Machine-Level Programming I: Basics Comp 21000: Introduction to Computer Organization & Systems Instructor: John Barr * Modified slides from the book “Computer Systems: a Programmer’s Perspective”, Randy Bryant & David O’Hallaron, 2011

Today: Machine Programming I: Basics History of Intel processors and architectures C, assembly, machine code Assembly Basics: Registers, operands, move Intro to x86-64

Turning C into Object Code Code in files p1.c p2.c Compile with command: gcc –O0 p1.c p2.c -o p Use basic optimizations (-O0) [-O0 means (almost) no optimization] Put resulting binary in file p compile to 64bit code text C program (p1.c p2.c) Compiler (gcc –O0 -S) (-S means stop after compiling) text Asm program (p1.s p2.s) Assembler (gcc or as) binary Object program (p1.o p2.o) Static libraries (.a) Linker (gcc or ld) binary Executable program (p) Use -march=x86-64 to compile 64-bit code on a 32-bit machine

Compiling Into Assembly C Code (sum.c) Generated x86-64 Assembly long plus(long x, long y); void sumstore(long x, long y, long *dest) { long t = plus(x, y); *dest = t; } sumstore: pushq %rbp movq %rdx, %rbx call plus movq %rax, (%rbx) popq %rbp ret Some compilers use instruction “leave” Obtain (on arda machine) with command gcc –O0 –S sum.c Produces file sum.s On 32-bit machines can force 64-bit code with the option -march=x86-64 Use the –O0 flag (dash uppercase ‘O’ number 0) to minimize optimization. Otherwise you’ll get strange register moving.

Compiling Into Assembly C Code (sum.c) Generated x86-64 Assembly long plus(long x, long y); void sumstore(long x, long y, long *dest) { long t = plus(x, y); *dest = t; } sumstore: pushq %rbx movq %rdx, %rbx call plus movq %rax, (%rbx) popq %rbx ret Some compilers use instruction “leave” Obtain (on server machine) with command gcc –O0 –g sum.c -g puts in hooks for gdb Can examine assembly code in gdb

How it used to be done pdp programming See https://youtu.be/XV-7J5y1TQc

gdb run with executable file a.out quit running gdb a.out run stop Reference: http://www.yolinux.com/TUTORIALS/GDB-Commands.html

gdb examine source code: break points debugging list firstLineNum, secondLineNum where firstLineNum is the first line number of the code that you want to examine secondLineNum is the ending line number of the code. break points break lineNum lineNum is the line number of the statement that you want to break at. debugging step // step one C instruction (steps into) stepi // step one assembly instruction next // steps one C instruction (steps over) where // info about where execution is stopped

gdb Viewing data print x This prints the value currently stored in the variable x. print &x This prints the memory address of the variable x. display x This command will print the value of variable x every time the program stops.

gdb Getting low level info about the program info line lineNum This command provides some information about line number lineNum including the memory address (in hex) where it is stored in RAM. disassem memAddress1 memAddress2 prints the assembly language instructions located in memory between memAddress1 and memAddress2. x memAddress This command prints the contents of the memAdress in hexadecimal notation. You can use this command to examine the contents of a piece of data. There are many more commands that you can use in gdb. While running you can type help to get a list of commands.

Assembly Characteristics: Data Types “Integer” data of 1, 2, 4 or 8 bytes Data values Addresses (untyped pointers) Floating point data of 4, 8, or 10 bytes Code: Byte sequences encoding series of instructions No aggregate types such as arrays or structures Just contiguously allocated bytes in memory

Assembly Characteristics: Operations Perform arithmetic function on register or memory data Transfer data between memory and register Load data from memory into register Store register data into memory Transfer control Unconditional jumps to/from procedures Conditional branches

Assembly Language Versions There is only one Intel x86-64 machine language (1’s and 0’s) There are two ways of writing assembly language on top of this: Intel version AT&T version UNIX/LINUX in general and the gcc compiler in particular uses the AT&T version. That’s also what the book uses and what we’ll use in these slides. Why? UNIX was developed at AT&T Bell Labs! Most reference material is in the Intel version

Assembly Language Versions There are slight differences, the most glaring being the placement of source and destination in an instruction: Intel: instruction dest, src AT&T: instruction src, dest Intel code omits the size designation suffixes mov instead of movl Intel code omits the % before a register eax instead of %eax Intel code has a different way of describing locations in memory [ebp+8] instead of 8(%ebp)

