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rust-raspberrypi-OS-tutorials/07_uart_chainloader/README.md

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# 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`.
[chainloader]: https://en.wikipedia.org/wiki/Chain_loading
[position independent code]: https://en.wikipedia.org/wiki/Position-independent_code
## Note
Please note that there is a lot of stuff going on in this tutorial that is very hard to grasp by
only looking at the source code changes.
Please bear with me until I find the time to write it all down here elaborately. For the time being,
please see this tutorial as an enabler for a convenience feature that allows booting the following
tutorials in a quick manner.
## Install and test it
Our chainloader is called `MiniLoad` and is inspired by [raspbootin].
You can try it with this tutorial already:
1. Depending on your target hardware:`make` or `BSP=rpi4 make`.
1. Copy `kernel8.img` to the SD card.
1. Execute `make chainboot` or `BSP=rpi4 make chainboot`.
1. Connect the USB serial to your host PC.
- Wiring diagram at [top-level README](../README.md#-usb-serial-output).
- Make sure that you **DID NOT** connect the power pin of the USB serial. Only RX/TX and GND.
1. Connect the RPi to the (USB) power cable.
1. Observe the loader fetching a kernel over `UART`:
> ❗ **NOTE**: By default, `make chainboot` tries to connect to `/dev/ttyUSB0`.
> Should the USB serial on your system have a different name, you have to provide it explicitly. For
> example:
>
> `DEV_SERIAL=/dev/tty.usbserial-0001 make chainboot`
[raspbootin]: https://github.com/mrvn/raspbootin
```console
$ make chainboot
[...]
Minipush 1.0
[MP] ⏳ Waiting for /dev/ttyUSB0
[MP] ✅ Serial connected
[MP] 🔌 Please power the target now
__ __ _ _ _ _
| \/ (_)_ _ (_) | ___ __ _ __| |
| |\/| | | ' \| | |__/ _ \/ _` / _` |
|_| |_|_|_||_|_|____\___/\__,_\__,_|
Raspberry Pi 3
[ML] Requesting binary
[MP] ⏩ Pushing 6 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 (`0x0200_0XXX`):
```console
$ make qemuasm
[...]
IN:
0x0008098c: b0000008 adrp x8, #0x81000
0x00080990: b0000000 adrp x0, #0x81000
0x00080994: 912a8000 add x0, x0, #0xaa0
0x00080998: f9471908 ldr x8, [x8, #0xe30]
0x0008099c: d63f0100 blr x8
----------------
IN:
0x02000b1c: b0000008 adrp x8, #0x2001000
0x02000b20: b0000009 adrp x9, #0x2001000
0x02000b24: f9475d08 ldr x8, [x8, #0xeb8]
0x02000b28: f9476129 ldr x9, [x9, #0xec0]
0x02000b2c: eb08013f cmp x9, x8
0x02000b30: 540000c2 b.hs #0x2000b48
[...]
```
## Diff to previous
```diff
Binary files 06_drivers_gpio_uart/demo_payload_rpi3.img and 07_uart_chainloader/demo_payload_rpi3.img differ
Binary files 06_drivers_gpio_uart/demo_payload_rpi4.img and 07_uart_chainloader/demo_payload_rpi4.img differ
diff -uNr 06_drivers_gpio_uart/Makefile 07_uart_chainloader/Makefile
--- 06_drivers_gpio_uart/Makefile
+++ 07_uart_chainloader/Makefile
@@ -21,7 +21,8 @@
OBJDUMP_BINARY = aarch64-none-elf-objdump
NM_BINARY = aarch64-none-elf-nm
