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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 9-IDA: IDA Pro — Read, Hack, and Patch Operator Immediates and a DHT11 Float Scale (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 the six C operator results as bare instruction immediates (movs r1, #50, movs r1, #5, …)
Read the DHT11 single-wire driver, including the inlined gpio_* and time_us_32 helpers ted value live t
Resolve the functions in the IDA GUI using the ELF symbol map, including the dht11.c symbols
Patch the arithmetic immediate (50 -> 99) and the DHT11 scaling constant (0.1f -> 5.0f)
Rename the printed label by patching the "arithmetic_operator:" string
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 9 lesson, and the Week 9 lesson is a Release build. Release optimizes the code the same way the original lesson was built: it inlines the static helpers print_operator_results, compute_arithmetic_ops, compute_operators, and print_dht11_reading straight into main, and it inlines the whole dht11.c static helper chain (send_start_signal, wait_for_level, wait_response, read_bit, read_40_bits, validate_checksum) into dht11_read. Every operator result is folded to a bare movs r1, #imm immediate. 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
`0x001a_operators`	six operator lines, then `Humidity: …%, Temperature: …°C`	DHT11 single-wire sensor on GPIO 4	`50 -> 99` (arithmetic immediate), `0.1f -> 5.0f` (DHT11 scale), and `"arithmetic_operator:" -> "hacked_operator:"`

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 how small the operator code is. The C source computes x * y, x > y, x << 1, and x += 5 in a helper function, but Release folds every one of those into a compile-time constant and stores it as a one-byte immediate inside a movs r1, #imm instruction. There is no variable in memory to change — the answer is baked into the instruction stream. The DHT11 reading is different: it is a real measurement scaled by a float constant (0.1f) sitting in the literal pool, and that one is a data word you can patch.

A note on the sensor reading. The humidity/temperature numbers depend on the physical DHT11 attached to GPIO 4. This guide's ground truth is about the code, not the sensor: the scale constant, where dht11_read lives, and which register holds the reading.

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 `0x001a_operators/`:

```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, "0x001a_operators")
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, "0x001a_operators")
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, "0x001a_operators")
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:

0x001a_operators/build/0x001a_operators.elf and .bin — the .bin is 17484 bytes (0x444c)

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/0x001a_operators.elf | grep -E ' [Tt] '

Windows x64:

arm-none-eabi-nm -n --defined-only build\0x001a_operators.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 (all four static helpers inlined)
`0x100002d4`	`dht11_init`	`void dht11_init(uint8_t)`	the `dht11.c` init function
`0x100002f4`	`dht11_read`	`bool dht11_read(float, float)`	the `dht11.c` read function (all six static helpers inlined)

The GPIO and timer functions main / dht11.c reach:

Address	ELF symbol	Signature	Role
`0x10000440`	`gpio_set_function`	`void gpio_set_function(uint, gpio_function_t)`	SDK GPIO function select (UART pins)
`0x1000047c`	`gpio_set_pulls`	`void gpio_set_pulls(uint, bool, bool)`	SDK pull config (`gpio_pull_up` inlined)
`0x100004a4`	`gpio_init`	`void gpio_init(uint)`	SDK GPIO init (`dht11_init` calls it)
`0x10000f00`	`sleep_us`	`void sleep_us(uint64_t)`	SDK microsecond delay (`dht11_read` calls it)
`0x10000fd8`	`sleep_ms`	`void sleep_ms(uint32_t)`	SDK millisecond delay (`main`, `dht11_read`)
