miri reports undefined behavior

[eric-seppanen]

This unit test reports undefined behavior in miri:

#[test]
fn oops() {
    executor::run(async {
        let (s, r) = async_channel::unbounded();

        std::thread::spawn(move || {
            s.send_blocking("42").unwrap();
        });

        // Receive the value.
        let res = r.recv().await;
        assert_eq!(res, Ok("42"));
    })
}

Miri output:

test tests::oops ... error: Undefined Behavior: trying to retag from <90602> for SharedReadWrite permission at alloc22794[0x0], but that tag does not exist in the borrow stack for this location
   --> /home/eric/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/non_null.rs:401:18
    |
401 |         unsafe { &*self.as_ptr().cast_const() }
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |                  |
    |                  trying to retag from <90602> for SharedReadWrite permission at alloc22794[0x0], but that tag does not exist in the borrow stack for this location
    |                  this error occurs as part of retag at alloc22794[0x0..0x28]
    |
    = 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: <90602> was created by a SharedReadWrite retag at offsets [0x18..0x51]
   --> src/lib.rs:6:9
    |
6   | /         executor::run(async {
7   | |             let (s, r) = async_channel::unbounded();
8   | |
9   | |             std::thread::spawn(move || {
...   |
15  | |             assert_eq!(res, Ok("42"));
16  | |         })
    | |__________^
    = note: BACKTRACE (of the first span) on thread `tests::oops`:
    = note: inside `std::ptr::NonNull::<alloc::sync::ArcInner<executor::Task<()>>>::as_ref::<'_>` at /home/eric/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/non_null.rs:401:18: 401:46
    = note: inside `std::sync::Arc::<executor::Task<()>>::inner` at /home/eric/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/sync.rs:1796:18: 1796:35
    = note: inside `<std::sync::Arc<executor::Task<()>> as std::clone::Clone>::clone` at /home/eric/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/sync.rs:2075:24: 2075:36
    = note: inside `<std::mem::ManuallyDrop<std::sync::Arc<executor::Task<()>>> as std::clone::Clone>::clone` at /home/eric/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/manually_drop.rs:51:5: 51:13
    = note: inside `woke::increase_refcount::<executor::Task<()>>` at /home/eric/.cargo/registry/src/index.crates.io-6f17d22bba15001f/woke-0.0.4/src/lib.rs:38:44: 38:55
    = note: inside `woke::clone_arc_raw::<executor::Task<()>>` at /home/eric/.cargo/registry/src/index.crates.io-6f17d22bba15001f/woke-0.0.4/src/lib.rs:42:5: 42:33
    = note: inside `<std::task::Waker as std::clone::Clone>::clone` at /home/eric/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/task/wake.rs:514:29: 514:71
    = note: inside `event_listener::TaskRef::<'_>::into_task` at /home/eric/.cargo/registry/src/index.crates.io-6f17d22bba15001f/event-listener-5.2.0/src/lib.rs:1322:47: 1322:60
    = note: inside `event_listener::sys::<impl event_listener::Inner<()>>::register` at /home/eric/.cargo/registry/src/index.crates.io-6f17d22bba15001f/event-listener-5.2.0/src/std.rs:165:45: 165:61
    = note: inside `event_listener::InnerListener::<(), std::sync::Arc<event_listener::Inner<()>>>::poll_internal` at /home/eric/.cargo/registry/src/index.crates.io-6f17d22bba15001f/event-listener-5.2.0/src/lib.rs:1159:15: 1160:65
    = note: inside `<event_listener::EventListener as std::future::Future>::poll` at /home/eric/.cargo/registry/src/index.crates.io-6f17d22bba15001f/event-listener-5.2.0/src/lib.rs:878:17: 878:54
    = note: inside `<event_listener_strategy::NonBlocking<'_> as event_listener_strategy::Strategy<'_>>::poll::<(), event_listener::EventListener>` at /home/eric/.cargo/registry/src/index.crates.io-6f17d22bba15001f/event-listener-strategy-0.5.0/src/lib.rs:467:20: 472:23
    = note: inside `<async_channel::RecvInner<'_, &str> as event_listener_strategy::EventListenerFuture>::poll_with_strategy::<'_, event_listener_strategy::NonBlocking<'_>>` at /home/eric/.cargo/registry/src/index.crates.io-6f17d22bba15001f/async-channel-2.2.0/src/lib.rs:1196:24: 1196:66
    = note: inside `<event_listener_strategy::FutureWrapper<async_channel::RecvInner<'_, &str>> as std::future::Future>::poll` at /home/eric/.cargo/registry/src/index.crates.io-6f17d22bba15001f/event-listener-strategy-0.5.0/src/lib.rs:381:9: 383:70
    = note: inside `<async_channel::Recv<'_, &str> as std::future::Future>::poll` at /home/eric/.cargo/registry/src/index.crates.io-6f17d22bba15001f/event-listener-strategy-0.5.0/src/lib.rs:259:17: 259:52
note: inside closure
   --> src/lib.rs:14:32
    |
14  |             let res = r.recv().await;
    |                                ^^^^^
    = note: inside `<std::sync::Arc<executor::Task<()>> as executor::Pendable>::is_pending` at /home/eric/.cargo/registry/src/index.crates.io-6f17d22bba15001f/executor-0.8.4/src/lib.rs:47:18: 47:47
    = note: inside `executor::Executor::poll_tasks` at /home/eric/.cargo/registry/src/index.crates.io-6f17d22bba15001f/executor-0.8.4/src/lib.rs:85:16: 85:33
    = note: inside `executor::Executor::run::<()>` at /home/eric/.cargo/registry/src/index.crates.io-6f17d22bba15001f/executor-0.8.4/src/lib.rs:58:9: 58:26
    = note: inside `executor::run::<(), {async block@src/lib.rs:6:23: 16:10}>` at /home/eric/.cargo/registry/src/index.crates.io-6f17d22bba15001f/executor-0.8.4/src/lib.rs:98:5: 98:50
note: inside `tests::oops`
   --> src/lib.rs:6:9
    |
6   | /         executor::run(async {
7   | |             let (s, r) = async_channel::unbounded();
8   | |
9   | |             std::thread::spawn(move || {
...   |
15  | |             assert_eq!(res, Ok("42"));
16  | |         })
    | |__________^

This is with async-channel = "2.2.0", executor = "0.8.4".

The toolchain that was used:

$ rustc +nightly -V
rustc 1.78.0-nightly (d18480b84 2024-03-04)

[TTTiNNNy]

hmmm, i retry run the same code, but with new syntax (executor::run func hasn't args) and got
help: <92347> was created by a SharedReadOnly retag at offsets [0x29..0x82] --> /home/tiny/workplace/executor/src/lib.rs:57:21 | 57 | let waker = waker_ref(self); | ^^^^^^^^^^^^^^^ = note: BACKTRACE (of the first span):
Don't actually understand potential issues with this code fragment, so guess that miri cant properly parse some code fragments. Same issues was with other guys like https://stackoverflow.com/questions/66164453/miri-complains-about-ub-when-initializing-and-manually-dropping-maybeuninit.
But since there is a potential problem, I think it's worth paying attention to in the future)

[TTTiNNNy]

Thanks for your report, Unfortunately, I still didn’t understand why miri was complaining about that section of the code, another issue with the same code block was find and fix. PR is open, waiting for review from the author of the project.