Merge #887

887: executor: miri fixes r=Dirbaio a=Dirbaio

Fixes a few MIRI errors due to loosely mixing `&TaskStorage<F>` and `&TaskHeader`. References "downgrade" the provenance. `TaskHeader` is smaller, so once you have a `&TaskHeader` you can't use pointer casts to access the whole `TaskStorage<F>`. This fixes it by always keeping the raw pointer around, which doesn't downgrade provenance.

The error was: 

```
[dirbaio@mars std]$ MIRIFLAGS=-Zmiri-disable-isolation cargo miri run --bin tick
    Finished dev [unoptimized + debuginfo] target(s) in 0.05s
     Running `/home/dirbaio/.rustup/toolchains/nightly-2022-07-13-x86_64-unknown-linux-gnu/bin/cargo-miri target/miri/x86_64-unknown-linux-gnu/debug/tick`
error: Undefined Behavior: trying to reborrow <12349> for SharedReadWrite permission at alloc2[0x30], but that tag does not exist in the borrow stack for this location
   --> /home/dirbaio/embassy/embassy/embassy-executor/src/executor/raw/mod.rs:162:20
    |
162 |         let this = &*(p.as_ptr() as *const TaskStorage<F>);
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |                    |
    |                    trying to reborrow <12349> for SharedReadWrite permission at alloc2[0x30], but that tag does not exist in the borrow stack for this location
    |                    this error occurs as part of a reborrow at alloc2[0x30..0x40]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <12349> was created by a retag at offsets [0x0..0x30]
   --> src/bin/tick.rs:15:1
    |
15  | #[embassy_executor::main]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: backtrace:
    = note: inside `embassy_executor::executor::raw::TaskStorage::<std::future::from_generator::GenFuture<[static generator@src/bin/tick.rs:15:1: 15:26]>>::poll` at /home/dirbaio/embassy/embassy/embassy-executor/src/executor/raw/mod.rs:162:20
    = note: inside closure at /home/dirbaio/embassy/embassy/embassy-executor/src/executor/raw/mod.rs:390:13
    = note: inside `embassy_executor::executor::raw::run_queue::RunQueue::dequeue_all::<[closure@embassy_executor::executor::raw::Executor::poll::{closure#1}]>` at /home/dirbaio/embassy/embassy/embassy-executor/src/executor/raw/run_queue.rs:69:13
    = note: inside `embassy_executor::executor::raw::Executor::poll` at /home/dirbaio/embassy/embassy/embassy-executor/src/executor/raw/mod.rs:373:9
    = note: inside `embassy_executor::executor::Executor::run::<[closure@src/bin/tick.rs:15:1: 15:26]>` at /home/dirbaio/embassy/embassy/embassy-executor/src/executor/arch/std.rs:52:22
```

Co-authored-by: Dario Nieuwenhuis <[email protected]>

1 files changed, 3 insertions, 3 deletions

diff --git a/embassy-executor/src/executor/raw/run_queue.rs b/embassy-executor/src/executor/raw/run_queue.rs
index 31615da7e..ed8c82a5c 100644
--- a/embassy-executor/src/executor/raw/run_queue.rs
+++ b/embassy-executor/src/executor/raw/run_queue.rs
@@ -46,10 +46,10 @@ impl RunQueue {
     ///
     /// `item` must NOT be already enqueued in any queue.
     #[inline(always)]
-    pub(crate) unsafe fn enqueue(&self, _cs: CriticalSection, task: *mut TaskHeader) -> bool {
+    pub(crate) unsafe fn enqueue(&self, _cs: CriticalSection, task: NonNull<TaskHeader>) -> bool {
         let prev = self.head.load(Ordering::Relaxed);
-        (*task).run_queue_item.next.store(prev, Ordering::Relaxed);
-        self.head.store(task, Ordering::Relaxed);
+        task.as_ref().run_queue_item.next.store(prev, Ordering::Relaxed);
+        self.head.store(task.as_ptr(), Ordering::Relaxed);
         prev.is_null()
     }