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Kevin Thomas 23b9f628ef Audit-clean C standard across all 5 cb challenges: fix 3 functions over 8 exec lines; rebuild bins; update CB-0x03 and CB-0x05 guides; verify all guides against new bins

2026-10-11 17:43:45 -04:00

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Week 7-IDA: IDA Pro — Read, Hack, and Patch Constants and a 1602 LCD String (Raw `.bin`)

LEGAL DISCLAIMER: The information, tools, and code provided in this repository and course are strictly for educational, research, and defensive purposes only.

You are explicitly prohibited from using any materials contained herein to access, test, modify, or exploit any device, network, or system that you do not own 100% or for which you do not have explicit, documented, and legally binding authorization to interact with.

By using this repository and course, you acknowledge and agree that:

Any illegal, unauthorized, or malicious use of this information is solely your responsibility.
The author(s) and contributor(s) of this repository and course shall not be held liable for any damages, legal repercussions, criminal charges, or unauthorized actions resulting from the use, misuse, or abuse of the contents herein.
You will comply with all applicable local, state, national, and international laws regarding cybersecurity and computer fraud.

IF YOU DO NOT AGREE WITH THESE TERMS, DO NOT USE THIS REPOSITORY AND COURSE.

What You'll Learn This Week

Build the lesson project with Release and get both an .elf and a raw .bin
Dump the ELF symbol map with arm-none-eabi-nm and use it as ground truth
Load the raw .bin into IDA at 0x10000000
Read how a #define macro and a const variable both fold to bare instruction immediates
Tell a 16-bit movs r1, #42 from a 32-bit movw r1, #1337 in the disassembly
Follow the I2C path through the i2c_inst_t struct: I2C_PORT -> i2c1 -> &i2c1_inst -> hw -> 0x40098000
Optionally hack the LCD string live by pointing r0 at a RAM replacement
Resolve the functions in the IDA GUI using the ELF symbol map, including the lcd_1602.c driver symbols
Patch both constants (42 -> 43, 1337 -> 1344) and the LCD string ("Reverse" -> "Exploit")
Export the patched image, convert it to UF2, and export it

How This Guide Works

The build produces two files for the project:

File	What it is	How we use it
`.elf`	The linked image with a full symbol table	Ground truth for every function address and name
`.bin`	The raw flash image, no headers, no symbols	The image we load into IDA and reverse

The .bin is built from the .elf, so the ELF tells you exactly what is at every address. We use the ELF symbol map to resolve functions in IDA, and we reverse-engineer the raw .bin the way a real extracted firmware image is reversed.

Build Release, not Debug. Every address in this guide matches the Week 7 lesson, and the Week 7 lesson is a Release build. Release optimizes the code the same way the original lesson was built: it inlines the static helpers init_i2c_and_lcd and write_lcd_greeting (and the whole lcd_1602.c static helper chain) straight into main or into the public lcd_* functions, and it folds both FAV_NUM and OTHER_FAV_NUM down to immediate values. If you build Debug, the SDK function addresses move and the helpers stay separate calls, so nothing lines up. Always build Release for this lesson.

The order is dynamic first, static second:

d prove what the code does. ge. 3. Resolve the functions in IDA using the ELF symbol map. 4. Patch the bytes, export, convert and export.

Project	Serial output	Also does	The hacks
`0x0017_constants`	`FAV_NUM: 42`, `OTHER_FAV_NUM: 1337`	I2C1 (SDA GP2, SCL GP3) drives a 1602 LCD, writing `Reverse` / `Engineering`	`42 -> 43`, `1337 -> 1344`, and `"Reverse" -> "Exploit"`

Addresses come from your build. Every address here is from the Release build produced in Step 3. Confirm against your own .elf with the command in Step 4.

The surprise of this week is that const is not in memory. #define FAV_NUM 42 becomes the 16-bit movs r1, #42; const int OTHER_FAV_NUM = 1337 also becomes an immediate, the 32-bit movw r1, #1337. The compiler only keeps a const in .rodata if the program takes its address (&OTHER_FAV_NUM); this program never does, so the const is inlined exactly like the macro. You patch an instruction operand, not a data word.

Part 1: Build and Get the Symbol Map

Step 1: Install the toolchain

Windows x64

Install the Raspberry Pi Pico extension in VS Code. It installs the ARM GNU toolchain, CMake, Ninja, and the Pico SDK.
Install IDA Pro and complete its license activation.
Install PuTTY for the serial monitor.

macOS Apple Silicon

All must report arm64. If any is x86_64, put the Apple Silicon prefix first for the session and check again:

