Locals aligned to greater than page size can cause unsound behavior

[retep998]

Forked from #70022

Minimal example

#[repr(align(0x10000))]
struct Aligned(u8);

fn main() {
    let x = Aligned(0);
    println!("{:#x}", &x as *const _ as usize);
}

Aligning the stack is done after the stack probe. Because stacks grow downwards and aligning the stack shifts it downwards, it can cause the end of the stack to extend past the guard page and cause invalid access exceptions or worse when those sections of the stack are touched.

Only confirmed that this occurs on pc-windows-msvc (pnkfelix edit: see comment thread, its a more general problem.)

[Amanieu]

This is most likely an LLVM bug.

[RalfJung]

Could someone who knows the technical details here report this bug upstream with LLVM?