feat(boot): ✨ Replace r0 dependency

Use pointer provenance to guarantee absence of UBs.

Cargo.lock

@@ -99,7 +99,6 @@ dependencies = [
  "cfg-if",
  "cortex-a",
  "machine",
- "r0",
  "seahash",
  "snafu",
  "tock-registers",
@@ -389,7 +388,6 @@ dependencies = [
  "cortex-a",
  "once_cell",
  "qemu-exit",
- "r0",
  "snafu",
  "tock-registers",
  "usize_conversions",
@@ -458,7 +456,6 @@ dependencies = [
  "cfg-if",
  "cortex-a",
  "machine",
- "r0",
  "snafu",
  "tock-registers",
  "usize_conversions",
@@ -546,12 +543,6 @@ dependencies = [
  "proc-macro2",
 ]
 
-[[package]]
-name = "r0"
-version = "1.0.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "bd7a31eed1591dcbc95d92ad7161908e72f4677f8fabf2a32ca49b4237cbf211"
-
 [[package]]
 name = "redox_syscall"
 version = "0.2.16"

bin/chainboot/Cargo.toml

@@ -27,7 +27,6 @@ rpi4 = ["machine/rpi4"]
 
 [dependencies]
 machine = { path = "../../machine" }
-r0 = "1.0"
 cortex-a = "7.5"
 tock-registers = "0.7"
 ux = { version = "0.1", default-features = false }

bin/chainboot/src/boot.rs

@@ -28,7 +28,7 @@ pub unsafe extern "C" fn _start() -> ! {
     extern "Rust" {
         // Boundaries of the .bss section, provided by the linker script
         static __bss_start: UnsafeCell<()>;
-        static __bss_end_exclusive: UnsafeCell<()>;
+        static __bss_size: UnsafeCell<()>;
         // Load address of the kernel binary
         static __binary_nonzero_lma: UnsafeCell<()>;
         // Address to relocate to and image size
@@ -42,7 +42,11 @@ pub unsafe extern "C" fn _start() -> ! {
     SP.set(__boot_core_stack_end_exclusive.get() as u64);
 
     // Zeroes the .bss section
-    r0::zero_bss(__bss_start.get() as u64, __bss_end_exclusive.get() as u64);
+    let bss =
+        core::slice::from_raw_parts_mut(__bss_start.get() as *mut u8, __bss_size.get() as usize);
+    for i in bss {
+        *i = 0;
+    }
 
     // Relocate the code
     core::ptr::copy_nonoverlapping(

bin/chainboot/src/boot.s

@@ -50,7 +50,8 @@ _start:
 
     // Initialize bss.
     ADR_ABS	x0, __bss_start
-    ADR_ABS x1, __bss_end_exclusive
+    ADR_ABS	x1, __bss_size
+    add x1, x0, x1
 
 .L_bss_init_loop:
     cmp	x0, x1

bin/chainboot/src/link.ld

@@ -93,6 +93,6 @@ SECTIONS
         __bss_start = .;
         *(.bss*);
         . = ALIGN(16);
-        __bss_end_exclusive = .;
+        __bss_size = . - __bss_start;
     } :segment_data
 }

linker/aarch64.ld

@@ -58,7 +58,8 @@ SECTIONS
         *(.bss .bss.*)
         *(COMMON)
         . = ALIGN(PAGE_SIZE); /* Align up to page size */
-        __BSS_END = .;
+        __BSS_SIZE = . - __BSS_START;
+        /* __BSS_END = .; unused */
     }
 
     /DISCARD/ : { *(.comment) *(.gnu*) *(.note*) *(.eh_frame*) }

machine/Cargo.toml

@@ -28,7 +28,6 @@ rpi3 = []
 rpi4 = []
 
 [dependencies]
-r0 = "1.0"
 qemu-exit = "3.0"
 cortex-a = "7.5"
 tock-registers = "0.7"

machine/src/arch/aarch64/boot.rs

@@ -10,6 +10,11 @@
 
 use {
     crate::endless_sleep,
+    core::{
+        cell::UnsafeCell,
+        slice,
+        sync::atomic::{self, Ordering},
+    },
     cortex_a::{asm, registers::*},
     tock_registers::interfaces::{Readable, Writeable},
 };
@@ -43,15 +48,33 @@ macro_rules! entry {
 /// Totally unsafe! We're in the hardware land.
 #[link_section = ".text.boot"]
 unsafe fn reset() -> ! {
-    extern "C" {
+    extern "Rust" {
-        // Boundaries of the .bss section, provided by the linker script
+        // Boundaries of the .bss section, provided by the linker script.
-        // The type, `u64`, indicates that the memory is 8-byte aligned
+        static __BSS_START: UnsafeCell<()>;
-        static mut __BSS_START: u64;
+        static __BSS_SIZE: UnsafeCell<()>;
-        static mut __BSS_END: u64;
     }
 
     // Zeroes the .bss section
-    r0::zero_bss(&mut __BSS_START, &mut __BSS_END);
+    // Based on https://gist.github.com/skoe/dbd3add2fc3baa600e9ebc995ddf0302 and discussions
+    // on pointer provenance in closing r0 issues (https://github.com/rust-embedded/cortex-m-rt/issues/300)
+
+    // NB: https://doc.rust-lang.org/nightly/core/ptr/index.html#provenance
+    // Importing pointers like `__BSS_START` and `__BSS_END` and performing pointer
+    // arithmetic on them directly may lead to Undefined Behavior, because the
+    // compiler may assume they come from different allocations and thus performing
+    // undesirable optimizations on them.
+    // So we use a painter-and-a-size as described in provenance section.
+
+    let bss = slice::from_raw_parts_mut(__BSS_START.get() as *mut u8, __BSS_SIZE.get() as usize);
+    for i in bss {
+        *i = 0;
+    }
+
+    // Don't cross this line with loads and stores. The initializations
+    // done above could be "invisible" to the compiler, because we write to the
+    // same memory location that is used by statics after this point.
+    // Additionally, we assume that no statics are accessed before this point.
+    atomic::compiler_fence(Ordering::SeqCst);
 
     extern "Rust" {
         fn main() -> !;

machine/src/lib.rs

@@ -4,6 +4,7 @@
 #![feature(allocator_api)]
 #![feature(format_args_nl)]
 #![feature(core_intrinsics)]
+#![feature(strict_provenance)]
 #![feature(stmt_expr_attributes)]
 #![feature(slice_ptr_get)]
 #![feature(nonnull_slice_from_raw_parts)]

nucleus/Cargo.toml

@@ -28,7 +28,6 @@ rpi4 = ["machine/rpi4"]
 
 [dependencies]
 machine = { path = "../machine" }
-r0 = "1.0"
 cortex-a = "7.5"
 tock-registers = "0.7"
 ux = { version = "0.1", default-features = false }

nucleus/src/main.rs

@@ -11,11 +11,14 @@
 #![no_main]
 #![feature(ptr_internals)]
 #![feature(format_args_nl)]
+#![feature(strict_provenance)]
 #![feature(custom_test_frameworks)]
 #![test_runner(machine::tests::test_runner)]
 #![reexport_test_harness_main = "test_main"]
 #![deny(missing_docs)]
 #![deny(warnings)]
+#![deny(unused)]
+#![feature(allocator_api)]
 
 #[cfg(not(test))]
 use core::panic::PanicInfo;