Introduction to debugging linux applications

Spenser Reinhardt mplsCTFgames.org DC612 SpenserReinhardt@gmail.com

General Overview Introduction to ELF (Executable and Linkable Format) Layout Assembly Primer Number Bases Registers and Memory Addressing Basic Instructions Debugging Tools and Ideas Principal of Confirmation GDB, GDBtui, DDD, Insight Working with GDB Example Errors

Executable & Linkable Format ELF is a format for storing programs or fragments of programs on disk, created as a result of compiling and linking. An ELF file is divided into sections. For an executable program, these are the text section for the code, the data section for global variables and the rodata section that usually contains constant strings. The ELF file contains headers that describe how these sections should be stored in memory. ELF is a format for storing programs or fragments of programs on disk, created as a result of compiling and linking. An ELF file is divided into sections. For an executable program, these are the text section for the code, the data section for global variables and the rodata section that usually contains constant strings. The ELF file contains headers that describe how these sections should be stored in memory. Types of Files: .O, regular executables, shared libraries and core dumps

ELF Structure Elf Header: Start of the file and a description of it’s organization. Program Header Table: Instructs the system how to execute the file. (optional) .text: Contains all program instructions .bss: Holds all uninitialized data .data: Holds all initialized data .rodata: Holds read only data .debug: Contains debug symbols (optional) Section Header Table: Assists in locating each internal section (optional)

What is Assembly Language? The language known as Assembly or ASM, is really a collection of CPU independent instructions that vary depending on the platform. Each different CPU and sometimes each revision within a family of processors, will have it's own version or interpretation of asm instructions. This variation in what we know as an individual language, is due to the close relationship of the hardware how machine instructions are almost directly derived from asm instructions. Assembly language is a translator language that allows total control over every individual machine instruction generated by the translator program or assembler.

Computers = Numbers Computers only speak in numbers, however they do not count with numbers as we think of them. They work in a mixed fashion of both binary and hexadecimal. Binary = Base 2 = Digits 0 1 Decimal = Base 10 = Digits 0 – 9 Hexadecimal = Base 16 = Digits 0 – 9, A - F

Decimal Calculations 1 15 100 870434 +2 +03 +86 + 37201 3 18 186 907635 9 53 234 829548 -5 - 36 - 75 - 829321 4 17 159 227

Binary Calculations 1B 1111B 1100100B 11010100100000100010B +10B +0011B +1010110B + 1001000101010001B 11B 10010B 10111010B 11011101100101110011B 1001B 110101B 11101010B 11001010100001101100B - 101B -100100B - 1001011B -11001010011110001001B 100B 10001B 10011111B 11100011B

Hexadecimal Calculations 1H 15H 64H D4822H +2H +03H +56H + 9151H 3H 12H BAH DD973H 9H 35H EAH CA86CH - 5H - 24H - 4BH - CA789H 4H 11H 9FH E3H

CPU Registers Within a CPU there are special small storage compartments for very fast access, these are called registers. Much like the rest of asm these registers are very processor specific, however many generalizations can be made. 8-bit 16-bit 32-bit 64-bit Description AL AX EAX RAX General purpose register BL BX EBX RBX General purpose register CL CX ECX RCX General purpose register DL DX EDX RDX General purpose register IP EIP RIP Points to current instruction location (Instruction Pointer) BP EBP RBP Points to bottom of current stack frame (Base Pointer) SP ESP RSP Points to top of current stack frame (Stack Pointer) SI ESI RSI Used for special operations (Source Index) DI EDI RDI Used for special operations (Destination Index) CS, DS, SS, ES, FS, GS Segment Registers (16-bit)

Endianness The order of importance and direction to read byte values. The systems CPU determines endianness. Little Endian: Read from right to left, with the most significant byte stored on the right. (x86, x86-64) Big Endian: Read from left to right, with the most significant byte stored on the left and not flipped when read. (PowerPC, IBM Mainframes) Bi Endian: Can potentially interpret either values either way. (MIPS, IA32, IA64)

The Stack • Stores data temporarily as an application may need it. • ESP = Top of the Stack EBP = Bottom of the Stack, or top of previous • Addressed by offsets of espebp or direct memory locations • Last in, First out (LIFO) or First in, Last out (FILO) • Push [value] – Adds to top of the Stack, then decreases ESP accordingly • Pop [value] – Removes from top of the Stack, then increases ESP • Dynamically allocated, 32 bits wide • Grows from higher memory down

