fix stack corruptions in startup routine

Author: max-niederman

compiler/rustc_target/src/spec/targets/armv7a_vex_v5_linker_script.ld

@@ -11,11 +11,7 @@ MEMORY {
 }
 
 __stack_length = 0x400000;
-/*
-It's currently unclear why subtracting anything is necessary, but it fixes memory permission errors.
-0x100 is an arbitrary number that works.
-*/
-__heap_end = __user_ram_end - __stack_length - 0x100;
+__heap_end = __user_ram_end - __stack_length;
 
 SECTIONS {
     .text : {
@@ -55,4 +51,4 @@ SECTIONS {
     /DISCARD/ : {
         *(.ARM.exidx)
     }
-}
+}

library/std/src/sys/pal/vexos/mod.rs

@@ -18,14 +18,24 @@ pub mod stdio;
 pub mod thread;
 pub mod time;
 
-use crate::arch::asm;
+use crate::arch::global_asm;
 use crate::hash::{DefaultHasher, Hasher};
 use crate::ptr::{self, addr_of_mut};
 use crate::time::{Duration, Instant};
 
+global_asm!(
+    r#"
+    .section .boot, "ax"
+    .global _boot
+
+    _boot:
+        ldr sp, =__stack_top @ Set up the user stack.
+        b _start             @ Jump to the Rust entrypoint.
+    "#
+);
+
 #[cfg(not(test))]
 #[no_mangle]
-#[link_section = ".boot"]
 pub unsafe extern "C" fn _start() -> ! {
     extern "C" {
         static mut __bss_start: u8;
@@ -34,9 +44,6 @@ pub unsafe extern "C" fn _start() -> ! {
         fn main() -> i32;
     }
 
-    // Setup the stack
-    asm!("ldr sp, =__stack_top", options(nostack));
-
     // VEXos doesn't explicitly clean out .bss.
     ptr::slice_from_raw_parts_mut(
         addr_of_mut!(__bss_start),