data_model: Implement Send + Sync for IoSliceMut

Pointers in rust don't implement Send or Sync and so any struct that
contains a raw pointer doesn't implement those traits either.  However,
there's nothing inherently unsafe about sending a pointer across thread
boundaries.  The only unsafety comes from actually de-referencing the
pointer, which already requires an unsafe block.

Explicitly implement Send + Sync for IoSliceMut since the iovec
internally holds a *mut c_void, which prevents those traits from being
auto-implemented.  Send + Sync is also implemented by
std::io::IoSliceMut, which is almost exactly the same as our IoSliceMut,
so that can provide some additional reassurance that this is safe.

Also add a missing copyright header in the file.

BUG=none
TEST=unit tests

Change-Id: Ic21fc0de9b29923420f36ab166fec80d4d4cf2e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2571146
Reviewed-by: Noah Gold <nkgold@google.com>
Reviewed-by: Dylan Reid <dgreid@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: Chirantan Ekbote <chirantan@chromium.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/data_model/src/sys.rs b/data_model/src/sys.rs
index 8201c4380..646cced4c 100644
--- a/data_model/src/sys.rs
+++ b/data_model/src/sys.rs
@@ -1,3 +1,7 @@
+// Copyright 2020 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 use std::ffi::c_void;
 use std::fmt::{self, Debug};
 use std::marker::PhantomData;
@@ -70,6 +74,13 @@ impl<'a> IoSliceMut<'a> {
     }
 }
 
+// It's safe to implement Send + Sync for this type for the same reason that `std::io::IoSliceMut`
+// is Send + Sync. Internally, it contains a pointer and a length. The integer length is safely Send
+// + Sync.  There's nothing wrong with sending a pointer between threads and de-referencing the
+// pointer requires an unsafe block anyway. See also https://github.com/rust-lang/rust/pull/70342.
+unsafe impl<'a> Send for IoSliceMut<'a> {}
+unsafe impl<'a> Sync for IoSliceMut<'a> {}
+
 struct DebugIovec(iovec);
 impl Debug for DebugIovec {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {