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Date:   Tue, 06 Jun 2023 15:01:30 +0800
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Subject: Re: [PATCH v2 1/3] rust: kernel: add ScatterList abstraction
To:     Boqun Feng <boqun.feng@gmail.com>
Cc:     <linux-kernel@vger.kernel.org>,
        "=?UTF-8?B?55Sw5rSq5Lqu?=" <tate.thl@antgroup.com>,
        "Miguel Ojeda" <ojeda@kernel.org>,
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        "=?UTF-8?Q?Bj=c3=b6rn_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>
References: <20230602101819.2134194-1-changxian.cqs@antgroup.com>
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 <ZH3+jARBbn2iG1dW@Boquns-Mac-mini.local>
From:   "Qingsong Chen" <changxian.cqs@antgroup.com>
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On 6/5/23 11:26 PM, Boqun Feng 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).

I understand that `reading` from uninitialized objects is UB, either
via references or raw pointers. But in this unsafe closure, we just do
the `writing` job to `slot` for initializing. This makes me a little
confused, why is there a difference between reference and raw pointer?
Is there any compiler magic on reference which may cause UB? Still, I
would fix this by `addr_of`. Thanks.