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RE🌾ED

notes about reverse engineering and exploit development

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REED | PTEH

This readme covers the absolute basics while more technical topics (linked throughout) are covered in seperate files. It's a good idea to read and understand this page entirely before delving into the specifics.

A couple of conventions to make orientation a bit easier:

toc

computer architectures

The following two subsections show the minimal, archetypal representation of both architectures. Caches (and for that matter cache levels) are ignored.

TODO source this section

Von Neumann

See [edvac].

Somewhat controversially named solely after John von Neumann this is still the base for most common CPU designs.

architecture-von-neumann.jpg

  • note the following:
    • data and instructions are stored in the same memory
    • data and instructions are transported over the same bus

Harvard

architecture-harvard.jpg

  • note the following:
    • data memory and instruction memory are separated
    • they travel over separate buses

modern processors

Most modern CPUs use the same memory for data and instructions, like in the Von Neumann architecture. To avoid the Von Neumann bottleneck multiple levels of caches are added between CPU and memory (L1-Ln cache) and a seperate cache for instructions and data is used in the lowest level (similar to the Harvard architecture).

The resulting architecture is called Modified Harvard or to be more specific split-cache/almost Von Neumann architecture.

As such modern CPUs are situated somewhere between a pure Von Neumann and a pure Harvard architecture.
The important takeaway here is that data and instructions are stored in the same place!

consequences

The - by the Von Neumann architecture - historically caused consolidation of data (that can be freely written by the process) and instructions (that are executed by the CPU) in the same memory has led to countless memory corruption exploits and in turn a myriad of countermeasures.

registers

As seen in the drawings of the Von Neumann and Harvard architectures, registers are part of the CPU. They store a small amount of data and instructions that the CPU can immediately operate on. As such they are much faster than any other form of storage (HDDs, SSDs, RAM, Cache).

Some registers might be read only or have a specific hardware function.

the instruction pointer

One important special purpose register is the one that stores the instruction pointer or program counter. The instruction pointer always points at the instruction that the CPU will execute next. As an internal register it is not usually directly read or writeable. (Some architectures such as 32-bit ARM do expose it.)

memory

Memory refers to fast, non persistant storage. Nowadays, (main) memory is usually RAM.

Bit and Byte order

See [inteldev, vol. 1, ch. 1, p. 5f].

There are several ways to store multi-Byte data in memory. Today, the two most common are:

  • Little Endian (used by x86_32, x86_64)
  • Big Endian (AVR32, z/Architecture)

ARM, IA-64 and PowerPC support both for data and instructions.

Endiannes only comes into play when storing multi-Byte data. Big endian stores the most significant Byte first. Little endian stores the least significan Byte first.

The big/little refer to the end that is stored first.

datastructure-byte-order.jpg

  • note the following:
    • higher addresses are towards the top
    • higher addresses are towards the left
    • each Byte has its own address
    • the first Byte (Byte 0) is stored first (it gets the lowest address)

Bits can be stored in two ways:

  • most significant bit first
  • least significant bit first

Since the smallest addressable unit is one Byte it usually does not matter. (Except when working with bitfields or serializing data Bit by Bit via SPI/I2C.) That said Bit order usually follows the Byte order of the system.

bit-order.jpg

  • note the following:
    • both Byte represent the same number (13 in base 10)
    • the drawing shows the order the bits would be stored in (starting left)
      • the leftmost bit gets the lowest address

virtual address space

See [pracbin, p. 27ff].

Modern operating systems create a virtual address space for each process. Behind the scenes the OS maps the virtual address space to real memory (or disk space). This is not visible to the process itself.

After setting up the VAS the interpreter is mapped into the newly created userspace.
The interpreter's job is it to perform the required relocations and start our program (by jumping to its entry point).

memory layout

See [compsec, p. 350].

