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

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Date:   Mon, 5 Jun 2023 08:26:04 -0700
From:   Boqun Feng <boqun.feng@...il.com>
To:     Qingsong Chen <changxian.cqs@...group.com>
Cc:     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>,
        Gary Guo <gary@...yguo.net>,
        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 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).

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