bash


**Windows x64** and **Linux x64** do not have this problem. Skip to Step 3.

### Step 3: Build the project with `Release`

Run this inside `0x0017_constants/`:

```bash
cmake -B build -G Ninja -DPICO_BOARD=pico2 -DPICO_PLATFORM=rp2350 -DCMAKE_BUILD_TYPE=Release
cmake --build build

Point IDA at this repository (once). Every console snippet below reads the repo root from ~/.embedded-hacking-repo, so IDA never needs a database open and nothing is hardcoded. From the repo root, run once:

macOS / Linux:

pwd > ~/.embedded-hacking-repo

Windows (PowerShell):

(Get-Location).Path | Set-Content "$env:USERPROFILE\.embedded-hacking-repo"

Then build from the IDA console, so the whole build -> patch -> flash loop stays inside IDA. The console inherits a minimal PATH — on macOS just /usr/bin:/bin:/usr/sbin:/sbin — so it does not see Homebrew; add your package manager's bin first, then run plain cmake.

macOS Apple Silicon:

import os, subprocess
root = open(os.path.expanduser("~/.embedded-hacking-repo")).read().strip()
os.environ["PATH"] = "/opt/homebrew/bin:" + os.environ["PATH"]  # the console's PATH omits Homebrew
proj = os.path.join(root, "0x0017_constants")
subprocess.run(["cmake", "-B", "build", "-G", "Ninja", "-DPICO_BOARD=pico2",
                "-DPICO_PLATFORM=rp2350", "-DCMAKE_BUILD_TYPE=Release"], cwd=proj)
subprocess.run(["cmake", "--build", "build"], cwd=proj)

Linux x64:

import os, subprocess
root = open(os.path.expanduser("~/.embedded-hacking-repo")).read().strip()
proj = os.path.join(root, "0x0017_constants")
subprocess.run(["cmake", "-B", "build", "-G", "Ninja", "-DPICO_BOARD=pico2",
                "-DPICO_PLATFORM=rp2350", "-DCMAKE_BUILD_TYPE=Release"], cwd=proj)
subprocess.run(["cmake", "--build", "build"], cwd=proj)

Windows x64:

import os, subprocess
root = open(os.path.expanduser("~/.embedded-hacking-repo")).read().strip()
proj = os.path.join(root, "0x0017_constants")
subprocess.run(["cmake", "-B", "build", "-G", "Ninja", "-DPICO_BOARD=pico2",
                "-DPICO_PLATFORM=rp2350", "-DCMAKE_BUILD_TYPE=Release"], cwd=proj)
subprocess.run(["cmake", "--build", "build"], cwd=proj)

The build directory now contains the pair we need:

0x0017_constants/build/0x0017_constants.elf and .bin — the .bin is 17980 bytes (0x463c)

If the ARM toolchain is not on your PATH, add -DPICO_TOOLCHAIN_PATH=...:

OS	Typical toolchain path
Windows x64	`C:/Program Files/Arm GNU Toolchain arm-none-eabi/14.2 rel1/bin`
macOS Apple Silicon	`~/.pico-sdk/toolchain/14_2_Rel1/bin`
Linux x64	`/usr`

This guide's toolchain lives at ~/.pico-sdk/toolchain/14_2_Rel1/bin. All of arm-none-eabi-nm, arm-none-eabi-objdump, and arm-none-eabi-gdb resolved in this document come from there. If your install is elsewhere, which arm-none-eabi-nm tells you where to point.

Step 4: Dump the ELF symbol map

This is the ground truth for the whole lesson. Run arm-none-eabi-nm on the ELF and keep the output in a terminal or a text file:

macOS Apple Silicon / Linux x64:

arm-none-eabi-nm -n --defined-only build/0x0017_constants.elf | grep -E ' [Tt] '

Windows x64:

arm-none-eabi-nm -n --defined-only build\0x0017_constants.elf | Select-String ' [Tt] '

Each line is address type name. The T/t type is a function. Here are the functions this lesson uses. The signatures come from the ELF's DWARF debug info queried with arm-none-eabi-gdb -batch -ex "ptype <name>", so they are exact.

Our code and the startup chain:

Address	ELF symbol	Signature	Role
`0x1000015c`	`_reset_handler`	`void _reset_handler(void)`	reset entry
`0x10000186`	`platform_entry`	`void platform_entry(void)`	calls `runtime_init`, `main`, `exit`
`0x1000019a`	`data_cpy`	`void data_cpy(void, void, void*)`	copies `.data` from flash to SRAM
`0x100001e4`	`_init`	`void _init(void)`	runs `.init_array`
`0x10000210`	`frame_dummy`	`void frame_dummy(void)`	C runtime boilerplate
`0x10000234`	`main`	`int main(void)`	the lesson function (`init_i2c_and_lcd` and `write_lcd_greeting` inlined)

Our I2C/LCD driver (lcd_1602.c) and the SDK I2C functions main reaches:

Address	ELF symbol	Signature	Role
`0x100002bc`	`lcd_i2c_init`	`void lcd_i2c_init(i2c_inst_t*, uint8_t, int, uint8_t)`	store config + HD44780 reset/configure
`0x100006f4`	`lcd_set_cursor`	`void lcd_set_cursor(int, int)`	move the HD44780 cursor
`0x100007f0`	`lcd_puts`	`void lcd_puts(const char*)`	write a string to the LCD
`0x10003cdc`	`i2c_init`	`uint i2c_init(i2c_inst_t*, uint)`	SDK I2C init (100 kHz)
`0x10003d28`	`i2c_write_blocking`	`int i2c_write_blocking(i2c_inst_t, uint8_t, const uint8_t, size_t, bool)`	one blocking I2C transfer
`0x100008f0`	`gpio_set_function`	`void gpio_set_function(uint, gpio_function_t)`	SDK GPIO function select (I2C pins)
`0x1000092c`	`gpio_set_pulls`	`void gpio_set_pulls(uint, bool, bool)`	SDK pull config (`gpio_pull_up` inlined)

The lcd_1602.c static helpers — pcf_write_byte, pcf_pulse_enable, lcd_write4, lcd_send, lcd_store_config, lcd_hd44780_reset, lcd_hd44780_configure — have no symbol of their own in the Release build. They are inlined into lcd_i2c_init, lcd_set_cursor, and lcd_puts, which is why those three functions are large and call i2c_write_blocking and the sleep_* helpers directly.

