LateStatic

[ammaraskar]

Currently LateStatic implements Send and Sync unconditionally for all types.

late-static/src/lib.rs

Lines 29 to 30 in 3b72ba5

	unsafe impl<T: Send> core::marker::Send for LateStatic<T> {}
	unsafe impl<T: Send> core::marker::Sync for LateStatic<T> {}

This should probably only be for Sync types (this is the same limitation that Rust applies to static types) otherwise there is a soundness issue for multithreaded programs . It is possible to wrap a type that isn't safe to use across threads like Cell in a LateStatic and then cause data races like so:

use late_static::LateStatic;

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

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

static STATIC_CELL: LateStatic<Cell<RefOrInt>> = LateStatic::new();

fn main() {
    unsafe {
        LateStatic::assign(&STATIC_CELL, Cell::new(RefOrInt::Ref(&SOME_INT)));
    }

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

    loop {
        if let RefOrInt::Ref(addr) = STATIC_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)

[Richard-W]

Thank you for the thorough report!

On first glance I think the line

unsafe impl<T: Send> core::marker::Sync for LateStatic<T> {}

may just be a typo and it should be

unsafe impl<T: Sync> core::marker::Sync for LateStatic<T> {}

I will verify and fix it (hopefully later today).

[ammaraskar]

Thank you for the quick response, the fix looks good!