2.4 KiB
Tutorial 07 - UART Chainloader
tl;dr
Running from an SD card was a nice experience, but it would be extremely tedious to do it for every
new binary. Let's write a chainloader using position independent code. This will be the last
binary you need to put on the SD card. Each following tutorial will provide a chainboot target in
the Makefile that lets you conveniently load the kernel over UART.
Install and test it
Our chainloader is called MiniLoad and is inspired by raspbootin.
You can try it with this tutorial already:
- Depending on your target hardware:
makeorBSP=rpi4 make. - Copy
kernel8.imgto the SD card. - Execute
make chainbootorBSP=rpi4 make chainboot. - Now plug in the USB Serial.
- Observe the loader fetching a kernel over
UART:
$ make chainboot
[...]
Minipush 1.0
[MP] ⏳ Waiting for /dev/ttyUSB0
[MP] ✅ Connected
__ __ _ _ _ _
| \/ (_)_ _ (_) | ___ __ _ __| |
| |\/| | | ' \| | |__/ _ \/ _` / _` |
|_| |_|_|_||_|_|____\___/\__,_\__,_|
Raspberry Pi 3
[ML] Requesting binary
[MP] ⏩ Pushing 7 KiB ==========================================🦀 100% 0 KiB/s Time: 00:00:00
[ML] Loaded! Executing the payload now
[0] Booting on: Raspberry Pi 3
[1] Drivers loaded:
1. BCM GPIO
2. BCM PL011 UART
[2] Chars written: 93
[3] Echoing input now
In this tutorial, a version of the kernel from the previous tutorial is loaded for demo purposes. In subsequent tuts, it will be the working directory's kernel.
Test it
The Makefile in this tutorial has an additional target, qemuasm, that lets
you nicely observe the jump from the loaded address (0x80_XXX) to the
relocated code at (0x3EFF_0XXX):
$ make qemuasm
[...]
IN:
0x00080990: d0000008 adrp x8, #0x82000
0x00080994: 52800020 movz w0, #0x1
0x00080998: f9416908 ldr x8, [x8, #0x2d0]
0x0008099c: d63f0100 blr x8
----------------
IN:
0x3eff0b10: d0000008 adrp x8, #0x3eff2000
0x3eff0b14: d0000009 adrp x9, #0x3eff2000
0x3eff0b18: f941ad08 ldr x8, [x8, #0x358]
0x3eff0b1c: f941b129 ldr x9, [x9, #0x360]
0x3eff0b20: eb08013f cmp x9, x8
0x3eff0b24: 540000c2 b.hs #0x3eff0b3c
[...]