Memory Layout • Almost identical to on-disk ELF layout • Definitions of sections in ELF, directly applies • Also has Stack and Heap sections • Heap space is dynamically allocated as programs request or deallocate it. • Heap is allocated in otherwise free space and does not need to be in any order or specific location • Application sees 4GB of virtual memory • Some or most space may be paged out

ASM Instructions - mnemonics • Usually one command per line • First or only operand is usually the destination operand, unless specifically noted in the instruction details. • R/8,16,32,64 Register size • M/8,16,32,64 Memory size • I/8,16,32,64 Immidate Data • D/8,16,32,64 Displacement • SR Segment Register mov eax, ‘WXYZ’ Save WXYZ into eax Move ZYXZ into eax, and zero any remaining space in the register

ASM Instructions - Arithmetic Instruction Description add r/m32, r/m32 Combines operands though addition and stores in first sub r/m32, r/m32 Subtracts operands and stores in first mul r/m32, eax Multiplies operands* and stores in ax and dx when operands are greater than 8 bits div r/m32, eax Divides operands* and * When mul and div are used the “A” register is used implicitly as the second operand. “A” register could be AL, AX, EAX, or RAX.

ASM Instructions – Unary Operators Instruction Description and r/m32, r/m32 Compares operands and sets to one if both are equal or zero if not. or r/m32, r/m32 Compares operands and sets to one if at least one, is not zero. xor r/m32, r/m32 Compares operands and sets to one if not equal and zero if equal. not r/m32 Sets one to zero, and zero to one. neg r/m32 Sets value equivalent negative value inc r/m32 Increments operand by 1. 1. dec r/m32 Decrements operand by 1. 1.

ASM Instructions – Bit Manipulation Instruction Description shl r/m32, count Shifts bits left [count] times, stores overflow in CF, inserts zero shr r/m32, count Shifts bits right [count] times, stores overflow in CF, inserts zero rol r/m32, count Rotates bits from left and inserts on right, no CF use ror r/m32, count Rotates bits from right and inserts on left, no CF use rcl r/m32, count Rotates left to right, storing the first value rotated off, and stored in CF, previous CF is set as right most value rcr r/m32, count Rotates left to right, storing the first value rotated off, and stored in CF, previous CF is set as right most value

ASM Instructions – Push Pop Mov Instruction Description push r/m32 Pushes data onto the stack and lowers ESP pusha Pushes all 16-bit general purpose registers at once pushad Pushes all 32-bit general purpose registers at once pushf Pushes Flags register onto the stack pop r/m32 Pull data from the stack, store at location provided and raise ESP popa Pull top 16 bytes from the stack and sets into each register !SP popad Pull top 32 bytes from stack and and sets into each register !ESP popf Pull top 2 bytes and store into Flags mov r/m32, r/m32 Moves data from one location of memory to another

Debugging? Debugging is the process within software development where applications and code are tested to be accurate to the developers expectations. This can include programmatic errors, unexpected data values, infinite loops, and potentially security risks. Debugging is generally a recursive process performed until all known bugs are located and corrected, and preformed again when new issues are found.

Principle of Confirmation The principle of confirmation, is a process of validating that assumptions you as a programmer make, actually are true within execution. If something is not as expected you have likely found a bug, or part of it.

GDB • TextCLI based by default • Semi GUI or uses other frontends • -tui or ctrl-X-A to access console analogue interface • Extremely fast • Low visual input

Insight • Red HatCent OSFedora based • Frontend to GDB • Removed from Debian repositories • Full GUI, including console • Fast and stable

DDD • Works in almost all distributions • Fast but not as stable (IMO) • Full GUI and supporting console • Virtually identical to Kdbg

GBD Commands Instruction Description -tui Used while starting for semi-gui Break [line] Stops execution at set line and allows for inspection Tbreak [line] Stops execution at set line the first time hit only Watch [condition] Performs commands for condition arguments set Print [variable] Displays a variables value while execution is stopped Frame [number] Diplays trace of set stack frame Backtrace Displays entire stack layout

GDB Instructions Run [arguments] Starts program execution with supplied arguments Continue Continues normal execution after being paused Step Executes line Stepi Executes next ASMmachine instruction Next Executes next line then pauses, skips over called functions Nexti Executes next ASMmachine instruction and pauses

Credits The Art of Debugging With GDB, DDD, and Eclipse Norman Mattloff and Peter Jay Salzman – No Starch Press 2008 Assembly Language Step by Step Programming With Linux Jeff Duntemann – Wiley 2009 C++ Programming Today Barbara Johnston – Pearson Prentice Hall 2008 Hacking The Art of Exploitation Jon Erickson – No Starch Press 2008

Introduction to debugging linux applications

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Introduction to debugging linux applications