Update linker script to put .data into FLASH

Previously .data's LMA was specified by a computated address instead of
placing it into FLASH explicitly, which means FLASH overflows caused by
.data would not be detected by the linker.

Fixes #86.

Author: adamgreig

cortex-m-rt/ci/script.sh

@@ -11,6 +11,9 @@ main() {
         main
         state
     )
+    local fail_examples=(
+        data_overflow
+    )
     if [ $TRAVIS_RUST_VERSION = nightly ]; then
         # linking with GNU LD
         for ex in "${examples[@]}"; do
@@ -22,6 +25,15 @@ main() {
                   -C link-arg=-nostartfiles \
                   -C link-arg=-Wl,-Tlink.x
         done
+        for ex in "${fail_examples[@]}"; do
+            ! cargo rustc --target $TARGET --example $ex -- \
+                  -C link-arg=-nostartfiles \
+                  -C link-arg=-Wl,-Tlink.x
+
+            ! cargo rustc --target $TARGET --example $ex --release -- \
+                  -C link-arg=-nostartfiles \
+                  -C link-arg=-Wl,-Tlink.x
+        done
 
         cargo rustc --target $TARGET --example device --features device -- \
               -C link-arg=-nostartfiles \
@@ -43,6 +55,17 @@ main() {
                   -Z linker-flavor=ld.lld \
                   -C link-arg=-Tlink.x
         done
+        for ex in "${fail_examples[@]}"; do
+            ! cargo rustc --target $TARGET --example $ex -- \
+                  -C linker=rust-lld \
+                  -Z linker-flavor=ld.lld \
+                  -C link-arg=-Tlink.x
+
+            ! cargo rustc --target $TARGET --example $ex --release -- \
+                  -C linker=rust-lld \
+                  -Z linker-flavor=ld.lld \
+                  -C link-arg=-Tlink.x
+        done
 
         cargo rustc --target $TARGET --example device --features device -- \
               -C linker=rust-lld \

cortex-m-rt/examples/data_overflow.rs

@@ -0,0 +1,42 @@
+//! This is not an example; this is a linker overflow detection test
+//! which should fail to link due to .data overflowing FLASH.
+
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+
+#[macro_use(entry, exception)]
+extern crate cortex_m_rt as rt;
+extern crate panic_abort;
+
+use core::ptr;
+
+use rt::ExceptionFrame;
+
+entry!(main);
+
+// This large static array uses most of .rodata
+static RODATA: [u8; 48*1024] = [1u8; 48*1024];
+
+// This large mutable array causes .data to use the rest of FLASH
+// without also overflowing RAM.
+static mut DATA: [u8; 16*1024] = [1u8; 16*1024];
+
+fn main() -> ! {
+    unsafe {
+        let _bigdata = ptr::read_volatile(&RODATA as *const u8);
+        let _bigdata = ptr::read_volatile(&DATA as *const u8);
+    }
+
+    loop {}
+}
+
+exception!(HardFault, hard_fault);
+
+fn hard_fault(_ef: &ExceptionFrame) -> ! {
+    loop {}
+}
+
+exception!(*, default_handler);
+
+fn default_handler(_irqn: i16) {}

cortex-m-rt/link.x.in

@@ -84,24 +84,24 @@ SECTIONS
   } > FLASH
 
   /* ### .rodata */
-  .rodata :
+  .rodata : ALIGN(4)
   {
     *(.rodata .rodata.*);
 
-    /* 4-byte align the end (VMA) of this section */
-    /* WHY? To my knowledge there's no way to indicate the alignment of *LMA* so we align *this*
-    section with the goal of using its end address as the LMA of .data */
+    /* 4-byte align the end (VMA) of this section.
+       This is required by LLD to ensure the LMA of the following .data
+       section will have the correct alignment. */
     . = ALIGN(4);
   } > FLASH
 
   /* ## Sections in RAM */
   /* ### .data */
-  .data : AT(ADDR(.rodata) + SIZEOF(.rodata)) /* LMA */
+  .data : ALIGN(4)
   {
     *(.data .data.*);
 
     . = ALIGN(4); /* 4-byte align the end (VMA) of this section */
-  } > RAM
+  } > RAM AT > FLASH
 
   /* VMA of .data */
   __sdata = ADDR(.data);
@@ -111,7 +111,7 @@ SECTIONS
   __sidata = LOADADDR(.data);
 
   /* ### .bss */
-  .bss :
+  .bss : ALIGN(4)
   {
     *(.bss .bss.*);
 

cortex-m-rt/memory.x

@@ -1,8 +1,12 @@
 /* Device specific memory layout */
 
+/* This file is used to build the cortex-m-rt examples,
+   but not other applications using cortex-m-rt. */
+
 MEMORY
 {
   /* FLASH and RAM are mandatory memory regions */
+  /* Update examples/data_overflow.rs if you change these sizes. */
   FLASH : ORIGIN = 0x08000000, LENGTH = 64K
   RAM : ORIGIN = 0x20000000, LENGTH = 20K