How to debug basic files

How to compile with gcc

gcc main.c -g # -g enables debug symbols
gdb a.out

Debug symbols enables additional information in binary like function, variable names

Debugging a running process

gdb -p <process id>

Running:

• tui: runs gdb with text user interface
• list: list code that's currently in gdb
• run: runs the binary
• break <breakpoint>: create a break point (can be used with line number and function name)
• info breakpoints: list breakpoints
• info registers: prints info about registers
• info proc mappings: prints memory map
• x/s <memory addr>: prints memory as string
• start: break main + start
• bt: prints backtrace
• print <expr>: prints stuff to stdout
• c: continues the running of the program, till next break point
• n / next: steps to next line in program
• step: steps into next line
• jump *<addr>: resumes program execution from a given address
• <enter>: repeats previous commands

Debugging without debug symbols:

• disassemble <function name / range>: dissassembles the function
• layout asm: prints assembly layout
• set disassembly-flavor <intel>: switches assembly to intel

Demo debug code, and debug a running process

• gdb -p <pid>: attach to running process
• frame / f: prints current line and line number

Reversable debugging

Command to run file while it does not crash

while ./buggy; do echo Ok; done

• b main: creates breakpoint at start of main
• b <exit>: creates breakpoint at end of main
• command 2: runs command when breakpoint 2 is hit
  • run: runs code
  • end: end recording
• command 1: runs command when breakpoint 2 is hit
  • record: records code steps
  • continue: continues debugging
• set pagination off: don't ask for input at end of large output
• reverse-stepi: step backwards

GEF:

https://github.com/hugsy/gef