Beginning Assembly, Part 2 The Assembling!

Beginning Assembly, Part 2The Assembling! Poorly Presented by Gleep

Dicks • How does Dicks work? • Zenitydependancy • Pushes text params onto the stack • Stores mem addresses of params in registers • Calls execve to start Zenity and passes regs to Zenity with params for pop-up text box

Binary Coded Decimal • Represents a decimal number by coding each digit in binary • Unpacked BCD is one byte per digit • 00000011 00000100 00001001 • 3 4 9 • Packed BCD is one byte per two digits • 00100100 10010001 • 2 4 9 1

Floating Point Maths • Floating point ops use special registers ST(0) thru ST(7)… 80 bit registers for data • 1 - 16 bit control register • 1 - 16 bit status register • 1 - 16 bit tag register to describe contents of data regs • 1 - 48 bit FIP register for next floating point op • 1 - 48 bit FDP FPU data pointer • 1 - 11 bit Opcode register

Floating Point Reg Ops • The floating point registers act as a stack. ST(0) is top of stack. • Fldsrc – loads 80 bit src onto FP stack • Fstdst – moves top of FP stack into dst • Info all will show all FP and SSE registers

Floating Point Maths ((55.24 / 31) + (83.11 * 2.4)) / ((14.35 * 9) – (251.5 / 77.62)) = OMGLOLWUT?!?!

Single Instruction Multiple Data • Aka – MMX, SSE, 3DNow! • MMX aliases the 8 FPU data regs as MM0-7 for 64 bit packed integers • SSE includes 8 new 128 bit regs XMM0-7 for 128 bit packed integers and floating point data • Useful for processing large amounts of data with one instruction

SIMD Continued • MMX regs can store 8x8 bit byte ints, 4x16 bit word ints, or 2x32 bit double word ints • Movqsrc, dst – where dst is %mm0-7 • SSE regs can store 16x8 bit byte packed ints, 8x16 bit word packed ints, 4x32 bit double word packed ints, or 2x64 bit quad word ints • Movdqusrc, dst – moves unaligned data into %xmm0-7 • Movdqasrc, dst – moves aligned data. Using this with unaligned data will throw an error.

SIMD Continued • This was supposed to be a meaningful slide but then I got bored with SIMD. • Research it yourself if you need to code it in natively.

C structures in ASM • For Loops • If/Then • While • Switch

Using C Libraries in Asm • When using C functions in asm, need to link in C libraries containing the functions • Ld –dynamic-linker /lib/ld-linux.so.2 –o <exe> -lx <obj file> - where x is /lib/libx.so • /lib/ld-linux.so.2 is a dynamic loader • -lc option would link to /lib/libc.so

Compiling Asm with GCC • Gcc –o <exe> <asmfile> • Automagically links everything • Must change _start label to main

Unconditional Branching • JMP <operand> • 3 types… Short, Near, Far • Equivalent of Goto: • Short JMP is less that 128 bytes • Far JMP is to another code segment • Near JMP is everything else • Takes one memory label as a parameter • Call <operand> • Equivalent to function call in C • Call pushes EIP to stack • Call returns to main with the RET command

Unconditional Branching Cont’d • Interrupt • Hardware – used for I/O functions • Software • Used for accessing kernel functions • In Linux, uses Interrupt 0x80 • In Windows, uses Interrupt 0x21 • Parameters in registers determine which function is being called with what parameters • Ex: MOVL $1, %EAX • MOVL $0, %EBX • INT $0x80

Conditional JMPs • JZ <label> – JMP if ZF is set • JNZ <label> – JMP if ZF is not set • JGE <label> – JMP if equal or greater • JLE <label> -JMP if less than or equal • There’s a bunch of em… use the Googles

Function Calls • .type <function label>, @function • <function label>: • Ret • Call <function label> • Functions can be defined anywhere within asm file • Return values can be returned in regs or global vars • Can put functions in separate file, just add .globl <function label> statement after .type statement, and add function object file to linker statement

Function Prologue and Epilogue Prologue • Function: • Pushl %ebp • Movl %esp, %ebp • Subl $8, %esp • Epilogue • Movl %ebp, %esp • Popl %ebp • Ret

Enter and Leave • Enter #bytes – used to perform function call prologue • Leave – used to perform function call epilogue

EBP • Main Process executes ESP • Main Process calls New Function • Call places RET addr on stack • Prologue pushes EBP to stack • Prologue sets EBP to ESP • Prologue decrements ESP to make room for variables • New Function loads variables • New Function executes • Epilogue sets ESP to EBP • Epilogue pops old EBP from stack • RET pops old RET address to EIP • Main Process resumes

System Calls • Can be found in /usr/include/asm/unistd.h • Look up the needed input and return values in man 2 <syscall> • Can use strace to see background syscalls when running a program • Strace –p <PID> will attach strace to a running process • Use man 3 <c lib func> to see C function calls

Inline Assembly • You can embed assembly within C programs • Asm ( “code goes here”); • Must enclose the code in quotes • Can use the “volatile” keyword to tell compiler not to optimize it • Must use \n newline char if embedding more than one command • If coding ANSI C, use __asm__ instead • Asm ( “movl $1, %eax\n\t” “movl $0, %ebx\n\t” “int $0x80”);

Calling Libraries • If using asm libraries in C code, add the asm files onto compile statement • Gcc –o testprogtestprog.c func1.s func2.s • Can also assemble code into object file then add the object file to gcc

Making Libraries • Can create a static library using the Ar command to add object files into an archive • Static library name syntax Libx.a • Ar r libmyfunc.a func1.o func2.o func3.o • Can see what is contained in a library using nm command • Nm –s libmyfunc.a • Compile by including the libx.a file on gcc command

Making shared libraries • Shared library name syntax libx.so.<version> • Create shared library with –shared option on gcc • Gcc –shared –o libmyfunc.so.1 func1.o func2.o func3.o • To compile using shared library, in same dir as program file use –L. option • Gcc –o testfunc –L. –lmyfunctestfunc.c

Using shared libraries • The dynamic loader must know where to find any libraries needed for program function • Change LD_LIBRARY_PATH environment variable • Export LD_LIBRARY_PATH= “$LD_LIBRARY_PATH:.” • Or change the /etc/ld.so.conf file • After adding path to ld.so.conf, must run ldconfig as root

Beginning Assembly, Part 2 The Assembling!