Exporting bigint slices panics

[daxpedda]

As of Rust v1.70, exporting a function like this:

#[wasm_bindgen]
pub fn test(_: &[u64]) {}

... will make it panic when called from JS:

test([]);

panicked at 'misaligned pointer dereference: address must be a multiple of 0x8 but is 0x4'

Here is a minimal reproducible example:
https://github.com/daxpedda/wasm-bindgen-bigint-slice-issue

I confirmed the following scenarios:

• Only u64 and i64 integer types cause this issue.
• Rust v1.69 does not have this issue.
• The recent Rust changelog doesn't note any significant changes other then the upgrade to LLVM 16, which added default target features to Wasm:
  • Compiling with -Ctarget-feature=-sign-ext,-mutable-globals and -Zbuild-std on nightly-2023-06-03 does not fix the issue.
  • Compiling with -Ctarget-feature=+sign-ext,+mutable-globals on Rust v1.69 does not cause this issue.

This issue was originally discovered here: https://github.com/rustwasm/wasm-bindgen/actions/runs/5176047134/jobs/9324382988#step:7:409.

Update: apparently the bug is coming from rust-lang/rust#98112.
It's possible to circumvent the check by just disabling debug_asserts.

[ranile]

Can you reproduce this error? It would be good to file an issue for Rust so the regression can be addressed.

[daxpedda]

It's pretty easy to reproduce, I made a whole example here: https://github.com/daxpedda/wasm-bindgen-bigint-slice-issue.

I don't think it's a Rust issue, but it definitely goes over my head. I don't know enough about the Rust/wasm-bindgen ABI surface to understand what's going on here let alone what needs to be done.

[Liamolucko]

It looks like this is happening because __wbindgen_malloc, the function that all the JS glue uses to allocate memory, always aligns everything to the alignment of a usize (so, 4 bytes):

wasm-bindgen/src/lib.rs

Lines 1571 to 1588 in 5453e33

	#[no_mangle]
	pub extern "C" fn __wbindgen_malloc(size: usize) -> *mut u8 {
	let align = mem::align_of::<usize>();
	if let Ok(layout) = Layout::from_size_align(size, align) {
	unsafe {
	if layout.size() > 0 {
	let ptr = alloc(layout);
	if !ptr.is_null() {
	return ptr
	}
	} else {
	return align as *mut u8
	}
	}
	}
	malloc_failure();
	}

The best fix is probably to add an align argument to __wbindgen_malloc (and __wbindgen_realloc and __wbindgen_free) instead of hard-coding it.