Stack overflow when using `zeroed_box()`

[Kimundi]

With a debug build, I'm consistently getting a stack overflow when using zeroed_box(). Minimal example:

const PAGE_SIZE: usize = 4096;
type BlockChunk = [u8; PAGE_SIZE];

const MIB_16: usize = 16 * 1024 * 1024;
const SUPER_SIZE: usize = MIB_16 / std::mem::size_of::<BlockChunk>();
type SuperPage = [BlockChunk; SUPER_SIZE];

#[test]
fn test_alloc() {
    // ABORT: thread '[...]::test_alloc' has overflowed its stack
    let _: Box<SuperPage> = bytemuck::zeroed_box();
}

EDIT: This happens on windows, have not checked anything else yet.

Reproducible on playpen: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=547372389428bf450c9bf885cf7ea6b8

[Lokathor]

As far as I can tell, zeroed_box, which is try_zeroed_box with an unwrap, can't possibly be using more than 5 or 6 usize worth of stack space itself. So the allocator must be doing something funky.

I'm putting you on the case!

[Kimundi]

So, I'm currently guessing there is undefined behavior in play. This repoduces the issue standalone:

use std::alloc;
use std::mem::{size_of, align_of, zeroed};
use std::alloc::{alloc_zeroed, Layout};

#[inline]
pub fn try_zeroed_box<T>() -> Result<Box<T>, ()> {
  if size_of::<T>() == 0 {
    return Ok(Box::new(unsafe{zeroed()}));
  }
  let layout =
    Layout::from_size_align(size_of::<T>(), align_of::<T>()).unwrap();
  let ptr = unsafe { alloc_zeroed(layout) };
  if ptr.is_null() {
    // we don't know what the error is because `alloc_zeroed` is a dumb API
    Err(())
  } else {
    Ok(unsafe { Box::<T>::from_raw(ptr as *mut T) })
  }
}

const PAGE_SIZE: usize = 4096;
type BlockChunk = [u8; PAGE_SIZE];

const MIB_16: usize = 16 * 1024 * 1024;
const SUPER_SIZE: usize = MIB_16 / std::mem::size_of::<BlockChunk>();
type SuperPage = [BlockChunk; SUPER_SIZE];

fn main() {
    // ABORT: thread '[...]::test_alloc' has overflowed its stack
    let _: Box<SuperPage> = try_zeroed_box().unwrap();
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=3622ed3090e28f58875d9932804c998a

Note in particual these lines:

if size_of::<T>() == 0 {
    return Ok(Box::new(unsafe{zeroed()}));
}

I'm guessing that this somehow is undefined behavior if expanded with a very large T

[Kimundi]

Actually its perfectly well define beheavior - its a normal branch, which contains a temporary stack variable that needs to be allocated in the stack frame. So the issue is just how to avoid it.

[Lokathor]

Fixed by #43