Re: [PATCH v3 09/13] rust: init: add `Zeroable` trait and `init::zeroed` function

[Gary Guo]

On Fri, 31 Mar 2023 00:40:34 +0200
Alice Ryhl <alice@xxxxxxx> wrote:

> On 3/30/23 00:33, y86-dev@xxxxxxxxxxxxxx wrote:
> > From: Benno Lossin <y86-dev@xxxxxxxxxxxxxx>
> > 
> > Add the `Zeroable` trait which marks types that can be initialized by
> > writing `0x00` to every byte of the type. Also add the `init::zeroed`
> > function that creates an initializer for a `Zeroable` type that writes
> > `0x00` to every byte.
> > 
> > Signed-off-by: Benno Lossin <y86-dev@xxxxxxxxxxxxxx>
> > ---
> > +// SAFETY: `null` pointer is valid.
> > +unsafe impl<T: ?Sized> Zeroable for *mut T {}
> > +unsafe impl<T: ?Sized> Zeroable for *const T {}  
> 
> Actually, I just realized that this is not ok for unsized types. When T 
> is unsized, the raw pointer is a fat pointer with a vtable, and the 
> vtable part is not necessarily zeroable.
> 
> However, it would be ok to do it for `*const [T]` since the fat part of 
> the pointer is just the length in this case, and a length of zero is fine.
> 
> See more here:
> https://github.com/Lokathor/bytemuck/blob/8391afa876ba2e99dffb0c991cc7fa775287d106/src/zeroable.rs#L56-L65

Good catch. vtable completely slipped my mind when I am reviewing this
code.

Vtable is not *not necessary zeroable*, but actually never zeroable.
Although currently not yet formally specified, the compiler has always
assumed vtable part of fat pointers to be non-null, well aligned and
dereferenceable.

Best,
Gary