The stdio/UART and printf chain main reaches:

Address	ELF symbol	Signature	Role
`0x10001368`	`sleep_us`	`void sleep_us(uint64_t)`	SDK microsecond delay
`0x10001440`	`sleep_ms`	`void sleep_ms(uint32_t)`	SDK millisecond delay
`0x10001624`	`time_us_64`	`uint64_t time_us_64(void)`	SDK microsecond clock
`0x10001638`	`busy_wait_us`	`void busy_wait_us(uint64_t)`	UART timing loop
`0x100016b8`	`uart_init`	`uint uart_init(uart_inst_t*, uint)`	SDK UART init
`0x1000188c`	`clock_get_hz`	`unsigned long clock_get_hz(clock_handle_t)`	UART clock lookup
`0x100035a4`	`exit`	`void exit(int)`	C runtime exit
`0x100035ac`	`runtime_init`	`void runtime_init(void)`	SDK runtime init
`0x100035d8`	`stdio_out_chars_crlf`	`void stdio_out_chars_crlf(stdio_driver_t, const char, int)`	CRLF output driver
`0x100036e8`	`stdio_put_string`	`int stdio_put_string(const char*, int, bool, bool)`	buffered string output
`0x100037d4`	`stdio_set_driver_enabled`	`void stdio_set_driver_enabled(stdio_driver_t*, bool)`	enable the UART driver
`0x100037fc`	`stdio_init_all`	`bool stdio_init_all(void)`	SDK serial init
`0x100038c8`	`__wrap_vprintf`	`int __wrap_vprintf(const char*, va_list)`	printf core
`0x1000398c`	`__wrap_printf`	`int __wrap_printf(const char*, ...)`	the `printf` wrapper
`0x10003b48`	`stdio_uart_init`	`void stdio_uart_init(void)`	SDK UART stdio init
`0x10003e24`	`strlen`	`size_t strlen(const char*)`	C runtime string length

SDK helpers the printf path reaches:

Address	ELF symbol	Signature	Role
`0x10003548`	`vfctprintf`	`int vfctprintf(void ()(char, void), void, const char, va_list)`	printf format engine
`0x10002b64`	`_vsnprintf`	`int _vsnprintf(out_fct_type, char, size_t, const char, va_list)`	the format dispatcher
`0x10001f48`	`_ntoa_format`	`unsigned _ntoa_format(out_fct_type, char, size_t, size_t, char, size_t, bool, unsigned, unsigned, unsigned, unsigned)`	number formatter
`0x10001eac`	`_out_rev`	`unsigned _out_rev(out_fct_type, char, size_t, size_t, const char, size_t, unsigned, unsigned)`	reversed-digit output
`0x1000211c`	`_out_char`	`void _out_char(char, void*, size_t, size_t)`	single-char sink

Two things in this project have no symbol of their own, because the compiler inlined them into main:

init_i2c_and_lcd and write_lcd_greeting — the two static helpers in our own source are inlined, so there is no address to rename. You see their bodies directly inside main.
gpio_pull_up — static inline in the SDK, so gpio_pull_up(2) and gpio_pull_up(3) compile to direct calls to gpio_set_pulls at 0x10000258 and 0x10000262.

Part 2: Load the Raw `.bin` into IDA

Start from a fresh IDA state. If you already have a database for this lesson, close it and start over; a stale database keeps old names and patches.

Step 7: Bring the raw `.bin` into IDA

A raw .bin has no headers, so IDA cannot know where it belongs or what architecture it is. You must supply both. If you just double-click the .bin, IDA may load it with a guessed architecture, and every address in this lesson will be wrong.

1.1 Open the file

File ▸ Open… (Ctrl/Cmd+O).
Select the .bin file and click Open.

IDA opens the "Load a new file" dialog because a raw .bin has no header.

1.2 Set the format and processor

In the Load a new file dialog:

Format: leave it as Binary file (IDA auto-detects a headerless image).
Processor type: click the … button next to the processor field. In the Processor type dialog:
- Family: ARM
- Type: ARM Little-endian (the 32-bit ARM module).
- Click OK.

If IDA asks "Do you want to disassemble it as 64-bit code?" — click No. The RP2350 is 32-bit; answering Yes loads AArch64 and you get X registers / data. (This is the single most common cause of the DCB wall.)

1.3 Configure the ARM architecture and Thumb mode

Still in the Load a new file dialog, open the processor options (the Processor options / Set… button, which opens the dialog titled ARM architecture options):

ARM architecture: choose ARMv8-M (this is the Cortex-M33 / RP2350 core).
Thumb instructions: select Thumb-2 (ARMv8-M is Thumb-only, so this is the correct mode).
Leave automatic ARM-THUMB switch off; we want Thumb throughout.

1.4 Set the loading address

Loading address: enter 10000000 (hex; the RP2350 XIP flash base). IDA rejects addresses that don't look like ROM/RAM with "The loading address should belong to RAM or ROM" — 0x10000000 is the correct flash window.

Leave Manual load and Create segments at their defaults. Click OK.

1.5 Force Thumb, then seed the code from the vector table

A raw .bin has no header, so IDA's loader has no entry point and defaults the segment to ARM mode (which cannot decode Thumb). Two required steps — in this order:

A. Set the segment to Thumb (do this first).

Edit ▸ Segments ▸ Set default segment register value… → register T = 1 (or press Alt+G). IDA reanalyzes the segment as Thumb automatically.

B. Seed the code from the vector table.

G → 10000000. The first two words are the initial stack pointer and the reset vector.
Read the 4-byte word at 0x10000004 (do not assume its value — it differs per build).
Clear bit 0 (the ARM Thumb flag) → the reset handler address.