LINKER_FILE = src/bsp/raspberrypi/link.ld
- RUSTC_MISC_ARGS = -C target-cpu=cortex-a53
+ RUSTC_MISC_ARGS = -C target-cpu=cortex-a53 -C relocation-model=pic
+ CHAINBOOT_DEMO_PAYLOAD = demo_payload_rpi3.img
else ifeq ($(BSP),rpi4)
TARGET = aarch64-unknown-none-softfloat
KERNEL_BIN = kernel8.img
@@ -31,7 +32,8 @@
OBJDUMP_BINARY = aarch64-none-elf-objdump
NM_BINARY = aarch64-none-elf-nm
LINKER_FILE = src/bsp/raspberrypi/link.ld
- RUSTC_MISC_ARGS = -C target-cpu=cortex-a72
+ RUSTC_MISC_ARGS = -C target-cpu=cortex-a72 -C relocation-model=pic
+ CHAINBOOT_DEMO_PAYLOAD = demo_payload_rpi4.img
endif
# Export for build.rs
@@ -67,13 +69,14 @@
ifeq ($(UNAME_S),Linux)
DOCKER_CMD_DEV = $(DOCKER_CMD_INTERACT) $(DOCKER_ARG_DEV)
- DOCKER_MINITERM = $(DOCKER_CMD_DEV) $(DOCKER_ARG_DIR_UTILS) $(DOCKER_IMAGE)
+ DOCKER_CHAINBOOT = $(DOCKER_CMD_DEV) $(DOCKER_ARG_DIR_UTILS) $(DOCKER_IMAGE)
endif
EXEC_QEMU = $(QEMU_BINARY) -M $(QEMU_MACHINE_TYPE)
-EXEC_MINITERM = ruby ../utils/miniterm.rb
+EXEC_MINIPUSH = ruby ../utils/minipush.rb
-.PHONY: all $(KERNEL_ELF) $(KERNEL_BIN) doc qemu miniterm clippy clean readelf objdump nm check
+.PHONY: all $(KERNEL_ELF) $(KERNEL_BIN) doc qemu qemuasm chainboot clippy clean readelf objdump nm \
+ check
all: $(KERNEL_BIN)
@@ -87,15 +90,18 @@
$(DOC_CMD) --document-private-items --open
ifeq ($(QEMU_MACHINE_TYPE),)
-qemu:
+qemu qemuasm:
@echo "This board is not yet supported for QEMU."
else
qemu: $(KERNEL_BIN)
@$(DOCKER_QEMU) $(EXEC_QEMU) $(QEMU_RELEASE_ARGS) -kernel $(KERNEL_BIN)
+
+qemuasm: $(KERNEL_BIN)
+ @$(DOCKER_QEMU) $(EXEC_QEMU) $(QEMU_RELEASE_ARGS) -kernel $(KERNEL_BIN) -d in_asm
endif
-miniterm:
- @$(DOCKER_MINITERM) $(EXEC_MINITERM) $(DEV_SERIAL)
+chainboot:
+ @$(DOCKER_CHAINBOOT) $(EXEC_MINIPUSH) $(DEV_SERIAL) $(CHAINBOOT_DEMO_PAYLOAD)
clippy:
RUSTFLAGS="$(RUSTFLAGS_PEDANTIC)" $(CLIPPY_CMD)
@@ -107,7 +113,10 @@
readelf --headers $(KERNEL_ELF)
objdump: $(KERNEL_ELF)
- @$(DOCKER_ELFTOOLS) $(OBJDUMP_BINARY) --disassemble --demangle $(KERNEL_ELF) | rustfilt
+ @$(DOCKER_ELFTOOLS) $(OBJDUMP_BINARY) --disassemble --demangle \
+ --section .text \
+ --section .got \
+ $(KERNEL_ELF) | rustfilt
nm: $(KERNEL_ELF)
@$(DOCKER_ELFTOOLS) $(NM_BINARY) --demangle --print-size $(KERNEL_ELF) | sort | rustfilt
diff -uNr 06_drivers_gpio_uart/src/_arch/aarch64/cpu.rs 07_uart_chainloader/src/_arch/aarch64/cpu.rs
--- 06_drivers_gpio_uart/src/_arch/aarch64/cpu.rs
+++ 07_uart_chainloader/src/_arch/aarch64/cpu.rs
@@ -22,12 +22,12 @@
/// actually set (`SP.set()`).
#[no_mangle]
pub unsafe fn _start() -> ! {
- use crate::runtime_init;
+ use crate::relocate;
// Expect the boot core to start in EL2.
if bsp::cpu::BOOT_CORE_ID == cpu::smp::core_id() {
SP.set(bsp::memory::boot_core_stack_end() as u64);
- runtime_init::runtime_init()
+ relocate::relocate_self()
} else {
// If not core0, infinitely wait for events.
wait_forever()
@@ -56,3 +56,19 @@
asm::wfe()
}
}
+
+/// Branch to a raw integer value.
+///
+/// # Safety
+///
+/// - This is highly unsafe. Use with care.
