Chunk API does not respect align requirement

[Qwaz]

chunky/src/value.rs

Lines 29 to 41 in ef8533a

	impl<V> ::std::ops::Deref for Value<V> {
	type Target = V;
	fn deref(&self) -> &V {
	unsafe { (self.chunk.as_ptr() as *const V).as_ref().unwrap() }
	}
	}
	impl<V> ::std::ops::DerefMut for Value<V> {
	fn deref_mut(&mut self) -> &mut V {
	unsafe { (self.chunk.as_mut_ptr() as *mut V).as_mut().unwrap() }
	}
	}

Description

Chunk API does not respect the align requirement of types. Unaligned reference can be created with the API, which is an undefined behavior.

Demonstration

• Crate: chunky
• Version: 0.3.7
• OS: Ubuntu 18.04.5 LTS
• Rust: rustc 1.47.0-nightly (bf4342114 2020-08-25)

#![forbid(unsafe_code)]

use chunky::{HeapStorage, Ident, Value};
use std::rc::Rc;

#[repr(align(256))]
struct LargeAlign(u8);

impl Drop for LargeAlign {
    fn drop(&mut self) {
        println!("Dropped");
    }
}

fn main() {
    let ident = Ident(String::from("ident"));
    let storage = Rc::new(HeapStorage);
    let value = Value::load_or_default(ident, LargeAlign(0), storage.clone());

    // Value reference does not have a correct alignment
    let v = &*value as *const _ as usize;
    println!("{:x}", v);
    assert!(v % std::mem::align_of::<LargeAlign>() == 0);

    // https://github.com/aeplay/chunky/blob/ef8533aec961eb5f415414dcd81ec4b395bae177/src/value.rs#L43-L49
    // Another bug that is not UB: `LargeAlign::Drop` is not called due to incorrect `drop_in_place()` in `Value::drop()`.
    // "Dropped" should be printed otherwise.
}

Output:

555e921bea80
thread 'main' panicked at 'assertion failed: v % std::mem::align_of::<LargeAlign>() == 0', src/main.rs:42:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Return Code: 101