`BlobVec` does not respect alignment for zero-sized types

[joseph-gio]

Bevy version

e48c05c73474a5f6a212257cecda7ceaf2465cf3

Problem

Like Vec, BlobVec uses a dangling pointer as the backing storage for zero-sized types. However, BlobVec does not respect alignment, which results in instant UB when using a ZST with alignment other than 1.

#[test]
fn aligned_zst() {
    #[derive(Component)]
    #[repr(align(32))]
    struct Zst;

    let mut world = World::default();
    world.spawn(Zst);
    world.spawn(Zst);
    world.spawn(Zst);

    let mut q = world.query::<&Zst>();
    for &Zst in q.iter(&world) {}
}

The offending line: it creates a dangling pointer which is well-aligned for u8, not for the type being stored.

Miri

error: Undefined Behavior: accessing memory with alignment 1, but alignment 32 is required
   --> C:\Users\joe10\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\slice\raw.rs:100:9
    |
100 |         &*ptr::slice_from_raw_parts(data, len)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ accessing memory with alignment 1, but alignment 32 is required
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
    = note: BACKTRACE:
    = note: inside `std::slice::from_raw_parts::<'_, std::cell::UnsafeCell<tests::aligned_zst::Zst>>` at C:\Users\joe10\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\slice\raw.rs:100:9
note: inside `storage::blob_vec::BlobVec::get_slice::<tests::aligned_zst::Zst>` at crates\bevy_ecs\src\storage\blob_vec.rs:309:9
   --> crates\bevy_ecs\src\storage\blob_vec.rs:309:9
    |
309 |         std::slice::from_raw_parts(self.data.as_ptr() as *const UnsafeCell<T>, self.len)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `storage::table::Column::get_data_slice::<tests::aligned_zst::Zst>` at crates\bevy_ecs\src\storage\table.rs:191:9
   --> crates\bevy_ecs\src\storage\table.rs:191:9
    |
191 |         self.data.get_slice()
    |         ^^^^^^^^^^^^^^^^^^^^^
note: inside `<&tests::aligned_zst::Zst as query::fetch::WorldQuery>::set_table` at crates\bevy_ecs\src\query\fetch.rs:586:13
   --> crates\bevy_ecs\src\query\fetch.rs:586:13
    |
586 | /             table
587 | |                 .get_column(component_id)
588 | |                 .debug_checked_unwrap()
589 | |                 .get_data_slice()
    | |_________________________________^
note: inside `query::iter::QueryIterationCursor::<'_, '_, &tests::aligned_zst::Zst, ()>::next` at crates\bevy_ecs\src\query\iter.rs:595:21
   --> crates\bevy_ecs\src\query\iter.rs:595:21
    |
595 |                     Q::set_table(&mut self.fetch, &query_state.fetch_state, table);
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `<query::iter::QueryIter<'_, '_, &tests::aligned_zst::Zst, ()> as std::iter::Iterator>::next` at crates\bevy_ecs\src\query\iter.rs:53:13
   --> crates\bevy_ecs\src\query\iter.rs:53:13
    |
53  | /             self.cursor
54  | |                 .next(self.tables, self.archetypes, self.query_state)
    | |_____________________________________________________________________^
note: inside `tests::aligned_zst` at crates\bevy_ecs\src\lib.rs:87:21
   --> crates\bevy_ecs\src\lib.rs:87:21
    |
87  |         for &Zst in q.iter(&world) {}
    |                     ^^^^^^^^^^^^^^
note: inside closure at crates\bevy_ecs\src\lib.rs:76:5
   --> crates\bevy_ecs\src\lib.rs:76:5
    |
75  |       #[test]
    |       ------- in this procedural macro expansion
76  | /     fn aligned_zst() {
77  | |         #[derive(Component)]
78  | |         #[repr(align(32))]
79  | |         struct Zst;
...   |
87  | |         for &Zst in q.iter(&world) {}
88  | |     }
    | |_____^
    = note: this error originates in the attribute macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to previous error

test tests::aligned_zst ... error: test failed, to rerun pass `-p bevy_ecs --lib`