Auto merge of #1065 - Aaron1011:fix/start-return, r=RalfJung

Propagate the return code from the `start` lang item

Fixes #1064

This ensures that we set the error code properly when a panic unwinds
past `main`.

I'm not sure what the best way to write a test for this is

src/bin/miri.rs

@@ -15,6 +15,7 @@ extern crate rustc_interface;
 extern crate syntax;
 
 use std::str::FromStr;
+use std::convert::TryFrom;
 use std::env;
 
 use hex::FromHexError;
@@ -39,7 +40,9 @@ impl rustc_driver::Callbacks for MiriCompilerCalls {
             // Add filename to `miri` arguments.
             config.args.insert(0, compiler.input().filestem().to_string());
 
-            miri::eval_main(tcx, entry_def_id, config);
+            if let Some(return_code) = miri::eval_main(tcx, entry_def_id, config) {
+                std::process::exit(i32::try_from(return_code).expect("Return value was too large!"));
+            }
         });
 
         compiler.session().abort_if_errors();

src/eval.rs

@@ -11,7 +11,7 @@ use syntax::source_map::DUMMY_SP;
 use crate::{
     EnvVars, Evaluator, FnVal, HelpersEvalContextExt, InterpCx, InterpError,
     InterpResult, MemoryExtra, MiriMemoryKind, Pointer, Scalar, StackPopCleanup, Tag,
-    TlsEvalContextExt,
+    TlsEvalContextExt, MPlaceTy
 };
 
 /// Configuration needed to spawn a Miri instance.
@@ -29,12 +29,15 @@ pub struct MiriConfig {
     pub seed: Option<u64>,
 }
 
-// Used by priroda.
+/// Returns a freshly created `InterpCx`, along with an `MPlaceTy` representing
+/// the location where the return value of the `start` lang item will be
+/// written to.
+/// Public because this is also used by `priroda`.
 pub fn create_ecx<'mir, 'tcx: 'mir>(
     tcx: TyCtxt<'tcx>,
     main_id: DefId,
     config: MiriConfig,
-) -> InterpResult<'tcx, InterpCx<'mir, 'tcx, Evaluator<'tcx>>> {
+) -> InterpResult<'tcx, (InterpCx<'mir, 'tcx, Evaluator<'tcx>>, MPlaceTy<'tcx, Tag>)> {
     let mut ecx = InterpCx::new(
         tcx.at(syntax::source_map::DUMMY_SP),
         ty::ParamEnv::reveal_all(),
@@ -170,40 +173,51 @@ pub fn create_ecx<'mir, 'tcx: 'mir>(
     ecx.write_scalar(Scalar::from_u32(0), errno_place.into())?;
     ecx.machine.last_error = Some(errno_place);
 
-    Ok(ecx)
+    Ok((ecx, ret_ptr))
 }
 
-pub fn eval_main<'tcx>(tcx: TyCtxt<'tcx>, main_id: DefId, config: MiriConfig) {
-    let mut ecx = match create_ecx(tcx, main_id, config) {
-        Ok(ecx) => ecx,
+/// Evaluates the main function specified by `main_id`.
+/// Returns `Some(return_code)` if program executed completed.
+/// Returns `None` if an evaluation error occured.
+pub fn eval_main<'tcx>(tcx: TyCtxt<'tcx>, main_id: DefId, config: MiriConfig) -> Option<i64> {
+    let (mut ecx, ret_ptr) = match create_ecx(tcx, main_id, config) {
+        Ok(v) => v,
         Err(mut err) => {
             err.print_backtrace();
             panic!("Miri initialziation error: {}", err.kind)
         }
     };
 
     // Perform the main execution.
-    let res: InterpResult<'_> = (|| {
+    let res: InterpResult<'_, i64> = (|| {
         ecx.run()?;
-        ecx.run_tls_dtors()
+        // Read the return code pointer *before* we run TLS destructors, to assert
+        // that it was written to by the time that `start` lang item returned.
+        let return_code = ecx.read_scalar(ret_ptr.into())?.not_undef()?.to_machine_isize(&ecx)?;
+        ecx.run_tls_dtors()?;
+        Ok(return_code)
     })();
 
     // Process the result.
     match res {
-        Ok(()) => {
+        Ok(return_code) => {
             let leaks = ecx.memory.leak_report();
             // Disable the leak test on some platforms where we do not
             // correctly implement TLS destructors.
             let target_os = ecx.tcx.tcx.sess.target.target.target_os.to_lowercase();
             let ignore_leaks = target_os == "windows" || target_os == "macos";
             if !ignore_leaks && leaks != 0 {
                 tcx.sess.err("the evaluated program leaked memory");
+                // Ignore the provided return code - let the reported error
+                // determine the return code.
+                return None;
             }
+            return Some(return_code)
         }
         Err(mut e) => {
             // Special treatment for some error kinds
             let msg = match e.kind {
-                InterpError::Exit(code) => std::process::exit(code),
+                InterpError::Exit(code) => return Some(code.into()),
                 err_unsup!(NoMirFor(..)) =>
                     format!("{}. Did you set `MIRI_SYSROOT` to a Miri-enabled sysroot? You can prepare one with `cargo miri setup`.", e),
                 _ => e.to_string()
@@ -246,6 +260,8 @@ pub fn eval_main<'tcx>(tcx: TyCtxt<'tcx>, main_id: DefId, config: MiriConfig) {
                     trace!("    local {}: {:?}", i, local.value);
                 }
             }
+            // Let the reported error determine the return code.
+            return None;
         }
     }
 }