+#[inline(always)]
+pub unsafe fn branch_to_raw_addr(addr: usize) -> ! {
+ asm!(
+ "blr {destination:x}",
+ destination = in(reg) addr,
+ options(nomem, nostack)
+ );
+
+ core::intrinsics::unreachable()
+}
diff -uNr 06_drivers_gpio_uart/src/bsp/device_driver/bcm/bcm2xxx_pl011_uart.rs 07_uart_chainloader/src/bsp/device_driver/bcm/bcm2xxx_pl011_uart.rs
--- 06_drivers_gpio_uart/src/bsp/device_driver/bcm/bcm2xxx_pl011_uart.rs
+++ 07_uart_chainloader/src/bsp/device_driver/bcm/bcm2xxx_pl011_uart.rs
@@ -268,6 +268,15 @@
// readability.
self.inner.lock(|inner| fmt::Write::write_fmt(inner, args))
}
+
+ fn flush(&self) {
+ // Spin until TX FIFO empty is set.
+ self.inner.lock(|inner| {
+ while !inner.registers.FR.matches_all(FR::TXFE::SET) {
+ cpu::nop();
+ }
+ });
+ }
}
impl console::interface::Read for PL011Uart {
@@ -278,18 +287,20 @@
cpu::nop();
}
- // Read one character.
- let mut ret = inner.registers.DR.get() as u8 as char;
-
- // Convert carrige return to newline.
- if ret == '\r' {
- ret = '\n'
- }
-
// Update statistics.
inner.chars_read += 1;
- ret
+ // Read one character.
+ inner.registers.DR.get() as u8 as char
+ })
+ }
+
+ fn clear(&self) {
+ self.inner.lock(|inner| {
+ // Read from the RX FIFO until it is indicating empty.
+ while !inner.registers.FR.matches_all(FR::RXFE::SET) {
+ inner.registers.DR.get();
+ }
})
}
}
diff -uNr 06_drivers_gpio_uart/src/bsp/raspberrypi/link.ld 07_uart_chainloader/src/bsp/raspberrypi/link.ld
--- 06_drivers_gpio_uart/src/bsp/raspberrypi/link.ld
+++ 07_uart_chainloader/src/bsp/raspberrypi/link.ld
@@ -5,12 +5,15 @@
SECTIONS
{
- /* Set current address to the value from which the RPi starts execution */
- . = 0x80000;
+ /* Set the link address to 32 MiB */
+ . = 0x2000000;
+ __binary_start = .;
.text :
{
- *(.text._start) *(.text*)
+ *(.text._start)
+ KEEP(*(.text.runtime_init))
+ *(.text*);
}
.rodata :
@@ -35,5 +38,16 @@
__bss_end_inclusive = . - 8;
}
+ .got :
+ {
+ *(.got*)
+ }
+
+ /* Fill up to 8 byte, b/c relocating the binary is done in u64 chunks */
+ . = ALIGN(8);
+ __binary_end_inclusive = . - 8;
+
+ __runtime_init_reloc = runtime_init;
+
/DISCARD/ : { *(.comment*) }
}
diff -uNr 06_drivers_gpio_uart/src/bsp/raspberrypi/memory.rs 07_uart_chainloader/src/bsp/raspberrypi/memory.rs
--- 06_drivers_gpio_uart/src/bsp/raspberrypi/memory.rs
+++ 07_uart_chainloader/src/bsp/raspberrypi/memory.rs
@@ -12,6 +12,9 @@
// Symbols from the linker script.
extern "Rust" {
+ static __binary_start: UnsafeCell<u64>;
+ static __binary_end_inclusive: UnsafeCell<u64>;
+ static __runtime_init_reloc: UnsafeCell<u64>;
static __bss_start: UnsafeCell<u64>;
static __bss_end_inclusive: UnsafeCell<u64>;
}
@@ -23,10 +26,12 @@
/// The board's memory map.
#[rustfmt::skip]
pub(super) mod map {
- pub const BOOT_CORE_STACK_END: usize = 0x8_0000;
+ pub const BOOT_CORE_STACK_END: usize = 0x8_0000;
- pub const GPIO_OFFSET: usize = 0x0020_0000;
- pub const UART_OFFSET: usize = 0x0020_1000;
+ pub const BOARD_DEFAULT_LOAD_ADDRESS: usize = 0x8_0000;
+
+ pub const GPIO_OFFSET: usize = 0x0020_0000;
+ pub const UART_OFFSET: usize = 0x0020_1000;
/// Physical devices.
#[cfg(feature = "bsp_rpi3")]
@@ -59,13 +64,35 @@
map::BOOT_CORE_STACK_END
}
-/// Return the inclusive range spanning the .bss section.
+/// The address on which the Raspberry firmware loads every binary by default.
+#[inline(always)]
+pub fn board_default_load_addr() -> *const u64 {
+ map::BOARD_DEFAULT_LOAD_ADDRESS as _
+}
+
+/// Return the inclusive range spanning the relocated kernel binary.