G to that address → press C (MakeCode). IDA disassembles the reset handler and follows its bls into main.

Once main shows code, the import is correct.

Why IDA needs this and others don't: IDA's generic raw-binary ARM loader treats the image as flat data and waits for you to set Thumb and mark the entry point. It's an IDA quirk, not a mistake on your part.

Step 8: The views you will use

Linear view: the disassembly listing. You navigate, read, and patch here.
Graph view: the control-flow graph of the current function.
Decompiler (HLIL): the pseudo-C decompilation.
Hex view: raw bytes, used for patching.
Function list: the sidebar list of every detected function.

Navigation: G go to address, N rename, Y set type or signature, ; add a comment.

macOS function keys: the top-row F keys are usually mapped to system functions. Every step here uses menu paths that work without them.

Part 3: Static — Resolve the Functions in IDA and Patch

Step 16: Resolve the functions in the IDA GUI

We now name the functions in IDA using the ELF symbol map from Step 4. IDA loaded the raw .bin with no symbols, so every function shows as sub_<addr> — resolving means giving each one its real name and signature.

Three keys do all the work:

Key	IDA action	Use it for
`G`	Go to address	Jump to a function's address
`Y`	Change Type	Set the function's signature. The dialog shows the full prototype, so this sets the name and the type in one step.
`N`	Rename	Rename only, when you just want the name and not the type

For each function below: G to its address, then Y (Change Type) and type the prototype from the table.

How to resolve a function in IDA (`Y`)

Y is the Change Type key, and it is what actually resolves the function — it turns void sub_100037fc() into bool stdio_init_all(void). The Change Type dialog shows the full declaration (name and type), so typing the prototype sets both:

G to the function's address. The cursor lands on the function.
Press Y. In the Change Type dialog, type the prototype from the table exactly — for example bool stdio_init_all(void) — and press Enter.

The decompiler header then shows the real prototype, and calls to the function read cleanly instead of sub_<addr>(). N is only for renaming without touching the type; Y alone sets both the name and the type.

If Y seems to do nothing, confirm the cursor is on the function, or right-click it and pick Change Type.... IDA parses what you type and silently keeps the old type if it does not parse, so glance at the header after each Y.

Worked example: `main`

Press G, type 0x10000234, press Enter. The view jumps there; the cursor lands on sub_10000234.
Press Y (Change Type), type int main(void), press Enter. That sets the name to main and the type to int(void).

IDA shows int32_t where Ghidra shows int. After you set int main(void), the decompiler header may read int32_t main(void). That is the same type — on this platform int is 32 bits and IDA's parser normalises it to int32_t. Do not fight it; it is not an error.

Worked example: `i2c_init`

G -> 0x10003cdc.
Y -> uint i2c_init(i2c_inst_t* i2c, uint baudrate).

i2c_init returns uint, not void. The SDK's i2c_init returns the actual configured baud rate; the ELF says unsigned int (i2c_inst_t *, uint). Keep the return type.

Worked example: `lcd_i2c_init`

G -> 0x100002bc.
Y -> void lcd_i2c_init(i2c_inst_t* i2c, uint8_t pcf_addr, int nibble_shift, uint8_t backlight_mask).

This is our own lcd_1602.c code. In this build it is one big function: the compiler inlined lcd_store_config, lcd_hd44780_reset, and lcd_hd44780_configure into it.

Worked example: `lcd_set_cursor`

G -> 0x100006f4.
Y -> void lcd_set_cursor(int line, int position).

Worked example: `lcd_puts`

G -> 0x100007f0.
Y -> void lcd_puts(const char* s).

Worked example: `gpio_set_function`

G -> 0x100008f0.
Y -> void gpio_set_function(uint gpio, gpio_function_t fn).

Worked example: `gpio_set_pulls`

G -> 0x1000092c.
Y -> void gpio_set_pulls(uint gpio, bool up, bool down).

This is what gpio_pull_up(2) in our source compiles to: the SDK's static inline gpio_pull_up disappears, and main calls gpio_set_pulls(2, true, false) directly at 0x10000258.

Worked example: `__wrap_printf`

G -> 0x1000398c.
Y -> int __wrap_printf(const char *fmt, ...). Keep the ... — printf is variadic.

Worked example: `stdio_init_all`

G -> 0x100037fc.
Y -> bool stdio_init_all(void).

printf in our source is __wrap_printf in the binary. The SDK links our printf calls to its __wrap_printf wrapper, which forwards to __wrap_vprintf. Rename it printf if you prefer the lesson's shorthand, but __wrap_printf is what the ELF says.

The rest of the chain is the same two keystrokes per function (G, then Y). This is our code plus the library functions it actually calls — not the whole SDK. main calls stdio_init_all, i2c_init, gpio_set_function, gpio_set_pulls, lcd_i2c_init, lcd_set_cursor, lcd_puts, and printf, so we follow that chain down.

The call chain for this project:

main
├── stdio_init_all ── stdio_uart_init ── gpio_set_function, uart_init, stdio_set_driver_enabled
│                                        └── uart_init ── clock_get_hz, busy_wait_us
├── i2c_init
├── gpio_set_function
