Merge #337

337: Fix unwinding through `Reset` r=thejpster a=jonas-schievink

Unwinders may detect the end of the program by seeing `0xFFFFFFFF` in `lr`, which is why code to ensure that it has that value was added in #293. However, the `bl main` overwrites that value with the current program counter. This PR saves the old `lr` value on the stack, and adds debuginfo entries to allow an external unwinder to restore the value.

This fixes knurling-rs/probe-run#277

Co-authored-by: Jonas Schievink <[email protected]>

asm.S

@@ -94,6 +94,14 @@ Reset:
 #endif
 
 4:
+  # Preserve `lr` and emit debuginfo that lets external tools restore it.
+  # This fixes unwinding past the `Reset` handler.
+  # See https://sourceware.org/binutils/docs/as/CFI-directives.html for an
+  # explanation of the directives.
+.cfi_def_cfa sp, 0
+  push {lr}
+.cfi_offset lr, 0
+
   # Jump to user main function. We use bl for the extended range, but the
   # user main function may not return.
   bl main

assemble.sh

@@ -9,25 +9,25 @@ crate=cortex-m-rt
 # remove existing blobs because otherwise this will append object files to the old blobs
 rm -f bin/*.a
 
-arm-none-eabi-gcc -c -march=armv6s-m asm.S -o bin/$crate.o
+arm-none-eabi-gcc -g -c -march=armv6s-m asm.S -o bin/$crate.o
 ar crs bin/thumbv6m-none-eabi.a bin/$crate.o
 
-arm-none-eabi-gcc -c -march=armv7-m asm.S -o bin/$crate.o
+arm-none-eabi-gcc -g -c -march=armv7-m asm.S -o bin/$crate.o
 ar crs bin/thumbv7m-none-eabi.a bin/$crate.o
 
-arm-none-eabi-gcc -c -march=armv7e-m asm.S -o bin/$crate.o
+arm-none-eabi-gcc -g -c -march=armv7e-m asm.S -o bin/$crate.o
 ar crs bin/thumbv7em-none-eabi.a bin/$crate.o
 
-arm-none-eabi-gcc -c -march=armv7e-m asm.S -DHAS_FPU -o bin/$crate.o
+arm-none-eabi-gcc -g -c -march=armv7e-m asm.S -DHAS_FPU -o bin/$crate.o
 ar crs bin/thumbv7em-none-eabihf.a bin/$crate.o
 
-arm-none-eabi-gcc -c -march=armv8-m.base asm.S -o bin/$crate.o
+arm-none-eabi-gcc -g -c -march=armv8-m.base asm.S -o bin/$crate.o
 ar crs bin/thumbv8m.base-none-eabi.a bin/$crate.o
 
-arm-none-eabi-gcc -c -march=armv8-m.main asm.S -o bin/$crate.o
+arm-none-eabi-gcc -g -c -march=armv8-m.main asm.S -o bin/$crate.o
 ar crs bin/thumbv8m.main-none-eabi.a bin/$crate.o
 
-arm-none-eabi-gcc -c -march=armv8-m.main -DHAS_FPU asm.S -o bin/$crate.o
+arm-none-eabi-gcc -g -c -march=armv8-m.main -DHAS_FPU asm.S -o bin/$crate.o
 ar crs bin/thumbv8m.main-none-eabihf.a bin/$crate.o
 
 rm bin/$crate.o

build.rs

@@ -19,12 +19,10 @@ fn main() {
     let out_dir = PathBuf::from(env::var("OUT_DIR").unwrap());
 
     if target.starts_with("thumbv") {
-        fs::copy(
-            format!("bin/{}.a", target),
-            out_dir.join("libcortex-m-rt.a"),
-        )
-        .unwrap();
+        let lib_path = format!("bin/{}.a", target);
+        fs::copy(&lib_path, out_dir.join("libcortex-m-rt.a")).unwrap();
         println!("cargo:rustc-link-lib=static=cortex-m-rt");
+        println!("cargo:rerun-if-changed={}", lib_path);
     }
 
     // Put the linker script somewhere the linker can find it