The Official Radare2 Book — страница 25 из 64

A target opcode is translated into a comma separated list of ESIL expressions.

xor eax, eax -> 0,eax,=,1,zf,=

Memory access is defined by brackets operation:

mov eax, [0x80480] -> 0x80480,[],eax,=

Default operand size is determined by size of operation destination.

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

The ? operator uses the value of its argument to decide whether to evaluate the expression in curly braces.

   1. Is the value zero? -> Skip it.

   2. Is the value non-zero? -> Evaluate it.

cmp eax, 123 -> 123,eax,==,$z,zf,=

jz eax -> zf,?{,eax,eip,=,}

If you want to run several expressions under a conditional, put them in curly braces:

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

Whitespaces, newlines and other chars are ignored. So the first thing when processing a ESIL program is to remove spaces:

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

Syscalls need special treatment. They are indicated by '$' at the beginning of an expression. You can pass an optional numeric value to specify a number of syscall. An ESIL emulator must handle syscalls. See (r_esil_syscall).

Arguments Order for Non-associative Operations

As discussed on IRC, the current implementation works like this:

a,b,- b - a

a,b,/= b /= a

This approach is more readable, but it is less stack-friendly.

Special Instructions

NOPs are represented as empty strings. As it was said previously, interrupts are marked by '$' command. For example, '0x80,$'. It delegates emulation from the ESIL machine to a callback which implements interrupt handler for a specific OS/kernel/platform.

Traps are implemented with the TRAP command. They are used to throw exceptions for invalid instructions, division by zero, memory read error, or any other needed by specific architectures.

Quick Analysis

Here is 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 ex: $z

indexOf("$") -> syscall ex: 1,$

indexOf("TRAP") -> can trap

indexOf('++') -> has iterator

indexOf('--') -> count to zero

indexOf("?{") -> conditional

equalsTo("") -> empty string, aka nop (wrong, if we append pc+=x)

Common operations:

   • Check dstreg

   • Check srcreg

   • Get destinaion

   • Is jump

   • Is conditional

   • Evaluate

   • Is syscall

CPU Flags

CPU flags are usually defined as single bit registers in the RReg profile. They are sometimes found under the 'flg' register type.

Variables

Properties of the VM variables:

   1. They have no predefined bit width. This way it should be easy to extend them to 128, 256 and 512 bits later, e.g. for MMX, SSE, AVX, Neon SIMD.

   2. There can be unbound number of variables. It is done 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.

Bit Arrays

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 Unit Support

At the moment of this writing, ESIL does not yet support FPU. But you can implement support for unsupported instructions using r2pipe. Eventually we will get proper support for multimedia and floating point.

Handling x86 REP Prefix in ESIL

ESIL specifies that the parsing control-flow commands must be uppercase. Bear in mind that some architectures have uppercase register names. The corresponding register profile should take care not to 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,--,0,GOTO

Unimplemented/Unhandled Instructions

Those are expressed with the 'TODO' command. They act as a 'BREAK', but displays a warning message describing that an instruction is not implemented and will not be emulated. For example:

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

ESIL Disassembly Example:

[0x1000010f8]> e asm.esil=true

[0x1000010f8]> pd $r @ entry0

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,=

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 that we want. For example, we may want to get the target address of a jump. The parser for ESIL expressions should offer 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 are more complex, like conditional ones. We need expressions to be able to get:

   • opcode type

   • destination of a 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 it to implement analysis without having to change it again and again. That is, every time an operation is about to be executed, a user hook is called. It can be used for example to determine if RIP is going to change, or if the instruction updates the stack. Later, we can split that callback into several ones to have an event-based analysis API that may be extended in JavaScript like this:

esil.on('regset', function(){..

esil.on('syscall', function(){esil.regset('rip'

For the API, see the functions hook_flag_read(), hook_execute() and hook_mem_read(). A callback should return true or 1 if you want to override the action that it takes. For example, to deny memory reads in a region, or voiding memory writes, effectively making it read-only. Return false or 0 if you want to trace ESIL expression parsing.

Other operations 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,=

Data and Code Analysis