This section assumes an x86_32 Linux with a 1GiB/3GiB Kernel/userspace split.

process-memory-layout.jpg

  • note the following:
    • high memory addresses are at the top (Intel notational convention, [inteldev, vol. 1, ch. 1, p. 5f])
    • the stack and heap both grow into free memory
    • the heap grows up
    • the stack grows down!
      • the more data is on the stack, the lower the last address
    • the size of argc, argv, and the environment influence the offset for addresses on the stack!
      • potential source of confusion when debugging or developing exploits
        (environment change results in address offset change)
    • 0xFFFFFFFF to 0xC0000000 is the kernelspace
    • 0xBFFFFFFF to 0x00000000 is the userspace
    • CPU instructions (.text, shared libs and the interpreter)
      and data (all the other sections) reside in the same memory

the stack

A stacks is a dynamic LIFO (last in, first out) data structure. It can be primarily interacted with in two ways:

  • push: add one item to the end
  • pop: remove the last item from the end (and put it somewhere)

"The" stack often refers to a specific instance of that data structure also known as the hardware stack.
Depending on the language used, its function is to provide a temporary space for local variables in the current scope, passing parameters between calls, storing return addresses and more.

Hardware stacks typically grow down which is counterintuitive to the name.

You can think of it as a stack of magnets on the ceiling:

the-stack-push-pop.jpg

  • note the following:
    • at the start the stack is empty (1.)
    • after we have pushed variable a the stack contains a single item (2.)
    • after we have pushed variable b the stack contain two items (3.)
      • at this point we have lost direct access to variable a (at least to pop it)
      • to pop variable a we first have to pop variable b
    • the first pop returns variable b to us and removes it from the stack (4.)
    • the second pop returns variable a, the stack is now empty again (5.)
    • we don't get to chose which item to pop, it's always the last one added!

Let's take a look at what is specifically stored on the stack.

the stack at <main+0>

This section assumes an x86_64 Linux. The following shows a partial dump of the stack at the start of main():

preparing for the stack dump:

# we will use the executable from the later 'stages of compilation' section.
# at this point it does not particularly matter though:
gdb a.out

# (gdb)
break *main
run "passed_along_argument"
# at this point we take a look at the stack.

stack-dump-main.jpg

  • note the following:
    • as we go further down the stack, memory addresses go down as well (the stack grows down!)
    • the lower the data on the screen, the later it was pushed to the stack
    • (a.) shows part of the environment, such as which shell we've used or the default editor
    • (b.) is the argument we started our program with (argv[1])
    • (c.) is the name of the executable (argv[0])
      • arrays grow up: argv[0] is further down the stack than argv[1]!
    • just after (c.) we can see argc is 2 (the size of our argv array)
    • (d.) contains the latest return addresses
      • __libc_start_main + 243 is the operation that will be executed once we return from main()

stack frames

The stack frame for the current function is the part of the stack that is used to store local variables for the current function scope.

The current stack frame is delimited by two pointers:

  1. The base pointer, that points to the start of the stack frame.
  2. And the frame pointer that points to the end of the current frame.

If a function calls another function a new stack frame is created and the pointers are updated.
As functions return to their calling function, stack frames are removed and the stack shrinks again.

In this section we will take a look at how this happens one assembly instruction at a time.

preparing for stepping through the creation of a stack frame:

# we will use the 'stages of compilation' binary again.
# compared to the previous example we don't pass any arguments. this will
# result in different addresses, even if everything else stays the same.
gdb a.out

# (gdb)
start # shortcut for `break main*` plus `run`
disas main # disassemble main function.

dissassembled main via (gdb) disas main:

=> 0x0000555555555160 <+0>:	endbr64 
   0x0000555555555164 <+4>:	push   rbp
   0x0000555555555165 <+5>:	mov    rbp,rsp
   0x0000555555555168 <+8>:	sub    rsp,0x20
   0x000055555555516c <+12>:	mov    DWORD PTR [rbp-0x4],edi
   0x000055555555516f <+15>:	mov    QWORD PTR [rbp-0x10],rsi
   0x0000555555555173 <+19>:	mov    QWORD PTR [rbp-0x18],rdx
   0x0000555555555177 <+23>:	mov    eax,0x0
   0x000055555555517c <+28>:	call   0x555555555149 <greet>
   0x0000555555555181 <+33>:	mov    eax,0x0
   0x0000555555555186 <+38>:	leave  
   0x0000555555555187 <+39>:	ret
  • note the following:
    • => marks the instruction, that will be executed next
    • <+n> shows the offset of an instruction from the start of the function
      • in our case main+28 would be the call instruction
    • main+4, main+5 and main+8 are what is called the function prologue