`0x100011bc`	`time_us_64`	`uint64_t time_us_64(void)`	SDK microsecond clock (`printf` path)
`0x100011d0`	`busy_wait_us`	`void busy_wait_us(uint64_t)`	UART timing loop
`0x10001250`	`uart_init`	`uint uart_init(uart_inst_t*, uint)`	SDK UART init
`0x10001424`	`clock_get_hz`	`unsigned long clock_get_hz(clock_handle_t)`	UART clock lookup

The stdio/UART and printf chain main reaches:

Address	ELF symbol	Signature	Role
`0x1000313c`	`exit`	`void exit(int)`	C runtime exit
`0x10003144`	`runtime_init`	`void runtime_init(void)`	SDK runtime init
`0x10003170`	`stdio_out_chars_crlf`	`void stdio_out_chars_crlf(stdio_driver_t, const char, int)`	CRLF output driver
`0x10003280`	`stdio_put_string`	`int stdio_put_string(const char*, int, bool, bool)`	buffered string output
`0x1000336c`	`stdio_set_driver_enabled`	`void stdio_set_driver_enabled(stdio_driver_t*, bool)`	enable the UART driver
`0x10003394`	`stdio_init_all`	`bool stdio_init_all(void)`	SDK serial init
`0x10003424`	`__wrap_puts`	`int __wrap_puts(const char*)`	the `puts` wrapper (`DHT11 read failed`)
`0x10003460`	`__wrap_vprintf`	`int __wrap_vprintf(const char*, va_list)`	printf core
`0x10003524`	`__wrap_printf`	`int __wrap_printf(const char*, ...)`	the `printf` wrapper (all seven calls)
`0x100036e0`	`stdio_uart_init`	`void stdio_uart_init(void)`	SDK UART stdio init
`0x10003820`	`strlen`	`size_t strlen(const char*)`	C runtime string length

SDK helpers the printf path reaches:

Address	ELF symbol	Signature	Role
`0x10001a44`	`_out_rev`	`unsigned _out_rev(out_fct_type, char, size_t, size_t, const char, size_t, unsigned, unsigned)`	reversed-digit output
`0x10001ae0`	`_ntoa_format`	`unsigned _ntoa_format(out_fct_type, char, size_t, size_t, char, size_t, bool, unsigned, unsigned, unsigned, unsigned)`	number formatter
`0x10001cb4`	`_out_char`	`void _out_char(char, void*, size_t, size_t)`	single-char sink
`0x100026fc`	`_vsnprintf`	`int _vsnprintf(out_fct_type, char, size_t, const char, va_list)`	the format dispatcher
`0x100030e0`	`vfctprintf`	`int vfctprintf(void ()(char, void), void, const char, va_list)`	printf format engine

Things in this project that have no symbol of their own, because the compiler inlined them:

print_operator_results, compute_arithmetic_ops, compute_operators, print_dht11_reading — the four static helpers in 0x001a_operators.c are inlined into main, so their bodies appear directly inside main.
send_start_signal, wait_for_level, wait_response, read_bit, read_40_bits, validate_checksum — the six static helpers in dht11.c are inlined into dht11_read, along with gpio_set_dir, gpio_put, gpio_get, and time_us_32 from the SDK headers. You see mcrr (SIO writes), ldr [.., #4] (SIO reads), and ldr [r0, #40] (TIMER0 read) directly inside dht11_read.
gpio_pull_up — static inline in the SDK, so dht11_init compiles to a direct tail-call to gpio_set_pulls.

dht_pin is a b symbol, not a function. arm-none-eabi-nm -n lists 20000b7c b dht_pin. That lowercase b is a local data symbol: the dht11.c file-scope static uint dht_pin; is parked in RAM .bss at 0x20000b7c. It holds the GPIO number (4) after dht11_init(4) runs. There is no function there; do not Y it with a prototype.

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.

Optional — load the RP2350 SVD for peripheral names. If you want IDA to label peripheral registers (TIMER0, SIO, UART0, …) instead of bare addresses, load the RP2350 SVD. It ships with Week 4, so it lives at WEEK04/rp2350.svd inside the repository — remember it is in Week 4, not this week. This lesson does not need it; the ELF symbol map already names every function.

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_10003394() 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: `dht11_init`

G -> 0x100002d4.
Y -> void dht11_init(uint8_t pin).