├── gpio_set_pulls                 (gpio_pull_up inlined)
├── lcd_i2c_init ── i2c_write_blocking, sleep_us, sleep_ms
│   └── pcf_write_byte / pcf_pulse_enable / lcd_write4 / lcd_send / lcd_store_config /
│       lcd_hd44780_reset / lcd_hd44780_configure   (all inlined; no calls)
├── lcd_set_cursor ── i2c_write_blocking, sleep_us
├── lcd_puts ── i2c_write_blocking, sleep_us
└── __wrap_printf ── __wrap_vprintf ── vfctprintf ── _vsnprintf
    │                                              └── _ntoa_format / _out_rev / _out_char
    ├── time_us_64
    └── stdio_out_chars_crlf

Resolve every function in that chain:

Address	Rename to (`N`)	Signature (`Y`)
`0x1000015c`	`_reset_handler`	`void _reset_handler(void)`
`0x10000186`	`platform_entry`	`void platform_entry(void)`
`0x1000019a`	`data_cpy`	`void data_cpy(void, void, void*)`
`0x100001e4`	`_init`	`void _init(void)`
`0x10000210`	`frame_dummy`	`void frame_dummy(void)`
`0x10000234`	`main`	`int main(void)`
`0x100002bc`	`lcd_i2c_init`	`void lcd_i2c_init(i2c_inst_t*, uint8_t, int, uint8_t)`
`0x100006f4`	`lcd_set_cursor`	`void lcd_set_cursor(int, int)`
`0x100007f0`	`lcd_puts`	`void lcd_puts(const char*)`
`0x100008f0`	`gpio_set_function`	`void gpio_set_function(uint, gpio_function_t)`
`0x1000092c`	`gpio_set_pulls`	`void gpio_set_pulls(uint, bool, bool)`
`0x10001368`	`sleep_us`	`void sleep_us(uint64_t)`
`0x10001440`	`sleep_ms`	`void sleep_ms(uint32_t)`
`0x10001624`	`time_us_64`	`uint64_t time_us_64(void)`
`0x10001638`	`busy_wait_us`	`void busy_wait_us(uint64_t)`
`0x100016b8`	`uart_init`	`uint uart_init(uart_inst_t*, uint)`
`0x1000188c`	`clock_get_hz`	`unsigned long clock_get_hz(clock_handle_t)`
`0x100035a4`	`exit`	`void exit(int)`
`0x100035ac`	`runtime_init`	`void runtime_init(void)`
`0x100035d8`	`stdio_out_chars_crlf`	`void stdio_out_chars_crlf(stdio_driver_t, const char, int)`
`0x100036e8`	`stdio_put_string`	`int stdio_put_string(const char*, int, bool, bool)`
`0x100037d4`	`stdio_set_driver_enabled`	`void stdio_set_driver_enabled(stdio_driver_t*, bool)`
`0x100037fc`	`stdio_init_all`	`bool stdio_init_all(void)`
`0x100038c8`	`__wrap_vprintf`	`int __wrap_vprintf(const char*, va_list)`
`0x1000398c`	`__wrap_printf`	`int __wrap_printf(const char*, ...)`
`0x10003b48`	`stdio_uart_init`	`void stdio_uart_init(void)`
`0x10003cdc`	`i2c_init`	`uint i2c_init(i2c_inst_t*, uint)`
`0x10003d28`	`i2c_write_blocking`	`int i2c_write_blocking(i2c_inst_t, uint8_t, const uint8_t, size_t, bool)`
`0x10003e24`	`strlen`	`size_t strlen(const char*)`

Resolve the SDK helpers the printf path reaches:

Address	Rename to (`N`)	Signature (`Y`)
`0x10003548`	`vfctprintf`	`int vfctprintf(void ()(char, void), void, const char, va_list)`
`0x10002b64`	`_vsnprintf`	`int _vsnprintf(out_fct_type, char, size_t, const char, va_list)`
`0x10001f48`	`_ntoa_format`	`unsigned _ntoa_format(out_fct_type, char, size_t, size_t, char, size_t, bool, unsigned, unsigned, unsigned, unsigned)`
`0x10001eac`	`_out_rev`	`unsigned _out_rev(out_fct_type, char, size_t, size_t, const char, size_t, unsigned, unsigned)`
`0x1000211c`	`_out_char`	`void _out_char(char, void*, size_t, size_t)`

A void return type may not stick — here is the fix. IDA treats void as low-confidence, and its analysis can override it with an inferred type — most often int32_t on this 32-bit target. It is most visible on _reset_handler (a hand-written assembly entry that never returns normally), but it can happen to any function whose return type IDA thinks it can infer.

Setting the full signature with Y reproduces the unwanted int32_t, and fn.return_type = ... fails too. What works is the return-value setter:
from binaryninja import ReturnValue, Type
fn = bv.get_function_at(0x1000015c)
if fn is not None:
    fn.return_value = ReturnValue(Type.void())
That holds _reset_handler at void even after reanalysis. If it still will not stick, leave it — it does not affect the rest of the lesson.

i2c1_inst is data, not a function. arm-none-eabi-nm -n lists 2000062c T i2c1_inst. The T is a global data symbol: the linker parks the i2c1_inst struct at RAM address 0x2000062c. Its first word is the hardware pointer 0x40098000, the I2C1 register base. There is no function there; do not Y it with a prototype.

Shortcut — resolves name and type for every function. Instead of doing N + Y by hand, paste this into IDA's Python console (Plugins -> Python Console). It sets each function's name and signature programmatically:

from binaryninja import Symbol, SymbolType
# The raw .bin has no headers, so these SDK types don't exist. set_user_type()
# re-parses each signature as C, so an undefined name raises
# "SyntaxError: unknown type name '...'". Define them first.
sdk = bv.parse_types_from_string("""
typedef unsigned int uint;
typedef char* va_list;
typedef unsigned long clock_handle_t;
typedef void (*out_fct_type)(char, void*, size_t, size_t);
struct stdio_driver;
typedef struct stdio_driver stdio_driver_t;
struct uart_inst;
typedef struct uart_inst uart_inst_t;
struct i2c_inst;
typedef struct i2c_inst i2c_inst_t;
enum gpio_function {
    GPIO_FUNC_XIP = 0, GPIO_FUNC_SPI = 1, GPIO_FUNC_UART = 2, GPIO_FUNC_I2C = 3,
    GPIO_FUNC_PWM = 4, GPIO_FUNC_SIO = 5, GPIO_FUNC_PIO0 = 6, GPIO_FUNC_PIO1 = 7,
