If I understand https://doc.rust-lang.org/nomicon/other-reprs.html#reprpacked correctly,
#[repr(packed)] forces byte alignment. And it sounds like this may be problematic:

most architectures strongly prefer values to be aligned.

I very new to Rust (and system level programming), but this sounds scary to me.

Could you explain, why the Buf32 should be packed?

#[repr(packed)]
pub struct Buf32<H> {
    pub ptr: *mut H,
    pub len: u32,
    pub cap: u32,
}