don't halt execution when we write to a read-only file

Author: RalfJung

src/shims/files.rs

@@ -334,6 +334,13 @@ impl FileDescription for FileHandle {
     ) -> InterpResult<'tcx> {
         assert!(communicate_allowed, "isolation should have prevented even opening a file");
 
+        if !self.writable {
+            // Linux returns EBADF here which we can't emulate via the platform-independent
+            // code since it does not map to any `io::ErrorKind`. So let's hard-code this.
+            if ecx.target_os_is_unix() {
+                return finish.call(ecx, Err(IoError::LibcError("EBADF")));
+            }
+        }
         let result = ecx.write_to_host(&self.file, len, ptr)?;
         finish.call(ecx, result)
     }

tests/pass/shims/fs.rs

@@ -67,6 +67,10 @@ fn test_file() {
 
     assert!(!file.is_terminal());
 
+    // Writing to a file opened for reading should error (and not stop interpretation). std does not
+    // categorize the error so we don't check for details.
+    file.write(&[]).unwrap_err();
+
     // Removing file should succeed.
     remove_file(&path).unwrap();
 }