Re: [PATCH v2 1/3] rust: kernel: add ScatterList abstraction

[Boqun Feng <boqun.feng@gmail.com>]

On Wed, Jun 07, 2023 at 07:42:10PM +0100, Gary Guo wrote:
> On Mon, 5 Jun 2023 08:26:04 -0700
> Boqun Feng <boqun.feng@gmail.com> wrote:
> 
> > On Fri, Jun 02, 2023 at 06:18:17PM +0800, Qingsong Chen wrote:
> > [...]
> > > +impl<'a> ScatterList<'a> {
> > > +    /// Construct a new initializer.
> > > +    pub fn new(buf: &'a Pin<&mut [u8]>) -> impl PinInit<ScatterList<'a>> {
> > > +        // SAFETY: `slot` is valid while the closure is called, the memory
> > > +        // buffer is pinned and valid.
> > > +        unsafe {
> > > +            init::pin_init_from_closure(move |slot: *mut Self| {
> > > +                (*slot).set_buf(buf);
> > > +                (*slot).mark_end();  
> > 
> > Benno can provide more information, but you cannot dereference or create
> > a reference to `*slot`, since `slot` points to an uninitialized object
> > (see `try_pin_init` implementations), and referencing uninitialized
> > objects is UB (or may cause UB).
> 
> This is fine for `Self`, because the only non-ZST field in there is
> `Opaque`, which can be uninitialised.
> 

OK, but if you do a

	// this is OK
	let slot_ref = unsafe { &(*slot) };

	// reads uninitialised data.
	let o = slot_ref.offset();

here (before any `set_buf()` or `mark_end()`), isn't it an UB?

Regards,
Boqun

> > 
> > Note that you could do the following for `set_buf`:
> > 
> > 	// `addr_of!`[1] is special since it won't create references
> > 	// (even temporary onces).
> > 	let opaque = addr_of!((*slot).opaque); // <- *const Opaque<bindings::scatterlist>
> > 
> > 	let ptr = Opaque::raw_get(opaque); // <- *mut bindings::scatterlist
> > 
> > 	// Maybe this can be wrapped as a Rust function with a
> > 	// parameter: *mut bindings::scatterlist.
> > 	unsafe {
> > 		bindings::sg_set_buf(ptr, buf.as_ptr(), buf.len());
> > 	}
> > 
> > [1]: https://doc.rust-lang.org/core/ptr/macro.addr_of.html
> > 
> > Regards,
> > Boqun
> > 
> > > +                Ok(())
> > > +            })
> > > +        }
> > > +    }
> > > +  
> > [...]
>