This is our own dht11.c code. In this build the SDK's gpio_init and gpio_pull_up are called directly (gpio_pull_up as a tail-call to gpio_set_pulls), so the function is small.

Worked example: `dht11_read`

G -> 0x100002f4.
Y -> bool dht11_read(float* humidity, float* temperature).

This is the big one: all six dht11.c static helpers are inlined here, along with gpio_set_dir / gpio_put / gpio_get / time_us_32, so dht11_read contains the entire single-wire protocol and the 0.1f scale.

Worked example: `gpio_init`

G -> 0x100004a4.
Y -> void gpio_init(uint gpio).

Worked example: `gpio_set_pulls`

G -> 0x1000047c.
Y -> void gpio_set_pulls(uint gpio, bool up, bool down).

This is what gpio_pull_up(4) in dht11_init compiles to.

Worked example: `__wrap_printf`

G -> 0x10003524.
Y -> int __wrap_printf(const char *fmt, ...). Keep the ... — printf is variadic.

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.

Worked example: `__wrap_puts`

G -> 0x10003424.
Y -> int __wrap_puts(const char *s).

This is the else branch of print_dht11_reading, which prints "DHT11 read failed\r\n". (__wrap_puts is aliased to stdio_puts at the same address.)

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, dht11_init, __wrap_printf, dht11_read, __wrap_puts, and sleep_ms, 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
├── dht11_init ── gpio_init, gpio_set_pulls        (gpio_pull_up inlined)
├── __wrap_printf ── __wrap_vprintf ── vfctprintf ── _vsnprintf
│   │                                              └── _ntoa_format / _out_rev / _out_char
│   ├── stdio_out_chars_crlf
│   └── time_us_64
├── dht11_read ── sleep_ms, sleep_us
│   └── send_start_signal / wait_for_level / wait_response / read_bit /
│       read_40_bits / validate_checksum / gpio_set_dir / gpio_put /
│       gpio_get / time_us_32   (all inlined; no calls)
└── __wrap_puts ── stdio_put_string

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)`
`0x100002d4`	`dht11_init`	`void dht11_init(uint8_t)`
`0x100002f4`	`dht11_read`	`bool dht11_read(float, float)`
`0x10000440`	`gpio_set_function`	`void gpio_set_function(uint, gpio_function_t)`
`0x1000047c`	`gpio_set_pulls`	`void gpio_set_pulls(uint, bool, bool)`
`0x100004a4`	`gpio_init`	`void gpio_init(uint)`
`0x10000f00`	`sleep_us`	`void sleep_us(uint64_t)`
`0x10000fd8`	`sleep_ms`	`void sleep_ms(uint32_t)`
`0x100011bc`	`time_us_64`	`uint64_t time_us_64(void)`
`0x100011d0`	`busy_wait_us`	`void busy_wait_us(uint64_t)`
`0x10001250`	`uart_init`	`uint uart_init(uart_inst_t*, uint)`
`0x10001424`	`clock_get_hz`	`unsigned long clock_get_hz(clock_handle_t)`
`0x1000313c`	`exit`	`void exit(int)`
`0x10003144`	`runtime_init`	`void runtime_init(void)`
`0x10003170`	`stdio_out_chars_crlf`	`void stdio_out_chars_crlf(stdio_driver_t, const char, int)`
`0x10003280`	`stdio_put_string`	`int stdio_put_string(const char*, int, bool, bool)`
`0x1000336c`	`stdio_set_driver_enabled`	`void stdio_set_driver_enabled(stdio_driver_t*, bool)`
`0x10003394`	`stdio_init_all`	`bool stdio_init_all(void)`
`0x10003424`	`__wrap_puts`	`int __wrap_puts(const char*)`
`0x10003460`	`__wrap_vprintf`	`int __wrap_vprintf(const char*, va_list)`
`0x10003524`	`__wrap_printf`	`int __wrap_printf(const char*, ...)`
`0x100036e0`	`stdio_uart_init`	`void stdio_uart_init(void)`
`0x10003820`	`strlen`	`size_t strlen(const char*)`

Resolve the SDK helpers the printf path reaches:

Address	Rename to (`N`)	Signature (`Y`)
`0x10001a44`	`_out_rev`	`unsigned _out_rev(out_fct_type, char, size_t, size_t, const char, size_t, unsigned, unsigned)`
`0x10001ae0`	`_ntoa_format`	`unsigned _ntoa_format(out_fct_type, char, size_t, size_t, char, size_t, bool, unsigned, unsigned, unsigned, unsigned)`
`0x10001cb4`	`_out_char`	`void _out_char(char, void*, size_t, size_t)`
`0x100026fc`	`_vsnprintf`	`int _vsnprintf(out_fct_type, char, size_t, const char, va_list)`
`0x100030e0`	`vfctprintf`	`int vfctprintf(void ()(char, void), void, const char, va_list)`

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.