    GPIO_FUNC_GPCK = 8, GPIO_FUNC_USB = 9, GPIO_FUNC_NULL = 0x1f,
};
typedef enum gpio_function gpio_function_t;
""")
for name, ty in sdk.types.items():
    bv.define_user_type(name, ty)

# address: (name, signature)
funcs = {
    0x1000015c: ("_reset_handler",            "void _reset_handler(void)"),
    0x10000186: ("platform_entry",            "void platform_entry(void)"),
    0x1000019a: ("data_cpy",                  "void data_cpy(void*, void*, void*)"),
    0x100001e4: ("_init",                     "void _init(void)"),
    0x10000210: ("frame_dummy",               "void frame_dummy(void)"),
    0x10000234: ("main",                      "int main(void)"),
    0x100002bc: ("lcd_i2c_init",              "void lcd_i2c_init(i2c_inst_t*, uint8_t, int, uint8_t)"),
    0x100006f4: ("lcd_set_cursor",            "void lcd_set_cursor(int, int)"),
    0x100007f0: ("lcd_puts",                  "void lcd_puts(const char*)"),
    0x100008f0: ("gpio_set_function",         "void gpio_set_function(uint, gpio_function_t)"),
    0x1000092c: ("gpio_set_pulls",            "void gpio_set_pulls(uint, bool, bool)"),
    0x10001368: ("sleep_us",                  "void sleep_us(uint64_t)"),
    0x10001440: ("sleep_ms",                  "void sleep_ms(uint32_t)"),
    0x10001624: ("time_us_64",                "uint64_t time_us_64(void)"),
    0x10001638: ("busy_wait_us",              "void busy_wait_us(uint64_t)"),
    0x100016b8: ("uart_init",                 "uint uart_init(uart_inst_t*, uint)"),
    0x1000188c: ("clock_get_hz",              "unsigned long clock_get_hz(clock_handle_t)"),
    0x100035a4: ("exit",                      "void exit(int)"),
    0x100035ac: ("runtime_init",              "void runtime_init(void)"),
    0x100035d8: ("stdio_out_chars_crlf",      "void stdio_out_chars_crlf(stdio_driver_t*, const char*, int)"),
    0x100036e8: ("stdio_put_string",          "int stdio_put_string(const char*, int, bool, bool)"),
    0x100037d4: ("stdio_set_driver_enabled",  "void stdio_set_driver_enabled(stdio_driver_t*, bool)"),
    0x100037fc: ("stdio_init_all",            "bool stdio_init_all(void)"),
    0x100038c8: ("__wrap_vprintf",            "int __wrap_vprintf(const char*, va_list)"),
    0x1000398c: ("__wrap_printf",             "int __wrap_printf(const char*, ...)"),
    0x10003b48: ("stdio_uart_init",           "void stdio_uart_init(void)"),
    0x10003cdc: ("i2c_init",                  "uint i2c_init(i2c_inst_t*, uint)"),
    0x10003d28: ("i2c_write_blocking",        "int i2c_write_blocking(i2c_inst_t*, uint8_t, const uint8_t*, size_t, bool)"),
    0x10003e24: ("strlen",                    "size_t strlen(const char*)"),
    0x10003548: ("vfctprintf",                "int vfctprintf(void (*)(char, void*), void*, const char*, va_list)"),
    0x10002b64: ("_vsnprintf",                "int _vsnprintf(out_fct_type, char*, size_t, const char*, va_list)"),
    0x10001f48: ("_ntoa_format",              "unsigned _ntoa_format(out_fct_type, char*, size_t, size_t, char*, size_t, bool, unsigned, unsigned, unsigned, unsigned)"),
    0x10001eac: ("_out_rev",                  "unsigned _out_rev(out_fct_type, char*, size_t, size_t, const char*, size_t, unsigned, unsigned)"),
    0x1000211c: ("_out_char",                 "void _out_char(char, void*, size_t, size_t)"),
}
for addr, (name, sig) in funcs.items():
    bv.define_user_symbol(Symbol(SymbolType.FunctionSymbol, addr, name))
    f = bv.get_function_at(addr)
    if f is not None:
        f.set_user_type(sig)

SDK type names (i2c_inst_t, uart_inst_t, stdio_driver_t, gpio_function_t, plus uint, va_list, clock_handle_t, and out_fct_type) are not in the raw .bin. set_user_type re-parses each signature as C, so an undefined name raises SyntaxError: unknown type name '...' and stops the loop — it is not harmless. The sdk block above defines them first (an opaque struct/enum/typedef is enough to parse). If you add a function that uses another SDK type, add a definition for it to that block too.

Step 17: Read `main` in the decompiler

Open the Decompiler view on main. Once the functions above are typed, it reads roughly:

int32_t main(void)
{
    stdio_init_all();
    i2c_init(&i2c1_inst, 0x186a0);              // i2c_init(i2c1, 100000)
    gpio_set_function(2, GPIO_FUNC_I2C);
    gpio_set_function(3, GPIO_FUNC_I2C);
    gpio_set_pulls(2, true, false);             // gpio_pull_up(2)
    gpio_set_pulls(3, true, false);             // gpio_pull_up(3)
    lcd_i2c_init(&i2c1_inst, 0x27, 4, 8);       // lcd_i2c_init(i2c1, 0x27, 4, 0x08)
    lcd_set_cursor(0, 0);
    lcd_puts("Reverse");
    lcd_set_cursor(1, 0);
    lcd_puts("Engineering");
    while (true) {
        __wrap_printf("FAV_NUM: %d\r\n", 0x2a);         // FAV_NUM  = 42
        __wrap_printf("OTHER_FAV_NUM: %d\r\n", 0x539);  // OTHER_FAV_NUM = 1337
    }
}

The 0x2a and 0x539 are the constants we will patch. Both are immediates in the instruction stream — there is no .rodata word to change, which is why the patch edits the instruction operand. Now make the hacks permanent.

Step 18: Patch 1 — change `FAV_NUM` from 42 to 43

Go to 0x1000028e:

1000028e: 2a 21   movs  r1, #42   @ 0x2a