The following images show the stack before we have executed any instructions (in main):

stack-frame-main-0.jpg

note the following:

  • the base pointer (rbp) points to 0x0
    • on entry GDB sets all registers (except rsp and rip) to zero
      • you can observe this behaviour with (gdb) starti
    • if we had ran the executable without a debugger it would contain a valid address
  • the stack pointer (rsp) points to the end of the previous frame
  • the most recently pushed item is a return address
    • it points to an instruction that is part of the C standard library
    • when main() exits, the program will continue from here

Now we repeatedly call stepi and see how the stack grows during the course of the program.

stack-frame-main-function-prologue-a.jpg

  • note the following:
    • (1.) after executing push rbp the stack grows by one word
      • a word is traditionally the smallest addressable size of memory
      • 1 word = 8 Byte = 64 bit (we're running on x86_64 after all)
      • we do this so we can restore the old base pointer
    • (1.) at the same time the stack pointer is updated to point to the new end of our frame
    • (2.) after executing mov rbp, rsp both pointers point at the same address (the backup)

If we take one more stepi we'll be the farthest away from the top we've ever been!

stack-frame-main-function-prologue-b.jpg

  • note the following:
    • (3.) after executing sub rsp,0x20 the stack pointer is updated and the stack grows by 32 Byte
      • we are substracting from the stack pointer because the stack grows down: 0xC0 - 0x20 == 0xA0
      • the memory is not initialized in any way, whatever was there before still is

Instructions main+12, main+15and main+19 write data to our new space on the stack frame. Instruction main+23 only touches a register.

Let's skip ahead to main+28, the function call.

stack-frame-main-greet.jpg

  • note the following:
    • (4.) as part of executing call 0x555555555149, the address for main+33 is pushed to the stack
      • this is the return address for the instruction that is next in line after we return from greet()
    • a call is basically an atomic push <address_after_call> plus jmp <target>

If we would continue the execution of greet() the process would be the same as for main():

  • backup the base pointer to the stack
  • move the base and stack pointers to grow the frame
  • push a return address as we call another function

programming paradigms

imperative

how to change the state

  • Forth
  • assembly

procedural

gouping instructions into procedures

  • C

object oriented

state and instructions are grouped together

  • C++

declerative

declare desired results

stack-based

  • Reverse Polish notation

stages of compilation

typing

  • Forth (typeless)
  • C (static)
  • Python (dynamic, duck)

See [pracbin, p. 12ff].

This section assumes compilation for the x86_64 architecture on Linux.

All intermediary files can be found in the stages-of-compilation directory.

Let's go through the stages of compilation step by step. We'll use the following example:

stages.c

#include <stdio.h>
#include <stdlib.h>

#define FORMAT_STRING "%s"
#define MESSAGE "hello, world!\n"

void greet() {
  printf(FORMAT_STRING, MESSAGE);
}

int main(int argc, char** argv, char** envp)
{
  greet();
  return EXIT_SUCCESS;
}
  • note the following:
    • using a main() signature with envp does not conform to POSIX but is widely supported by Unix-like systems and mentioned as a common alternative in the C standard ([c11, Annex J.5.1, p. 575])

run through all stages and save intermediary files:

uname -a
# Linux ubuntu 5.4.0-48-generic #52-Ubuntu SMP Thu Sep 10 10:58:49 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

gcc -masm=intel --save-temps stages.c

tree
# .
# ├── a.out
# ├── stages.c
# ├── stages.i
# ├── stages.o
# └── stages.s

stages-of-compilation.jpg

  • note the following:
    • produced filenames assume compilation with --save-temps
    • on macOS gcc is llvm in disguise and will produce an additional .bc bitcode file

preprocessor

In this first stage the preprocessor does the following:

  • all included headers (#include) and our source file are (recursively) concatenated in place
  • preprocessor macros (#define) are expanded

stages.i (partial)