dht_pin is data, not a function. arm-none-eabi-nm -n lists 20000b7c b dht_pin. That lowercase b is a local data symbol: the linker parks the dht11.c file-scope static at RAM address 0x20000b7c. dht11_init(4) stores 4 into it. 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;
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)"),
    0x100002d4: ("dht11_init",                "void dht11_init(uint8_t)"),
    0x100002f4: ("dht11_read",                "bool dht11_read(float*, float*)"),
    0x10000440: ("gpio_set_function",         "void gpio_set_function(uint, gpio_function_t)"),
    0x1000047c: ("gpio_set_pulls",            "void gpio_set_pulls(uint, bool, bool)"),
    0x100004a4: ("gpio_init",                 "void gpio_init(uint)"),
    0x10000f00: ("sleep_us",                  "void sleep_us(uint64_t)"),
    0x10000fd8: ("sleep_ms",                  "void sleep_ms(uint32_t)"),
    0x100011bc: ("time_us_64",                "uint64_t time_us_64(void)"),
    0x100011d0: ("busy_wait_us",              "void busy_wait_us(uint64_t)"),
    0x10001250: ("uart_init",                 "uint uart_init(uart_inst_t*, uint)"),
    0x10001424: ("clock_get_hz",              "unsigned long clock_get_hz(clock_handle_t)"),
    0x1000313c: ("exit",                      "void exit(int)"),
    0x10003144: ("runtime_init",              "void runtime_init(void)"),
    0x10003170: ("stdio_out_chars_crlf",      "void stdio_out_chars_crlf(stdio_driver_t*, const char*, int)"),
    0x10003280: ("stdio_put_string",          "int stdio_put_string(const char*, int, bool, bool)"),
    0x1000336c: ("stdio_set_driver_enabled",  "void stdio_set_driver_enabled(stdio_driver_t*, bool)"),
    0x10003394: ("stdio_init_all",            "bool stdio_init_all(void)"),
    0x10003424: ("__wrap_puts",               "int __wrap_puts(const char*)"),
    0x10003460: ("__wrap_vprintf",            "int __wrap_vprintf(const char*, va_list)"),
    0x10003524: ("__wrap_printf",             "int __wrap_printf(const char*, ...)"),
    0x100036e0: ("stdio_uart_init",           "void stdio_uart_init(void)"),
    0x10003820: ("strlen",                    "size_t strlen(const char*)"),
    0x10001a44: ("_out_rev",                  "unsigned _out_rev(out_fct_type, char*, size_t, size_t, const char*, size_t, unsigned, unsigned)"),
    0x10001ae0: ("_ntoa_format",              "unsigned _ntoa_format(out_fct_type, char*, size_t, size_t, char*, size_t, bool, unsigned, unsigned, unsigned, unsigned)"),
    0x10001cb4: ("_out_char",                 "void _out_char(char, void*, size_t, size_t)"),
    0x100026fc: ("_vsnprintf",                "int _vsnprintf(out_fct_type, char*, size_t, const char*, va_list)"),
    0x100030e0: ("vfctprintf",                "int vfctprintf(void (*)(char, void*), void*, const char*, va_list)"),
}
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 (stdio_driver_t, uart_inst_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). Standard names (uint8_t, uint32_t, uint64_t, bool, size_t) are built in. 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();
    dht11_init(4);
    while (true) {
        __wrap_printf("arithmetic_operator: %d\r\n", 0x32);   // 50
        __wrap_printf("increment_operator: %d\r\n", 5);       // 5
        __wrap_printf("relational_operator: %d\r\n", 0);      // false
        __wrap_printf("logical_operator: %d\r\n", 0);         // false
        __wrap_printf("bitwise_operator: %d\r\n", 0xa);       // 10
        __wrap_printf("assignment_operator: %d\r\n", 0xa);    // 10
