elf + intro to x86

Lazy Binding

• A program, loaded to memory, functions from shared library are given dummy addresses
  • Process of resolving them is relocation
    • Relocations done when an unresolved symbol is first referenced
• Requires two sections: Procedure Linkage Table (.plt) and Global Offset Table (.got/.got.plt)

Relocation Sections

• Indicated by SHT_REL and SHT_RELA
  • SHT_REL -> Table of relocation entries + instructions on how to resolve the value
• Linker performs the relocations

Program Headers

• Provides a segment view as opposed to a section view
  • Contain information about what segments need to be loaded to what sections in memory and what flags need to be assigned
• Segment consists of zero or more sections
• Only used in executable ELF

Section vs Segment

• A section is a collection of bytes in file itself
• A segment is a collection of sections

Intro to x86

Layout

Has the following:

• CPU Instructions
• Directions -> Commands to tell the assembler to produce/place a piece of data in a particular section
• Labels -> Symbolic names for instructions/data
• Comments -> Human readable

Memory Operands

• Specified memory address where CPU should fetch one or more bytes

• Of the form: [base] + [index*scale] + [displacement]

  • base, index -> Register or memory location
  • scale -> Multiplier for index. An integer value with value 1,2,4 or 8, Default is 1
  • displacement -> a 32-bit constant or symbol. Default is 0

The Stack

• Downward growing
• Last-in-first-out (LIFO) order
  • Pop or push
• Lower memory addresses are usually at the top of the stack

Function calls and frames

• Each function has its own function frame or stack frame

• rbp -> Base Pointer (points to the bottom of the stack)

• rsp -> Stack Pointer (points to the top of the stack)

• Access to elements in a stack frame is usually specified as an offset relative to the rbp

  • rsp relative offsetting also possible

Function Prologues and Local variables

• Function prologues are the first instructions of a function

  • Save content of rbp register on stack
  • Copy rsp into rbp

• After prologue, space for local variables is reserved by decrementing rsp

  • i.e. sub rsp, 0x10 -> reserves 0x10 (16) bytes for local variable(s)

Passing variables

• Passed in registers: rdi, rsi, rdx, rcx, r8, r9
  • If more than 6 arguments, remaining arguments are stored on the stack

Returning from a function

• Return values stored in rax
• To exit, an epilogue is called to clean up the frame, see also the instruction leave

mov rsp, rbp; # restore stack pointer
pop rbp; # restore base pointer

Conditional jumps

• Implemented using cmp (compare) instruction
  • Status:
    • Zero flag (ZF)
    • Sign flag (SF)
    • Overflow flag (OF)