GDB Basic Commands

2019-10-31 | Read: times | Author: Junpeng Wan

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Here is a simple tutorial of GDB I wrote before.

gdb level1: use gab to debug binary level1
run: execute this binary
disas f_A: disassembly function f_A
break *0xdeedbeef: set a new breakpoint at position 0xdeedbeef
info breakpoint: check all the breakpoints
info register: check the states of registers
x/wx address: check the contents at address
- w could be b/h/w/g as 1/2/4/8 bytes
- x/100wx: show 100 four-byte word once a time
- the second x could be u/d/s/x/w (determines the type of the memory address)
  - u: unsigned int
  - d: show as decimal number
  - x: show as hexadecimal number
  - s: show as strings
  - i: show as instructions
ni – finish next instruction(if it is a call, it will execute until it returns)
si - execute next instruction(if it is a call, it will execute the first instruction of the function next)
backtrace - show all the stack frames of the calling chain
continue - execute the process until it ends, crashes, or is at a breakpoint
set *address = value
- set 4 bytes at address
- change it to char, short, int, and long to represent 1,2, 4, and 8 bytes
- e.g. set [int]0x80408000 = 666
attach [pid]: attach a running process

checksec – check protection mechanisms of the binary
elfsymbol - get every PLT addresses(useful at rop)
vmmap - check all the memory segments and permissions(read, write,execute)
readelf - check positions of important elf data structures(.plt, .plt.got, .bss)
find /bin/sh - find the address of string “/bin/sh”