        if (!dht11_read(&hum, &temp)) {
            __wrap_puts("DHT11 read failed\r\n");
        } else {
            __wrap_printf("Humidity: %.1f%%, Temperature: %.1f°C\r\n", hum, temp);
        }
        sleep_ms(2000);
    }
}

The 0x32, 5, 0, 0xa, and 0xa are the constants we will patch first. Every one is an immediate in the instruction stream — there is no variable in memory to change, which is why the patch edits the instruction operand.

Step 18: Patch 1 — change `arithmetic_operator` from 50 to 99

Go to 0x1000026e:

1000026e: 32 21   movs  r1, #50   @ 0x32

movs r1, #imm8 is a 16-bit Thumb instruction. Its encoding is 0x21XX (movs r1, #imm8), stored little-endian as XX 21 — so the immediate is the byte at the instruction's own address, 0x1000026e. Change 0x32 (50) to 0x63 (99).

This is the opposite byte from the Week 7 movw patch. For a 16-bit movs, the immediate is the first byte. For the 32-bit movw, the low immediate byte was the third byte. Always decode the instruction before you decide which byte to touch.

Project	Address	Before	After	Effect
`0x001a`	`0x1000026e`	`32`	`63`	`movs r1, #50` -> `#99`, prints `arithmetic_operator: 99`

Option A — Hex view:

Switch to the Hex view (View -> Hex).
Toggle the lock off so editing is enabled.
Go to 0x1000026e and change the byte 32 to 63.
Return to the linear view, right-click the function -> Reanalyze.

Option B — Python console:

bv.write(0x1000026e, b"\x63")
print(hex(bv.read(0x1000026e, 1)[0]))  # -> 0x63

After reanalysis the instruction reads movs r1, #99.

Step 18b: Patch 2 — change the DHT11 scale from `0.1f` to `5.0f`

The DHT11 driver converts the integer and decimal bytes to a float and scales the decimal part:

humidity    = data[0] + data[1] * 0.1f
temperature = data[2] + data[3] * 0.1f

dht11_read computes both with a fused multiply-add against a single 0.1f constant. Look at the end of dht11_read:

100003ee:	vmov	s15, r6
100003f2:	vcvt.f32.s32	s12, s15
100003f6:	vmov	s15, r1
100003fa:	vcvt.f32.s32	s14, s15
100003fe:	vmov	s15, r0
10000402:	vcvt.f32.s32	s13, s15
10000406:	vmov	s15, r2
1000040a:	vldr	s11, [pc, #44]	@ 10000438 <dht11_read+0x144>
1000040e:	vcvt.f32.s32	s15, s15
10000412:	movs	r0, #1
10000414:	vfma.f32	s14, s12, s11
10000418:	vfma.f32	s15, s13, s11
1000041c:	vstr	s14, [r5]
10000420:	vstr	s15, [fp]

The constant s11 is loaded by the vldr s11, [pc, #44] at 0x1000040a, which reads the literal-pool word at 0x10000438. That word is 0x3dcccccd (float 0.1), stored little-endian as cd cc cc 3d. Change it to 0x40a00000 (float 5.0), bytes 00 00 a0 40:

Project	Address	Before	After	Effect
`0x001a`	`0x10000438`	`cd cc cc 3d`	`00 00 a0 40`	`0.1f` -> `5.0f`; the humidity/temperature decimal parts read ~50x larger

+-----------------------------------------------------------------+
|  IEEE-754 little-endian change                                  |
|                                                                 |
|  0.1f = 0x3dcccccd  ->  bytes  cd cc cc 3d                      |
|  5.0f = 0x40a00000  ->  bytes  00 00 a0 40                      |
|                                                                 |
|  humidity    = int + (decimal * 0.1f)   becomes                  |
|  humidity    = int + (decimal * 5.0f)                           |
|                                                                 |
+-----------------------------------------------------------------+

Option A — Hex view:

Switch to the Hex view (View -> Hex).
