Support user thread mode in hard fault handler

Author: crawford

asm.s

@@ -12,7 +12,7 @@ fn main() {
     let is_armv6m = is_armv6m(&target);
 
     if target.starts_with("thumbv") {
-        cc::Build::new().file("asm.s").compile("asm");
+        cc::Build::new().file("hardfault.s").compile("asm");
     }
 
     // Put the linker script somewhere the linker can find it

build.rs

@@ -0,0 +1,14 @@
+  .global HardFault
+  .thumb_func
+HardFault:
+  movs r0, lr
+  lsl  r0, r0, #29     // Test bit[2] of EXC_RETURN to determine thread mode
+  bmi  HardFault.PSP
+
+HardFault.MSP:
+  mrs r0, MSP          // Use Main Stack Pointer (if in privileged mode)
+  bl  UserHardFault    // Jump to hard fault handler
+
+HardFault.PSP:
+  mrs r0, PSP          // Use Process Stack Pointer (if in user mode)
+  bl  UserHardFault    // Jump to hard fault handler