Object Code Code for sumstore Assembler Linker Translates .s into .o Binary encoding of each instruction Nearly-complete image of executable code Missing linkages between code in different files Linker Resolves references between files Combines with static run-time libraries E.g., code for malloc, printf Some libraries are dynamically linked Linking occurs when program begins execution 0x0400595: 0x53 0x48 0x89 0xd3 0xe8 0xf2 0xff 0x03 0x5b 0xc3 Total of 14 bytes Each instruction 1, 3, or 5 bytes Starts at address 0x0400595

Machine Instruction Example C Code Store value t where designated by dest Assembly Move 8-byte value to memory Quad words in x86-64 parlance Operands: t: Register %rax dest: Register %rbx *dest: Memory M[%rbx] Object Code 3-byte instruction Stored at address 0x40059e Symbol table Assembler must change labels into memory locations To do this, keeps a table that associates a memory location for every label Called a symbol table *dest = t; movq %rax, (%rbx) 0x40059e: 48 89 03

Disassembling Object Code Disassembled 0000000000400595 <sumstore>: 400595: 53 push %rbx 400596: 48 89 d3 mov %rdx,%rbx 400599: e8 f2 ff ff ff callq 400590 <plus> 40059e: 48 89 03 mov %rax,(%rbx) 4005a1: 5b pop %rbx 4005a2: c3 retq Disassembler objdump –d sum Otool –tV sum // in Mac OS Useful tool for examining object code -d means disassemble sum is the file name (e.g., could be a.out) Analyzes bit pattern of series of instructions Produces approximate rendition of assembly code Can be run on either a.out (complete executable) or .o file

Alternate Disassembly Disassembled Object 0x0400595: 0x53 0x48 0x89 0xd3 0xe8 0xf2 0xff 0x03 0x5b 0xc3 Dump of assembler code for function sumstore: 0x0000000000400595 <+0>: push %rbx 0x0000000000400596 <+1>: mov %rdx,%rbx 0x0000000000400599 <+4>: callq 0x400590 <plus> 0x000000000040059e <+9>: mov %rax,(%rbx) 0x00000000004005a1 <+12>:pop %rbx 0x00000000004005a2 <+13>:retq Within gdb Debugger gdb sum disassemble sumstore Disassemble procedure x/14xb sum Examine the 14 bytes starting at sumstore

Direct creation of assembly programs In .s file % gcc –S p.c Result is an assembly program in the file p.s .file "assemTest.c" .text .type sum.0, @function sum.0: pushl %ebp movl %esp, %ebp subl $4, %esp movl 12(%ebp), %eax addl 8(%ebp), %eax leave ret .size sum.0, .-sum.0 .globl main .type main, @function main: subl $8, %esp andl $-16, %esp subl $16, %esp movl $0, %eax .size main, .-main .section .note.GNU-stack,"",@progbits .ident "GCC: (GNU) 3.4.6 Assembler directives Assembly commands Symbols

Features of Disassemblers Disassemblers determine the assembly code based purely on the byte sequences in the object file. Do not need source file Disassemblers use a different naming convention than the GAS assembler. Example: omits the “l” from the suffix of many instructions. Disassembler uses nop instruction (no operation). Does nothing; just fills space. Necessary in some machines because of branch prediction Necessary in some machines because of addressing restrictions And sometimes the disassembler just can’t figure out what’s going on

What Can be Disassembled? % objdump -d WINWORD.EXE WINWORD.EXE: file format pei-i386 No symbols in "WINWORD.EXE". Disassembly of section .text: 30001000 <.text>: 30001000: 55 push %ebp 30001001: 8b ec mov %esp,%ebp 30001003: 6a ff push $0xffffffff 30001005: 68 90 10 00 30 push $0x30001090 3000100a: 68 91 dc 4c 30 push $0x304cdc91 Reverse engineering forbidden by Microsoft End User License Agreement Anything that can be interpreted as executable code Disassembler examines bytes and reconstructs assembly source

Machine Programming I: Summary History of Intel processors and architectures Evolutionary design leads to many quirks and artifacts C, assembly, machine code Compiler must transform statements, expressions, procedures into low-level instruction sequences Assembly Basics: Registers, operands, move The x86 move instructions cover wide range of data movement forms Intro to x86-64 A major departure from the style of code seen in IA32