Go to 0x10000438 and change cd cc cc 3d to 00 00 a0 40.
Return to the linear view and reanalyze.

Option B — Python console:

bv.write(0x10000438, bytes.fromhex("0000a040"))
print(bv.read(0x10000438, 4).hex())  # -> 0000a040

The scale is applied to both humidity and temperature. The two vfma.f32 instructions both read s11, so patching the one constant changes both readings, not just temperature.

Step 18c: Patch 3 — rename the printed label

The string "arithmetic_operator: %d\r\n" starts at 0x100038e8. Its first 26 bytes are:

61 72 69 74 68 6d 65 74 69 63 5f 6f 70 65 72 61 74 6f 72 3a 20 25 64 0d 0a 00
 a  r  i  t  h  m  e  t  i  c  _  o  p  e  r  a  t  o  r  :     %  d \r \n \0

We replace it with "hacked_operator: %d\r\n\0", which is four bytes shorter, and pad the tail with NULs:

68 61 63 6b 65 64 5f 6f 70 65 72 61 74 6f 72 3a 20 25 64 0d 0a 00 00 00 00 00
 h  a  c  k  e  d  _  o  p  e  r  a  t  o  r  :     %  d \r \n \0 <pad>

The \0 at offset 21 terminates the string, so printf stops there and the four padding bytes are never read. The next format string ("increment_operator: …") begins at 0x10003904, which is 28 bytes past 0x100038e8, so our 26-byte write cannot reach it.

Project	Address	Before	After	Effect
`0x001a`	`0x100038e8`	`61 72 69 74 68 6d 65 74 69 63 5f 6f 70 65 72 61 74 6f 72 3a 20 25 64 0d 0a 00`	`68 61 63 6b 65 64 5f 6f 70 65 72 61 74 6f 72 3a 20 25 64 0d 0a 00 00 00 00 00`	prints `hacked_operator:` instead of `arithmetic_operator:`

Option A — Hex view:

Switch to the Hex view (View -> Hex).
Go to 0x100038e8 and replace the 26 bytes above.
Return to the linear view and reanalyze.

Option B — Python console:

bv.write(0x100038e8, b"hacked_operator: %d\r\n\x00\x00\x00\x00")
print(bv.read(0x100038e8, 26))  # -> b'hacked_operator: %d\r\n\x00\x00\x00\x00'

Keep the NUL. printf walks the format string until it hits \0. If you omit it, the printer runs on into the padding and then into the next string, producing garbage. The four padding bytes guarantee the terminator.

