Embedded-Hacking/WEEK02/WEEK02-04.md

Embedded Systems Reverse Engineering

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Week 2

Hello, World - Debugging and Hacking Basics: Debugging and Hacking a Basic Program for the Pico 2

Exercise 4: Automate the Hack

Objective

Create a reusable GDB command that injects a string into SRAM, repoints r0, and resumes execution with a single call.

Prerequisites

• Pico 2 connected with OpenOCD, GDB, and serial monitor ready
• build/0x0001_hello-world.elf available
• Familiarity with breaking at 0x1000023c and injecting strings from prior exercises

Task Description

You will define a custom GDB command hack that writes a payload to 0x20000000, repoints r0, and continues execution automatically.

Step-by-Step Instructions

Step 1: Connect, Halt, and Break

(gdb) target extended-remote :3333
(gdb) monitor reset halt
(gdb) b *0x1000023c
(gdb) c

Step 2: Define the hack Command

(gdb) define hack
> set {char[14]} 0x20000000 = {'h','a','c','k','y',',',' ','w','o','r','l','d','\r','\0'}
> set $r0 = 0x20000000
> c
> end

Step 3: Invoke the Command

(gdb) hack

Step 4: Observe Output

• PuTTY should immediately show your injected string after the command runs.
• The breakpoint will be re-hit on the next loop iteration; rerun hack if you want to reapply after changes.

Expected Output

• hack executes without errors, writes the payload, updates r0, and resumes execution.
• Serial output reflects the injected message.

Questions for Reflection

Question 1: How could you parameterize the command to accept different strings or addresses?

Question 2: What happens if you define hack before setting the breakpoint—will it still work as expected?

Question 3: How would you adapt this pattern for multi-step routines (e.g., patch, dump, continue)?

Tips and Hints

• Redefine hack any time you want a different payload; GDB will overwrite the prior definition.
• Keep the payload length aligned with the buffer size to avoid stray bytes.
• If the target keeps running past the breakpoint, ensure hardware breakpoints are available and set correctly.

Next Steps

• Create additional helper commands (e.g., dumpstr, retarget) to streamline experiments.
• Explore GDB scripting files (.gdbinit) to auto-load your helpers on startup.
• Try combining hack with watchpoints to observe memory changes live.