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

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Date:   Wed, 7 Jun 2023 19:42:10 +0100
From:   Gary Guo <gary@...yguo.net>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Qingsong Chen <changxian.cqs@...group.com>,
        linux-kernel@...r.kernel.org,
        田洪亮 <tate.thl@...group.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>,
        Martin Rodriguez Reboredo <yakoyoku@...il.com>,
        Alice Ryhl <aliceryhl@...gle.com>,
        Asahi Lina <lina@...hilina.net>, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v2 1/3] rust: kernel: add ScatterList abstraction

On Mon, 5 Jun 2023 08:26:04 -0700
Boqun Feng <boqun.feng@...il.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.

> 
> 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(())
> > +            })
> > +        }
> > +    }
> > +  
> [...]