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85 lines
2.9 KiB
Rust
85 lines
2.9 KiB
Rust
// SPDX-License-Identifier: MIT OR Apache-2.0
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//
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// Copyright (c) 2018-2020 Andre Richter <andre.o.richter@gmail.com>
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//! Rust runtime initialization code.
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use crate::memory;
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use core::ops::Range;
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//--------------------------------------------------------------------------------------------------
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// Private Definitions
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//--------------------------------------------------------------------------------------------------
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struct Traitor;
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//--------------------------------------------------------------------------------------------------
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// Public Definitions
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//--------------------------------------------------------------------------------------------------
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/// We are outsmarting the compiler here by using a trait as a layer of indirection. Because we are
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/// generating PIC code, a static dispatch to `init()` would generate a relative jump from the
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/// callee to `init()`. However, when calling `init()`, code just finished copying the binary to the
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/// actual link-time address, and hence is still running at whatever location the previous loader
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/// has put it. So we do not want a relative jump, because it would not jump to the relocated code.
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///
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/// By indirecting through a trait object, we can make use of the property that vtables store
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/// absolute addresses. So calling `init()` this way will kick execution to the relocated binary.
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pub trait RunTimeInit {
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/// Equivalent to `crt0` or `c0` code in C/C++ world. Clears the `bss` section, then jumps to
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/// kernel init code.
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///
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/// # Safety
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///
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/// - Only a single core must be active and running this function.
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unsafe fn runtime_init(&self) -> ! {
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zero_bss();
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crate::kernel_init()
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}
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}
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//--------------------------------------------------------------------------------------------------
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// Private Code
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//--------------------------------------------------------------------------------------------------
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impl RunTimeInit for Traitor {}
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/// Return the range spanning the .bss section.
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///
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/// # Safety
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///
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/// - The symbol-provided addresses must be valid.
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/// - The symbol-provided addresses must be usize aligned.
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unsafe fn bss_range() -> Range<*mut usize> {
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extern "C" {
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// Boundaries of the .bss section, provided by linker script symbols.
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static mut __bss_start: usize;
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static mut __bss_end: usize;
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}
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Range {
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start: &mut __bss_start,
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end: &mut __bss_end,
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}
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}
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/// Zero out the .bss section.
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///
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/// # Safety
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///
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/// - Must only be called pre `kernel_init()`.
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#[inline(always)]
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unsafe fn zero_bss() {
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memory::zero_volatile(bss_range());
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}
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//--------------------------------------------------------------------------------------------------
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// Public Code
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//--------------------------------------------------------------------------------------------------
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/// Give the callee a `RunTimeInit` trait object.
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pub fn get() -> &'static dyn RunTimeInit {
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&Traitor {}
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}
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