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, "0x001a_operators", "build"), "0x001a_operators-h.bin")
open(out, "wb").write(data)
print(len(data), out)  # -> 17484 /.../build/0x001a_operators-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 (0x444c = 17484). 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("0x001a_operators-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 0x001a_operators-h.bin \
  --base 0x10000000 --family 0xe48bff59 --output hacked.uf2

Windows x64:

python ..\uf2conv.py 0x001a_operators-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, "0x001a_operators", "build"))   # the project build dir (writable)
sys.argv = ["uf2conv.py", "0x001a_operators-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
Arithmetic operator	C operators for math (`+`, `-`, ``, `/`, `%`); here `x y` folds to `#50`
Assignment operator	Compound assignment (`+=`, `-=`, …); here `x += 5` folds to `#10`
Bitwise operator	Operators on individual bits (`<<`, `>>`, `&`, `\|`, `^`); here `x << 1` folds to `#10`
`.bss`	Section for uninitialized (or zero-initialized) static/global variables; `dht_pin` lives here
`.data`	Section for initialized static/global variables; copied from flash to SRAM at boot
DHT11	Low-cost single-wire digital humidity and temperature sensor
`.elf`	Linked image with the symbol table; the ground truth for addresses and names
Fused multiply-add	`vfma.f32 Sd, Sn, Sm` computes `Sd = Sd + (Sn * Sm)` in one operation
Immediate value	A constant embedded directly in an instruction, not fetched from memory
Increment operator	`x++` (post) or `++x` (pre); here `x++` returns the old value, `5`
IEEE-754	Standard for floating-point representation; `0.1f` is `0x3dcccccd`
Literal pool	A block of 32-bit constants that Thumb-2 code reaches with PC-relative `ldr`
Logical operator	Operators combining conditions (`&&`, `\|\|`, `!`); here `false`
`movs`	16-bit Thumb move that loads an 8-bit immediate (0-255); immediate is the first byte
`movw`	32-bit Thumb-2 "move wide" that loads a 16-bit immediate; low imm8 is the third byte
PCF8574	An I2C I/O expander; not used this week (that was the LCD, Week 7)
Relational operator	Comparison operators (`<`, `>`, `==`, `!=`); here `5 > 10` is `false`
`.rodata`	Read-only section for constants and string literals; stays in flash
SIO	Single-cycle I/O block; `gpio_set_dir` / `gpio_put` write it with `mcrr`, `gpio_get` reads it
SVD	System View Description file — register names for a peripheral; the RP2350 one is at `WEEK04/rp2350.svd`
Thumb bit	Bit 0 of a Cortex-M function pointer; selects Thumb instruction mode
TIMER0	RP2350 timer block at `0x400b0000`; `time_us_32` reads its `TIMERAWL` register (`+0x28`)
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 — themovs immediate (0x32 -> 0x63), the DHT11 float word (0.1f -> 5.0f), and the format string (arithmetic_operator:->hacked_operator:`) — and export the patched artifact.

44 KiB Raw Blame History

Week 9-IDA: IDA Pro — Read, Hack, and Patch Operator Immediates and a DHT11 Float Scale (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: dht11_init

Worked example: dht11_read

Worked example: gpio_init

Worked example: gpio_set_pulls

Worked example: __wrap_printf

Worked example: __wrap_puts

Step 17: Read main in the decompiler

Step 18: Patch 1 — change arithmetic_operator from 50 to 99

Step 18b: Patch 2 — change the DHT11 scale from 0.1f to 5.0f

Step 18c: Patch 3 — rename the printed label

Step 19: Export the patched .bin

Step 20: Convert to UF2

Static stopping point

Glossary

44 KiB

Raw Blame History

Week 9-IDA: IDA Pro — Read, Hack, and Patch Operator Immediates and a DHT11 Float Scale (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: `dht11_init`

Worked example: `dht11_read`

Worked example: `gpio_init`

Worked example: `gpio_set_pulls`

Worked example: `__wrap_printf`

Worked example: `__wrap_puts`

Step 17: Read `main` in the decompiler

Step 18: Patch 1 — change `arithmetic_operator` from 50 to 99

Step 18b: Patch 2 — change the DHT11 scale from `0.1f` to `5.0f`

Step 19: Export the patched `.bin`