Obstack generates unaligned references

[Qwaz]

rust-obstack/src/lib.rs

Lines 317 to 337 in e1dde0d

	#[inline]
	unsafe fn alloc(&mut self, size: usize, alignment: usize) -> *mut A {
	let start_ptr = self.tip.as_mut_ptr()
	.offset(self.tip.len_items() as isize);
	let padding = start_ptr as usize % alignment;
	debug_assert!(padding < alignment);
	debug_assert_eq!(padding, 0);
	let start_ptr = start_ptr.offset(AlignedVec::<A>::bytes_to_items(padding) as isize);
	let new_used = self.tip.len_items() + padding + AlignedVec::<A>::bytes_to_items(size);
	if new_used <= self.tip.capacity_items() {
	self.tip.set_len_items(new_used);
	start_ptr as *mut A
	} else {
	self.alloc_from_new_slab(size, alignment)
	}
	}

Description

Obstack generates unaligned references for types that require a large alignment.

Demonstration

• Crate: obstack
• Version: 0.1.3
• OS: Ubuntu 18.04.5 LTS
• Rust: rustc 1.46.0 (04488afe3 2020-08-24)
• Cargo flags: --release

#![forbid(unsafe_code)]

use obstack::Obstack;

#[repr(align(256))]
#[derive(Copy, Clone)]
struct LargeAlign(u8);

fn main() {
    // https://github.com/petertodd/rust-obstack/blob/e1dde0dbed709ebdea9bd1f79ec718e80c5d0bf6/src/lib.rs#L317-L337
    // Line 322: Incorrect padding bytes calculation. It should be `(alignment - (start_ptr % alignment)) % alignment`.
    // Line 329: Wasted memory due to `bytes_to_item()` not being applied to `padding`.

    // Due to the incorrect padding calculation, the code generates unaligned reference in release mode.
    let obstack = Obstack::new();
    let val_ref = obstack.push_copy(LargeAlign(0));

    let address = val_ref as *mut _ as usize;
    println!("{:x}", address);
    assert!(address % std::mem::align_of::<LargeAlign>() == 0);
}

Output:

55bc0c2fc520
thread 'main' panicked at 'assertion failed: address % std::mem::align_of::<LargeAlign>() == 0', src/main.rs:39:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Return Code: 101

[petertodd]

Thanks!

Looks like in debug mode it'd bail if alignment padding was needed at all; obviously forgot to finish that part or something.

Made a new release, v0.1.4, which fixes this.

[Qwaz]

Thank you for a quick fix :)