rust-raspberrypi-OS-tutorials/07_uart_chainloader

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:

1. Depending on your target hardware:make or BSP=rpi4 make.
2. Copy kernel8.img to the SD card.
3. Execute make chainboot or BSP=rpi4 make chainboot.
4. Now plug in the USB Serial.
5. 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

$ 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 (0x0200_0XXX):

$ 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

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
@@ -5,6 +5,12 @@
 # Default to the RPi3
 BSP ?= rpi3

+# Default to a serial device name that is common in Linux.
+DEV_SERIAL ?= /dev/ttyUSB0
+
+# Query the host system's kernel name
+UNAME_S = $(shell uname -s)
+
 # BSP-specific arguments
 ifeq ($(BSP),rpi3)
     TARGET            = aarch64-unknown-none-softfloat
@@ -13,7 +19,8 @@
     QEMU_MACHINE_TYPE = raspi3
     QEMU_RELEASE_ARGS = -serial stdio -display none
     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
@@ -21,7 +28,8 @@
     QEMU_MACHINE_TYPE =
     QEMU_RELEASE_ARGS = -serial stdio -display none
     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
@@ -46,12 +54,23 @@

 DOCKER_IMAGE         = rustembedded/osdev-utils
 DOCKER_CMD           = docker run -it --rm -v $(shell pwd):/work/tutorial -w /work/tutorial
+DOCKER_ARG_DIR_UTILS = -v $(shell pwd)/../utils:/work/utils
+DOCKER_ARG_DEV       = --privileged -v /dev:/dev

 DOCKER_QEMU = $(DOCKER_CMD) $(DOCKER_IMAGE)

-EXEC_QEMU = $(QEMU_BINARY) -M $(QEMU_MACHINE_TYPE)
+# Dockerize commands that require USB device passthrough only on Linux
+ifeq ($(UNAME_S),Linux)
+    DOCKER_CMD_DEV = $(DOCKER_CMD) $(DOCKER_ARG_DEV)
+
+    DOCKER_CHAINBOOT = $(DOCKER_CMD_DEV) $(DOCKER_ARG_DIR_UTILS) $(DOCKER_IMAGE)
+endif

-.PHONY: all $(KERNEL_ELF) $(KERNEL_BIN) doc qemu clippy clean readelf objdump nm check
+EXEC_QEMU     = $(QEMU_BINARY) -M $(QEMU_MACHINE_TYPE)
+EXEC_MINIPUSH = ruby ../utils/minipush.rb
+
+.PHONY: all $(KERNEL_ELF) $(KERNEL_BIN) doc qemu qemuasm chainboot clippy clean readelf objdump nm \
+    check

 all: $(KERNEL_BIN)

@@ -65,13 +84,19 @@
 	$(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

+chainboot:
+	@$(DOCKER_CHAINBOOT) $(EXEC_MINIPUSH) $(DEV_SERIAL) $(CHAINBOOT_DEMO_PAYLOAD)
+
 clippy:
 	RUSTFLAGS="$(RUSTFLAGS_PEDANTIC)" $(CLIPPY_CMD)


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
@@ -21,12 +21,12 @@
 #[naked]
 #[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::<u64>()
     } else {
         // If not core0, infinitely wait for events.
         wait_forever()

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
@@ -271,6 +271,16 @@
         let mut r = &self.inner;
         r.lock(|inner| fmt::Write::write_fmt(inner, args))
     }
+
+    fn flush(&self) {
+        // Spin until TX FIFO empty is set.
+        let mut r = &self.inner;
+        r.lock(|inner| {
+            while !inner.registers.FR.matches_all(FR::TXFE::SET) {
+                cpu::nop();
+            }
+        });
+    }
 }

 impl console::interface::Read for PL011Uart {
@@ -282,18 +292,21 @@
                 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) {
+        let mut r = &self.inner;
+        r.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
@@ -5,9 +5,10 @@

 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*)
@@ -32,5 +33,14 @@
         __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 = .;
+
     /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
@@ -23,10 +23,12 @@
 /// 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")]
@@ -59,6 +61,12 @@
     map::BOOT_CORE_STACK_END
 }

+/// The address on which the Raspberry firmware loads every binary by default.
+#[inline(always)]
+pub fn board_default_load_addr() -> usize {
+    map::BOARD_DEFAULT_LOAD_ADDRESS
+}
+
 /// Return the inclusive range spanning the .bss section.
 ///
 /// # Safety

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
@@ -108,7 +108,8 @@
 #![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;
@@ -117,6 +118,7 @@
 mod memory;
 mod panic_wait;
 mod print;
+mod relocate;
 mod runtime_init;
 mod synchronization;

@@ -143,35 +145,52 @@

 /// The main function running after the early init.
 fn kernel_main() -> ! {
+    use bsp::console::console;
     use console::interface::All;
-    use driver::interface::DriverManager;

