Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Feedback-ID: iad51458e:Fastmail
Date:   Wed, 7 Jun 2023 12:40:43 -0700
From:   Boqun Feng <boqun.feng@gmail.com>
To:     Gary Guo <gary@garyguo.net>
Cc:     Qingsong Chen <changxian.cqs@antgroup.com>,
        linux-kernel@vger.kernel.org,
        =?utf-8?B?55Sw5rSq5Lqu?= <tate.thl@antgroup.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        =?iso-8859-1?Q?Bj=F6rn?= Roy Baron <bjorn3_gh@protonmail.com>,
        Benno Lossin <benno.lossin@proton.me>,
        Martin Rodriguez Reboredo <yakoyoku@gmail.com>,
        Alice Ryhl <aliceryhl@google.com>,
        Asahi Lina <lina@asahilina.net>, rust-for-linux@vger.kernel.org
Subject: Re: [PATCH v2 1/3] rust: kernel: add ScatterList abstraction
Message-ID: <ZIDdO7SPvZh8fD7z@boqun-archlinux>
References: <20230602101819.2134194-1-changxian.cqs@antgroup.com>
 <20230602101819.2134194-2-changxian.cqs@antgroup.com>
 <ZH3+jARBbn2iG1dW@Boquns-Mac-mini.local>
 <20230607194210.380d195f.gary@garyguo.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20230607194210.380d195f.gary@garyguo.net>
Precedence: bulk

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