# 1 "stages.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 31 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4

/* ... */

typedef signed char __int8_t;
typedef unsigned char __uint8_t;

/* ... */

extern FILE *stdin;
extern FILE *stdout;
extern FILE *stderr;

/* ... */

# 7 "stages.c"
void greet() {
  printf("%s", "hello, world!\n");
}

int main(int argc, char** argv, char** penv)
{
  greet();
  return 
# 14 "stages.c" 3 4
        0
# 14 "stages.c"
                    ;
}
  • note the following:
    • our 15 line program ballooned to 1837 lines (wc -l stages.i)
      • all neccessary (uncompiled) code for the following stages is now contained within
    • if you grep the file for include or define directives there won't be any (egrep '#include|#define' stages.i)
    • the greet() function no longer contains the macros for our strings but the strings themselves

compiler

In the compilation stage our concatenated source is translated into assembly language. Depending on flags more or less optimization takes place (-O0 to -O3).

stages.s (Intel syntax)

	.file	"stages.c"
	.intel_syntax noprefix
	.text
	.section	.rodata
.LC0:
	.string	"hello, world!"
	.text
	.globl	greet
	.type	greet, @function
greet:
.LFB6:
	.cfi_startproc
	endbr64
	push	rbp
	.cfi_def_cfa_offset 16
	.cfi_offset 6, -16
	mov	rbp, rsp
	.cfi_def_cfa_register 6
	lea	rdi, .LC0[rip]
	call	puts@PLT
	nop
	pop	rbp
	.cfi_def_cfa 7, 8
	ret
	.cfi_endproc
.LFE6:
	.size	greet, .-greet
	.globl	main
	.type	main, @function
main:
.LFB7:
	.cfi_startproc
	endbr64
	push	rbp
	.cfi_def_cfa_offset 16
	.cfi_offset 6, -16
	mov	rbp, rsp
	.cfi_def_cfa_register 6
	sub	rsp, 32
	mov	DWORD PTR -4[rbp], edi
	mov	QWORD PTR -16[rbp], rsi
	mov	QWORD PTR -24[rbp], rdx
	mov	eax, 0
	call	greet
	mov	eax, 0
	leave
	.cfi_def_cfa 7, 8
	ret
	.cfi_endproc
.LFE7:
	.size	main, .-main
	.ident	"GCC: (Ubuntu 9.3.0-10ubuntu2) 9.3.0"
	.section	.note.GNU-stack,"",@progbits
	.section	.note.gnu.property,"a"
	.align 8
	.long	 1f - 0f
	.long	 4f - 1f
	.long	 5
0:
	.string	 "GNU"
1:
	.align 8
	.long	 0xc0000002
	.long	 3f - 2f
2:
	.long	 0x3
3:
	.align 8
4:
  • note the following:
    • if not told otherwise (with -masm=intel) gcc creates assembly with AT&T syntax
    • constants and variables have symbolic names and not just addresses (.LC0 for the hello world string)
    • constants and variables have types
    • functions are easily identified by their labels (greet:, main:)
    • calls to functions happen via names and not addresses (call puts@PLT)
    • our call to printf was optimized to puts
    • there's not much left of the 1.8k lines from the preprocessor step

assembler

Now we create a binary file for the first time. But while this object file contains machine code it is not quite executable just yet.

inspecting the object file:

file stages.o
# stages.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped

xxd stages.o | head -n 1 # hex dump.
# 00000000: 7f45 4c46 0201 0100 0000 0000 0000 0000  .ELF............
  • note the following:
    • the file utility tells us a couple of interesting things:
      • ELF stands for Executable and Linkable Format, a file format for executables and object files (among others)
      • 64-bit since our object file is targeted at x86_64
      • LSB stands for Least Significant Byte first (when storing integers)
      • relocatable since object files don't have a fixed address in memory
      • not stripped means that debug symbols are present
    • each object files is compiled on its own and thus has no idea of memory addresses of other object files
    • ELF files start with a bit of magic, the bytes: 0x7F and then the ASCII characters E, L, F

trying to execute an object file

chmod +x stages.o
./stages.o
# -bash: ./stages.o: cannot execute binary file: Exec format error

That did not work, let's see why:

C and other high level languages refer to functions and variables with human readable (symbolic) names. This makes it easier for the programmer. The CPU on the other hand refers to them directly via memory addresses.