-    // Wait for user to hit Enter.
-    loop {
-        if bsp::console::console().read_char() == '\n' {
-            break;
+    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);
+    }
+
+    // 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 {
+            *kernel_addr.offset(i as isize) = console().read_char() as u8;
         }
     }

-    println!("[0] Booting on: {}", bsp::board_name());
+    println!("[ML] Loaded! Executing the payload now\n");
+    console().flush();

-    println!("[1] Drivers loaded:");
-    for (i, driver) in bsp::driver::driver_manager()
-        .all_device_drivers()
-        .iter()
-        .enumerate()
-    {
-        println!("      {}. {}", i + 1, driver.compatible());
-    }
+    // Use black magic to get a function pointer.
+    let kernel: extern "C" fn() -> ! = unsafe { core::mem::transmute(kernel_addr as *const ()) };

-    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);
-    }
+    // 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 +1,52 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+//
+// Copyright (c) 2018-2020 Andre Richter <andre.o.richter@gmail.com>
+
+//! Relocation code.
+
+use crate::{bsp, runtime_init};
+
+//--------------------------------------------------------------------------------------------------
+// Public Code
+//--------------------------------------------------------------------------------------------------
+
+/// Relocates the own binary from `bsp::cpu::BOARD_DEFAULT_LOAD_ADDRESS` 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.
+pub unsafe fn relocate_self<T>() -> ! {
+    extern "C" {
+        static __binary_start: usize;
+        static __binary_end: usize;
+    }
+
+    let binary_start_addr: usize = &__binary_start as *const _ as _;
+    let binary_end_addr: usize = &__binary_end as *const _ as _;
+    let binary_size_in_byte: usize = binary_end_addr - binary_start_addr;
+
+    // Get the relocation destination address from the linker symbol.
+    let mut reloc_dst_addr: *mut T = binary_start_addr as *mut T;
+
+    // The address of where the previous firmware loaded us.
+    let mut src_addr: *const T = bsp::memory::board_default_load_addr() as *const _;
+
+    // Copy the whole binary.
+    //
+    // This is essentially a `memcpy()` optimized for throughput by transferring in chunks of T.
+    let n = binary_size_in_byte / core::mem::size_of::<T>();
+    for _ in 0..n {
+        use core::ptr;
+
+        ptr::write_volatile::<T>(reloc_dst_addr, ptr::read_volatile::<T>(src_addr));
+        reloc_dst_addr = reloc_dst_addr.offset(1);
+        src_addr = src_addr.offset(1);
+    }
+
+    // Call `runtime_init()` through a trait object, causing the jump to use an absolute address to
+    // reach the relocated binary. An elaborate explanation can be found in the `runtime_init.rs`
+    // source comments.
+    runtime_init::get().runtime_init()
+}

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
@@ -7,9 +7,43 @@
 use crate::{bsp, memory};

 //--------------------------------------------------------------------------------------------------
+// Private Definitions
+//--------------------------------------------------------------------------------------------------
+
+struct Traitor;
+
+//--------------------------------------------------------------------------------------------------
+// Public Definitions
+//--------------------------------------------------------------------------------------------------
+
+/// We are outsmarting the compiler here by using a trait as a layer of indirection. Because we are
+/// generating PIC code, a static dispatch to `init()` would generate a relative jump from the
+/// callee to `init()`. However, when calling `init()`, code just finished copying the binary to the
+/// actual link-time address, and hence is still running at whatever location the previous loader
+/// has put it. So we do not want a relative jump, because it would not jump to the relocated code.
+///
+/// By indirecting through a trait object, we can make use of the property that vtables store
+/// absolute addresses. So calling `init()` this way will kick execution to the relocated binary.
+pub trait RunTimeInit {
+    /// Equivalent to `crt0` or `c0` code in C/C++ world. Clears the `bss` section, then jumps to
+    /// kernel init code.
+    ///
+    /// # Safety
+    ///
+    /// - Only a single core must be active and running this function.
+    unsafe fn runtime_init(&self) -> ! {
+        zero_bss();
+
+        crate::kernel_init()
+    }
+}
+
+//--------------------------------------------------------------------------------------------------
 // Private Code
 //--------------------------------------------------------------------------------------------------

+impl RunTimeInit for Traitor {}
+
 /// Zero out the .bss section.
 ///
 /// # Safety
@@ -24,14 +58,7 @@
 // Public Code
 //--------------------------------------------------------------------------------------------------

-/// Equivalent to `crt0` or `c0` code in C/C++ world. Clears the `bss` section, then jumps to kernel
-/// init code.
-///
-/// # Safety
-///
-/// - Only a single core must be active and running this function.
-pub unsafe fn runtime_init() -> ! {
-    zero_bss();
-
-    crate::kernel_init()
+/// Give the callee a `RunTimeInit` trait object.
+pub fn get() -> &'static dyn RunTimeInit {
+    &Traitor {}
 }