AtomicBox should have bounds on its Send/Sync traits

[ammaraskar]

Currently AtomicBox implements the Sync/Send traits unconditionally:

lever/src/sync/atomics.rs

Lines 104 to 105 in 15a96c7

	unsafe impl<T: Sized> Sync for AtomicBox<T> {}
	unsafe impl<T: Sized> Send for AtomicBox<T> {}

I think this should only be when T: Sized + Sync and T: Sized + Send respectively. Otherwise, this makes it possible to send across types that aren't safe to use across threads such as Cells. Here's a demonstration that causes a data-race:

#![forbid(unsafe_code)]

use lever::sync::atomics::AtomicBox;

use std::cell::Cell;
use crossbeam_utils::thread;

#[derive(Debug, Clone, Copy)]
enum RefOrInt<'a> {
    Ref(&'a u64),
    Int(u64),
}
static SOME_INT: u64 = 123;

fn main() {
    let cell = Cell::new(RefOrInt::Ref(&SOME_INT));
    let atomic_box = AtomicBox::new(&cell);

    thread::scope(|s| {
        s.spawn(move |_| {
            let smuggled_cell = atomic_box.get();

            loop {
                // Repeatedly write Ref(&addr) and Int(0xdeadbeef) into the cell.
                smuggled_cell.set(RefOrInt::Ref(&SOME_INT));
                smuggled_cell.set(RefOrInt::Int(0xdeadbeef));
            }
        });

        loop {
            if let RefOrInt::Ref(addr) = cell.get() {
                // Hope that between the time we pattern match the object as a
                // `Ref`, it gets written to by the other thread.
                if addr as *const u64 == &SOME_INT as *const u64 {
                    continue;
                }

                // Due to the data race, obtaining Ref(0xdeadbeef) is possible
                println!("Pointer is now: {:p}", addr);
                println!("Dereferencing addr will now segfault: {}", *addr);
            }
        }
    });
}

This outputs:

Pointer is now: 0xdeadbeef

Return Code: -11 (SIGSEGV)

(Issue found by @sslab-gatech's Rust group)

[Shnatsel]

Heads up: this issue has been included in the RustSec advisory database. It will be surfaced by tools such as cargo-audit or cargo-deny from now on.

Once a fix is released to crates.io, please open a pull request to update the advisory with the patched version, or file an issue on the advisory database repository.

[o0Ignition0o]

Hey! thanks for opening the issue.

You're correct! I think @vertexclique has a patch and we can roll it out hopefully soon

[o0Ignition0o]

Added a regression test thanks to trybuild, super nice crate, i didn't know about it!

[o0Ignition0o]

@vertexclique can we close this now ? :) I'm going to submit a aptch to the rustsec advisory database

[vertexclique]

Truly, let's close it.