A symbol table is a mapping between the two.

symbol table via readelf --syms stages.o

Symbol table '.symtab' contains 14 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS stages.c
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    3
     4: 0000000000000000     0 SECTION LOCAL  DEFAULT    4
     5: 0000000000000000     0 SECTION LOCAL  DEFAULT    5
     6: 0000000000000000     0 SECTION LOCAL  DEFAULT    7
     7: 0000000000000000     0 SECTION LOCAL  DEFAULT    8
     8: 0000000000000000     0 SECTION LOCAL  DEFAULT    9
     9: 0000000000000000     0 SECTION LOCAL  DEFAULT    6
    10: 0000000000000000    23 FUNC    GLOBAL DEFAULT    1 greet
    11: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND puts
    12: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND _GLOBAL_OFFSET_TABLE_
    13: 0000000000000017    40 FUNC    GLOBAL DEFAULT    1 main
  • note the following:
    • for relocatables most symbols point to just zeroes
      • this is because before the linking stage it's unclear where they'll land in memory/the file
      • we will have to resolve them before we can run our binary

linker

In this stage all object files will be linked together. The result is a single executable. The process is as follows:

  1. merge all object files into a single executable
  2. resolve (static) symbolic references to now known fixed locations

Static libraries (.a) are merged into the executable. Dynamic/shared libraries (.so) are left unresolved. The dynamic linker (interpreter) will resolve these at runtime.

inspecting the executable:

file a.out
# a.out: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=41087481fb19eebf518ec7ff727fde7395cdc927, for GNU/Linux 3.2.0, not stripped

# let's finally run it:
./a.out
# hello, world!
  • note the following:
    • pie Position Independent Executable, code does not rely on being located in a specific place in memory
      • you can ignore this for now, we'll talk about it later
    • executable instead of relocatable, which means we can actually run it
    • dynamically linked at least some of the used libraries are shared ones
    • interpreter [...] which dynamic linker will be used to resolve shared libraries

symbol table via readelf --syms a.out

Symbol table '.dynsym' contains 7 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_deregisterTMCloneTab
     2: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND puts@GLIBC_2.2.5 (2)
     3: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_main@GLIBC_2.2.5 (2)
     4: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
     5: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_registerTMCloneTable
     6: 0000000000000000     0 FUNC    WEAK   DEFAULT  UND __cxa_finalize@GLIBC_2.2.5 (2)

