       ││   0x100001134    4c394230     rdx,48,+,[8],r8,==,cz,?=
      ┌───< 0x100001138    7f1a         sf,of,!,^,zf,!,&,?{,0x1154,rip,=,} ;[2]
     ┌────< 0x10000113a    7d07         of,!,sf,^,?{,0x1143,rip,} ;[3]
     ││││   0x10000113c    b8ffffffff   0xffffffff,eax,= ;  0xffffffff                              ┌─────< 0x100001141    eb11         0x1154,rip,= ;[2]
    │└────> 0x100001143    488b4938     rcx,56,+,[8],rcx,=
    │ │││   0x100001147    48394a38     rdx,56,+,[8],rcx,==,cz,?=

Introspection

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.

  • Io Get/Set Out ax, 44 44,ax,:ou
  • Selectors (cs,ds,gs...) Mov eax, ds:[ebp+8] Ebp,8,+,:ds,eax,=
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