The halfword is 0x212a, stored little-endian as 2a 21. The immediate is the low byte, so the byte at the instruction's own address is 0x2a. Change it to 0x2b (43).

Project	Address	Before	After	Effect
`0x0017`	`0x1000028e`	`2a`	`2b`	`movs r1, #42` -> `#43`, prints `FAV_NUM: 43`

Option A — Hex view:

Switch to the Hex view (View -> Hex).
Toggle the lock off so editing is enabled.
Go to 0x1000028e and change the byte 2A to 2B.
Return to the linear view, right-click the function -> Reanalyze.

Option B — Python console:

bv.write(0x1000028e, b"\x2b")
print(hex(bv.read(0x1000028e, 1)[0]))  # -> 0x2b

After reanalysis the instruction reads movs r1, #43.

Step 18b: Patch 2 — change `OTHER_FAV_NUM` from 1337 to 1344

Go to 0x10000296:

10000296: 40 f2 39 51   movw  r1, #1337   @ 0x539

This is the 32-bit Thumb-2 encoding of movw r1, #0x539. The four bytes and their roles:

+-----------------------------------------------------------------+
|  movw r1, #0x539  ->  bytes: 40 F2 39 51                        |
|                                                                 |
|  Byte 0: 0x40  -+                                               |
|  Byte 1: 0xF2  -+   First halfword (opcode + upper imm bits)    |
|  Byte 2: 0x39  ---- Lower 8 bits of immediate (imm8) <- CHANGE  |
|  Byte 3: 0x51  ---- Destination register (r1) + upper imm bits  |
|                                                                 |
|  imm16 = 0x0539 = 1337 decimal                                  |
|  imm8 field = 0x39 (lower 8 bits of the value)                  |
|                                                                 |
+-----------------------------------------------------------------+

The imm8 byte is the third byte of the 4-byte instruction: the instruction starts at 0x10000296, so the byte to change is 0x10000296 + 2 = 0x10000298. Change 0x39 to 0x40, which changes the value from 0x539 (1337) to 0x540 (1344).

Project	Address	Before	After	Effect
`0x0017`	`0x10000298`	`39`	`40`	`movw r1, #1337` -> `#1344`, prints `OTHER_FAV_NUM: 1344`

Option A — Hex view:

Switch to the Hex view (View -> Hex).
Go to 0x10000298 and change the byte 39 to 40.
Return to the linear view and reanalyze.

Option B — Python console:

bv.write(0x10000298, b"\x40")
print(bv.read(0x10000296, 4).hex())  # -> 40f24051

Do not patch 0x10000296 itself. That is the instruction's first byte (the opcode), not the immediate. The immediate's low 8 bits are at 0x10000298; patching the opcode corrupts the instruction.

Step 18c: Patch 3 — change the LCD text from `"Reverse"` to `"Exploit"`

The string "Reverse" starts at 0x10003ee8. Its eight bytes are 52 65 76 65 72 73 65 00 (Reverse\0). Change them to 45 78 70 6c 6f 69 74 00 (Exploit\0). Both strings are exactly seven characters, so the replacement fits without touching "Engineering" at 0x10003ef0.

Project	Address	Before	After	Effect
`0x0017`	`0x10003ee8`	`52 65 76 65 72 73 65 00`	`45 78 70 6c 6f 69 74 00`	LCD line 1 prints `Exploit` instead of `Reverse`

ASCII reference:

Character	Hex
E	`0x45`
x	`0x78`
p	`0x70`
l	`0x6c`
o	`0x6f`
i	`0x69`
t	`0x74`

Option A — Hex view:

Switch to the Hex view (View -> Hex).
Go to 0x10003ee8 and change the eight bytes 52 65 76 65 72 73 65 00 to 45 78 70 6c 6f 69 74 00.
Return to the linear view and reanalyze.

Option B — Python console:

bv.write(0x10003ee8, b"Exploit\x00")
print(bv.read(0x10003ee8, 8))  # -> b'Exploit\x00'

Keep the replacement exactly eight bytes. If you use a shorter string you must pad it and keep the terminating \0, or lcd_puts will run into the "Engineering" string that follows at 0x10003ef0.

Step 19: Export the patched `.bin`

import os
seg = next(s for s in bv.segments if s.data_length)  # the loadable image segment
data = bv.read(seg.start, seg.data_length)  # base + size come from the view itself
out = os.path.join(os.path.join(root, "0x0017_constants", "build"), "0x0017_constants-h.bin")
open(out, "wb").write(data)
print(len(data), out)  # -> 17980 /.../build/0x0017_constants-h.bin

Where the two numbers come from — nothing is hardcoded:

seg.start is the image base IDA loaded the .bin at (0x10000000), the same value you pass to uf2conv --base.
seg.data_length is the segment's size in the file (0x463c = 17980). Exactly one segment carries data (the image); every peripheral and synthetic segment has data_length == 0, so next(...) picks the image.
Reading seg.start for seg.data_length bytes therefore grabs exactly the image.

Two gotchas this avoids:

No relative path. IDA's Python console runs with a read-only working directory (inside the app bundle), so open("0x0017_constants-h.bin", "wb") fails with OSError: [Errno 30] Read-only file system. root (from ~/.embedded-hacking-repo, Step 3) is the repo, so the file is written into the project's build/ — no machine-specific path and no database needed.
Read the image, not the whole view. bv.read(bv.start, bv.length) spans the entire mapped range, which is not the image. The segment's data_length is the image size.

A different size means you exported a partial view.

Step 20: Convert to UF2

Run from the project directory:

macOS Apple Silicon / Linux x64:

python3 ../uf2conv.py 0x0017_constants-h.bin \
  --base 0x10000000 --family 0xe48bff59 --output hacked.uf2

Windows x64:

python ..\uf2conv.py 0x0017_constants-h.bin ^
  --base 0x10000000 --family 0xe48bff59 --output hacked.uf2

Or convert from the IDA console — it is a normal Python interpreter, so you never have to leave the app. chdir to a writable directory first (the default one is read-only), then run the script:
import os, sys, runpy
os.chdir(os.path.join(root, "0x0017_constants", "build"))   # the project build dir (writable)
sys.argv = ["uf2conv.py", "0x0017_constants-h.bin",
            "--base", "0x10000000", "--family", "0xe48bff59", "--output", "hacked.uf2"]
runpy.run_path("../../uf2conv.py", run_name="__main__")   # path to your uf2conv.py
This writes hacked.uf2 next to the .bin, ready to drag onto the Pico.

Static stopping point

The patched image has been exported and converted to UF2. This IDA guide ends at static verification; it does not connect to, flash, or debug a live target.

Glossary

Term	Definition
AAPCS	ARM Architecture Procedure Call Standard — `r0`-`r3` for the first four arguments, `r0` for the return value
`.bss`	Section for uninitialized (or zero-initialized) static/global variables; zeroed by startup code
`const`	A source-level "read-only" qualifier; the compiler may still inline it as an immediate
`.data`	Section for initialized static/global variables; copied from flash to SRAM at boot
`#define`	Preprocessor text replacement performed before compilation; consumed by the compiler as a literal
`.elf`	Linked image with the symbol table; the ground truth for addresses and names
`imm8`	The low 8 bits of a `movw` immediate, stored in the third byte of the 32-bit instruction
Immediate value	A constant embedded directly in an instruction, not fetched from memory
I2C	Inter-Integrated Circuit — a two-wire (SDA/SCL) serial bus; open-drain, needs pull-ups
Literal pool	A block of 32-bit constants that Thumb-2 code reaches with PC-relative `ldr`
`movs`	16-bit Thumb move that loads an 8-bit immediate (0-255)
`movw`	32-bit Thumb-2 "move wide" that loads any 16-bit immediate (0-65535)
Open-drain	An output that can only pull a line LOW, not drive it HIGH; pull-ups restore HIGH
PCF8574	The I2C I/O expander on a typical 1602 LCD backpack; commonly at `0x27`
`.rodata`	Read-only section for constants and string literals; stays in flash
SCL / SDA	I2C Serial Clock and Serial Data lines
Struct	A user-defined type that groups related variables; the SDK uses one per I2C controller
Thumb bit	Bit 0 of a Cortex-M function pointer; selects Thumb instruction mode
`typedef`	Creates an alias for a type (for example `typedef struct i2c_inst i2c_inst_t`)
UF2	USB Flashing Format — the file format the Pico 2 bootloader accepts
Vector table	The first words of flash: initial stack pointer and exception vectors

Remember: the ELF tells you what every address is, and the .bin is what you actually patch. Verify the static behavior from the recovered instructions.csymbols), then patch the bytes —0x2a -> 0x2b, 0x39 -> 0x40`, and the eight-byte LCD string — and export the patched artifact.

42 KiB Raw Blame History

Week 7-IDA: IDA Pro — Read, Hack, and Patch Constants and a 1602 LCD String (Raw .bin)

What You'll Learn This Week

How This Guide Works

Part 1: Build and Get the Symbol Map

Step 1: Install the toolchain

Step 4: Dump the ELF symbol map

Part 2: Load the Raw .bin into IDA

Step 7: Bring the raw .bin into IDA

1.1 Open the file

1.2 Set the format and processor

1.3 Configure the ARM architecture and Thumb mode

1.4 Set the loading address

1.5 Force Thumb, then seed the code from the vector table

Step 8: The views you will use

Part 3: Static — Resolve the Functions in IDA and Patch

Step 16: Resolve the functions in the IDA GUI

How to resolve a function in IDA (Y)

Worked example: main

Worked example: i2c_init

Worked example: lcd_i2c_init

Worked example: lcd_set_cursor

Worked example: lcd_puts

Worked example: gpio_set_function

Worked example: gpio_set_pulls

Worked example: __wrap_printf

Worked example: stdio_init_all

Step 17: Read main in the decompiler

Step 18: Patch 1 — change FAV_NUM from 42 to 43

Step 18b: Patch 2 — change OTHER_FAV_NUM from 1337 to 1344

Step 18c: Patch 3 — change the LCD text from "Reverse" to "Exploit"

Step 19: Export the patched .bin

Step 20: Convert to UF2

Static stopping point

Glossary

42 KiB

Raw Blame History

Week 7-IDA: IDA Pro — Read, Hack, and Patch Constants and a 1602 LCD String (Raw `.bin`)

Part 2: Load the Raw `.bin` into IDA

Step 7: Bring the raw `.bin` into IDA

How to resolve a function in IDA (`Y`)

Worked example: `main`

Worked example: `i2c_init`

Worked example: `lcd_i2c_init`

Worked example: `lcd_set_cursor`

Worked example: `lcd_puts`

Worked example: `gpio_set_function`

Worked example: `gpio_set_pulls`

Worked example: `__wrap_printf`

Worked example: `stdio_init_all`

Step 17: Read `main` in the decompiler

Step 18: Patch 1 — change `FAV_NUM` from 42 to 43

Step 18b: Patch 2 — change `OTHER_FAV_NUM` from 1337 to 1344

Step 18c: Patch 3 — change the LCD text from `"Reverse"` to `"Exploit"`

Step 19: Export the patched `.bin`