Symbol table '.symtab' contains 66 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000318     0 SECTION LOCAL  DEFAULT    1 
     2: 0000000000000338     0 SECTION LOCAL  DEFAULT    2 
     3: 0000000000000358     0 SECTION LOCAL  DEFAULT    3 
     4: 000000000000037c     0 SECTION LOCAL  DEFAULT    4 
     5: 00000000000003a0     0 SECTION LOCAL  DEFAULT    5 
     6: 00000000000003c8     0 SECTION LOCAL  DEFAULT    6 
     7: 0000000000000470     0 SECTION LOCAL  DEFAULT    7 
     8: 00000000000004f2     0 SECTION LOCAL  DEFAULT    8 
     9: 0000000000000500     0 SECTION LOCAL  DEFAULT    9 
    10: 0000000000000520     0 SECTION LOCAL  DEFAULT   10 
    11: 00000000000005e0     0 SECTION LOCAL  DEFAULT   11 
    12: 0000000000001000     0 SECTION LOCAL  DEFAULT   12 
    13: 0000000000001020     0 SECTION LOCAL  DEFAULT   13 
    14: 0000000000001040     0 SECTION LOCAL  DEFAULT   14 
    15: 0000000000001050     0 SECTION LOCAL  DEFAULT   15 
    16: 0000000000001060     0 SECTION LOCAL  DEFAULT   16 
    17: 0000000000001208     0 SECTION LOCAL  DEFAULT   17 
    18: 0000000000002000     0 SECTION LOCAL  DEFAULT   18 
    19: 0000000000002014     0 SECTION LOCAL  DEFAULT   19 
    20: 0000000000002060     0 SECTION LOCAL  DEFAULT   20 
    21: 0000000000003db8     0 SECTION LOCAL  DEFAULT   21 
    22: 0000000000003dc0     0 SECTION LOCAL  DEFAULT   22 
    23: 0000000000003dc8     0 SECTION LOCAL  DEFAULT   23 
    24: 0000000000003fb8     0 SECTION LOCAL  DEFAULT   24 
    25: 0000000000004000     0 SECTION LOCAL  DEFAULT   25 
    26: 0000000000004010     0 SECTION LOCAL  DEFAULT   26 
    27: 0000000000000000     0 SECTION LOCAL  DEFAULT   27 
    28: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c
    29: 0000000000001090     0 FUNC    LOCAL  DEFAULT   16 deregister_tm_clones
    30: 00000000000010c0     0 FUNC    LOCAL  DEFAULT   16 register_tm_clones
    31: 0000000000001100     0 FUNC    LOCAL  DEFAULT   16 __do_global_dtors_aux
    32: 0000000000004010     1 OBJECT  LOCAL  DEFAULT   26 completed.8059
    33: 0000000000003dc0     0 OBJECT  LOCAL  DEFAULT   22 __do_global_dtors_aux_fin
    34: 0000000000001140     0 FUNC    LOCAL  DEFAULT   16 frame_dummy
    35: 0000000000003db8     0 OBJECT  LOCAL  DEFAULT   21 __frame_dummy_init_array_
    36: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS stages.c
    37: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c
    38: 0000000000002184     0 OBJECT  LOCAL  DEFAULT   20 __FRAME_END__
    39: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS 
    40: 0000000000003dc0     0 NOTYPE  LOCAL  DEFAULT   21 __init_array_end
    41: 0000000000003dc8     0 OBJECT  LOCAL  DEFAULT   23 _DYNAMIC
    42: 0000000000003db8     0 NOTYPE  LOCAL  DEFAULT   21 __init_array_start
    43: 0000000000002014     0 NOTYPE  LOCAL  DEFAULT   19 __GNU_EH_FRAME_HDR
    44: 0000000000003fb8     0 OBJECT  LOCAL  DEFAULT   24 _GLOBAL_OFFSET_TABLE_
    45: 0000000000001000     0 FUNC    LOCAL  DEFAULT   12 _init
    46: 0000000000001200     5 FUNC    GLOBAL DEFAULT   16 __libc_csu_fini
    47: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_deregisterTMCloneTab
    48: 0000000000004000     0 NOTYPE  WEAK   DEFAULT   25 data_start
    49: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND puts@@GLIBC_2.2.5
    50: 0000000000004010     0 NOTYPE  GLOBAL DEFAULT   25 _edata
    51: 0000000000001208     0 FUNC    GLOBAL HIDDEN    17 _fini
    52: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_main@@GLIBC_
    53: 0000000000001149    23 FUNC    GLOBAL DEFAULT   16 greet
    54: 0000000000004000     0 NOTYPE  GLOBAL DEFAULT   25 __data_start
    55: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
    56: 0000000000004008     0 OBJECT  GLOBAL HIDDEN    25 __dso_handle
    57: 0000000000002000     4 OBJECT  GLOBAL DEFAULT   18 _IO_stdin_used
    58: 0000000000001190   101 FUNC    GLOBAL DEFAULT   16 __libc_csu_init
    59: 0000000000004018     0 NOTYPE  GLOBAL DEFAULT   26 _end
    60: 0000000000001060    47 FUNC    GLOBAL DEFAULT   16 _start
    61: 0000000000004010     0 NOTYPE  GLOBAL DEFAULT   26 __bss_start
    62: 0000000000001160    40 FUNC    GLOBAL DEFAULT   16 main
    63: 0000000000004010     0 OBJECT  GLOBAL HIDDEN    25 __TMC_END__
    64: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_registerTMCloneTable
    65: 0000000000000000     0 FUNC    WEAK   DEFAULT  UND __cxa_finalize@@GLIBC_2.2
  • note the following:
    • we have two sybol tables now
      • .dynsym is used by the dynamic linker
      • .symtab includes the same symbols as .dynsym (and more)
      • .symtab is optional at this point (not necessary for process creation)
    • the functions we've written ourselves (main, greet) now have locations/offsets
    • the puts function from before still has no location (dynamically linked)