+///
+/// # Safety
+///
+/// - Values are provided by the linker script and must be trusted as-is.
+/// - The linker-provided addresses must be u64 aligned.
+pub fn relocated_binary_range_inclusive() -> RangeInclusive<*mut u64> {
+ unsafe { RangeInclusive::new(__binary_start.get(), __binary_end_inclusive.get()) }
+}
+
+/// The relocated address of function `runtime_init()`.
+#[inline(always)]
+pub fn relocated_runtime_init_addr() -> *const u64 {
+ unsafe { __runtime_init_reloc.get() as _ }
+}
+
+/// Return the inclusive range spanning the relocated .bss section.
///
/// # Safety
///
/// - Values are provided by the linker script and must be trusted as-is.
/// - The linker-provided addresses must be u64 aligned.
-pub fn bss_range_inclusive() -> RangeInclusive<*mut u64> {
+pub fn relocated_bss_range_inclusive() -> RangeInclusive<*mut u64> {
let range;
unsafe {
range = RangeInclusive::new(__bss_start.get(), __bss_end_inclusive.get());
diff -uNr 06_drivers_gpio_uart/src/console.rs 07_uart_chainloader/src/console.rs
--- 06_drivers_gpio_uart/src/console.rs
+++ 07_uart_chainloader/src/console.rs
@@ -19,6 +19,10 @@
/// Write a Rust format string.
fn write_fmt(&self, args: fmt::Arguments) -> fmt::Result;
+
+ /// Block execution until the last character has been physically put on the TX wire
+ /// (draining TX buffers/FIFOs, if any).
+ fn flush(&self);
}
/// Console read functions.
@@ -27,6 +31,9 @@
fn read_char(&self) -> char {
' '
}
+
+ /// Clear RX buffers, if any.
+ fn clear(&self);
}
/// Console statistics.
diff -uNr 06_drivers_gpio_uart/src/main.rs 07_uart_chainloader/src/main.rs
--- 06_drivers_gpio_uart/src/main.rs
+++ 07_uart_chainloader/src/main.rs
@@ -100,7 +100,9 @@
//! - `crate::memory::*`
//! - `crate::bsp::memory::*`
+#![feature(asm)]
#![feature(const_fn_fn_ptr_basics)]
+#![feature(core_intrinsics)]
#![feature(format_args_nl)]
#![feature(naked_functions)]
#![feature(panic_info_message)]
@@ -109,7 +111,8 @@
#![no_std]
// `mod cpu` provides the `_start()` function, the first function to run. `_start()` then calls
-// `runtime_init()`, which jumps to `kernel_init()`.
+// `relocate::relocate_self()`. `relocate::relocate_self()` calls `runtime_init()`, which jumps to
+// `kernel_init()`.
mod bsp;
mod console;
@@ -118,6 +121,7 @@
mod memory;
mod panic_wait;
mod print;
+mod relocate;
mod runtime_init;
mod synchronization;
@@ -144,28 +148,52 @@
/// The main function running after the early init.
fn kernel_main() -> ! {
+ use bsp::console::console;
use console::interface::All;
- use driver::interface::DriverManager;
-
- println!("[0] Booting on: {}", bsp::board_name());
- println!("[1] Drivers loaded:");
- for (i, driver) in bsp::driver::driver_manager()
- .all_device_drivers()
- .iter()
- .enumerate()
- {
- println!(" {}. {}", i + 1, driver.compatible());
+ println!(" __ __ _ _ _ _ ");
+ println!("| \\/ (_)_ _ (_) | ___ __ _ __| |");
+ println!("| |\\/| | | ' \\| | |__/ _ \\/ _` / _` |");
+ println!("|_| |_|_|_||_|_|____\\___/\\__,_\\__,_|");
+ println!();
+ println!("{:^37}", bsp::board_name());
+ println!();
+ println!("[ML] Requesting binary");
+ console().flush();
+
+ // Clear the RX FIFOs, if any, of spurious received characters before starting with the loader
+ // protocol.
+ console().clear();
+
+ // Notify `Minipush` to send the binary.
+ for _ in 0..3 {
+ console().write_char(3 as char);
}
- println!(
- "[2] Chars written: {}",
- bsp::console::console().chars_written()
- );
- println!("[3] Echoing input now");
-
- loop {
- let c = bsp::console::console().read_char();
- bsp::console::console().write_char(c);
+ // Read the binary's size.