I've mentioned in the notes above that the .symtab symbol table is optional. Let's strip it.

stripped symbol table via strip -s a.out && readelf --syms a.out:

Symbol table '.dynsym' contains 7 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_deregisterTMCloneTab
     2: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND puts@GLIBC_2.2.5 (2)
     3: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_main@GLIBC_2.2.5 (2)
     4: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
     5: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_registerTMCloneTable
     6: 0000000000000000     0 FUNC    WEAK   DEFAULT  UND __cxa_finalize@GLIBC_2.2.5 (2)

running a stripped binary works just as well:

./a.out
# hello, world!
  • note the following:
    • while it still runs the same it is much harder to debug now
    • most binaries you encounter in the wild are stripped

instruction set architectures

aka. ISA

types

CISC

RISC

instruction sets

x86

See [inteldev, vol. 1, ch. 2, p. 1ff], [86hist].

AT&T and Intel syntax

x86_32

aka. x86

x86_64

aka. x64

ARM

executable file formats

ELF

Mach-O

PE

binary analysis

See [pracbin, p. 2f].

static

Static analysis is the process of gaining information about a binary without running it.

Pros

  • no need for a CPU with a fitting architecture
  • no need for additional software to run the binary (Kernel, etc.)

Cons

  • more difficult to reason about due to missing runtime state

dynamic

Dynamic analysis on the other hand executes the binary.

Pros

  • easier, due to additional information (runtime state)

Cons

  • might miss some code paths

tools

GDB

GDB ↣

LLVM

GCC

NASM

Netwide Assembler.

Radare2

Radare2 ↣

Ghidra

binutils

See [binutils].

binwalk

BinDiff

dd

xxd

ldd

print shared object dependencies

ldd rot13 # prints shared objects and their addresses in memory.

pwntools

ltrace and strace

pmap

Python 3

Compiler Explorer

  • generates and compares binaries with different compilers and settings
  • godbolt.org

binvis.io

dotfiles

dotfiles ↣

exploit-development

exploit-development ↣

reverse-engineering

reverse engineering ↣

sources and further reading

  • [86hist] Morse, S. P., Ravenel, B. W., Mazor, S., & Pohlman, W. B. (1980). Intel Microprocessors — 8008 to 8086. Computer, 13(10), 42–60. https://doi.org/10.1109/MC.1980.1653375
  • [binutils] GNU Binary Utilities Documentation. (2002). Retrieved from http://www.gnu.org/software/binutils/manual/
  • [c11] ISO, & IEC. (2010). ISO/IEC 9899:201x, International Standard Programming languages — C, Committee Draft (N1570 ed.). ISO/IEC. Retrieved from http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
  • [compsec] Stallings, W. (2018). Computer Security: Principles and Practice, Global Edition (4th ed.). Pearson.
  • [edvac] Von Neumann, J., & Godfrey, M. D. (1993). First Draft of a Report on the EDVAC. IEEE Annals of the History of Computing, 15(4), 27–75. https://doi.org/10.1109/85.238389
  • [inteldev] Intel. (2011). Intel 64 and IA-32 Architectures Software Developer’s Manual Combined Volumes. System, 3(253665). https://doi.org/10.1109/MAHC.2010.22
  • [pracbin] Andriesse, D., & Francisco, S. (2018). PRACTICAL BINARY ANALYSIS Build Your Own Linux Tools for Binary Instrumentation, Analysis, and Disassembly (2nd ed.). No Starch Press.
  • [x64beg] Hoey, J. Van. (2019). Beginning x64 Assembly Programming: From Novice to AVX Professional Paperback (1st ed.). Apress.

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