+ let mut size: u32 = u32::from(console().read_char() as u8);
+ size |= u32::from(console().read_char() as u8) << 8;
+ size |= u32::from(console().read_char() as u8) << 16;
+ size |= u32::from(console().read_char() as u8) << 24;
+
+ // Trust it's not too big.
+ console().write_char('O');
+ console().write_char('K');
+
+ let kernel_addr: *mut u8 = bsp::memory::board_default_load_addr() as *mut u8;
+ unsafe {
+ // Read the kernel byte by byte.
+ for i in 0..size {
+ core::ptr::write_volatile(kernel_addr.offset(i as isize), console().read_char() as u8)
+ }
}
+
+ println!("[ML] Loaded! Executing the payload now\n");
+ console().flush();
+
+ // Use black magic to get a function pointer.
+ let kernel: fn() -> ! = unsafe { core::mem::transmute(kernel_addr) };
+
+ // Jump to loaded kernel!
+ kernel()
}
diff -uNr 06_drivers_gpio_uart/src/relocate.rs 07_uart_chainloader/src/relocate.rs
--- 06_drivers_gpio_uart/src/relocate.rs
+++ 07_uart_chainloader/src/relocate.rs
@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+//
+// Copyright (c) 2018-2020 Andre Richter <andre.o.richter@gmail.com>
+
+//! Relocation code.
+
+use crate::{bsp, cpu};
+
+//--------------------------------------------------------------------------------------------------
+// Public Code
+//--------------------------------------------------------------------------------------------------
+
+/// Relocates the own binary from `bsp::memory::board_default_load_addr()` to the `__binary_start`
+/// address from the linker script.
+///
+/// # Safety
+///
+/// - Only a single core must be active and running this function.
+/// - Function must not use the `bss` section.
+#[inline(never)]
+pub unsafe fn relocate_self() -> ! {
+ let range = bsp::memory::relocated_binary_range_inclusive();
+ let mut relocated_binary_start_addr = *range.start();
+ let relocated_binary_end_addr_inclusive = *range.end();
+
+ // The address of where the previous firmware loaded us.
+ let mut current_binary_start_addr = bsp::memory::board_default_load_addr();
+
+ // Copy the whole binary.
+ while relocated_binary_start_addr <= relocated_binary_end_addr_inclusive {
+ core::ptr::write_volatile(
+ relocated_binary_start_addr,
+ core::ptr::read_volatile(current_binary_start_addr),
+ );
+ relocated_binary_start_addr = relocated_binary_start_addr.offset(1);
+ current_binary_start_addr = current_binary_start_addr.offset(1);
+ }
+
+ // The following function calls form a hack to achieve an "absolute jump" to
+ // `runtime_init::runtime_init()` by forcing an indirection through the global offset table
+ // (GOT), so that execution continues from the relocated binary.
+ //
+ // Without this, the address of `runtime_init()` would be calculated as a relative offset from
+ // the current program counter, since we are compiling as `position independent code`. This
+ // would cause us to keep executing from the address to which the firmware loaded us, instead of
+ // the relocated position.
+ //
+ // There likely is a more elegant way to do this.
+ let relocated_runtime_init_addr = bsp::memory::relocated_runtime_init_addr() as usize;
+ cpu::branch_to_raw_addr(relocated_runtime_init_addr)
+}
diff -uNr 06_drivers_gpio_uart/src/runtime_init.rs 07_uart_chainloader/src/runtime_init.rs
--- 06_drivers_gpio_uart/src/runtime_init.rs
+++ 07_uart_chainloader/src/runtime_init.rs
@@ -17,7 +17,7 @@
/// - Must only be called pre `kernel_init()`.
#[inline(always)]
unsafe fn zero_bss() {
- memory::zero_volatile(bsp::memory::bss_range_inclusive());
+ memory::zero_volatile(bsp::memory::relocated_bss_range_inclusive());
}
//--------------------------------------------------------------------------------------------------
@@ -30,6 +30,7 @@
/// # Safety
///
/// - Only a single core must be active and running this function.
+#[no_mangle]
pub unsafe fn runtime_init() -> ! {
zero_bss();
diff -uNr 06_drivers_gpio_uart/update.sh 07_uart_chainloader/update.sh
--- 06_drivers_gpio_uart/update.sh
+++ 07_uart_chainloader/update.sh
@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+
+cd ../06_drivers_gpio_uart
+BSP=rpi4 make
+cp kernel8.img ../07_uart_chainloader/demo_payload_rpi4.img
+make
+cp kernel8.img ../07_uart_chainloader/demo_payload_rpi3.img
+rm kernel8.img
```
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