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Subject: Re: [PATCH] rust: init: remove impl Zeroable for Infallible
From: Laine Taffin Altman <alexanderaltman@me.com>
In-Reply-To: <6857bb37-c4ee-4817-9b6a-e40e549b6402@proton.me>
Date: Wed, 20 Mar 2024 21:53:42 -0700
Cc: Boqun Feng <boqun.feng@gmail.com>,
 Miguel Ojeda <ojeda@kernel.org>,
 Alex Gaynor <alex.gaynor@gmail.com>,
 Wedson Almeida Filho <wedsonaf@gmail.com>,
 Gary Guo <gary@garyguo.net>,
 =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
 Andreas Hindborg <a.hindborg@samsung.com>,
 Alice Ryhl <aliceryhl@google.com>,
 Martin Rodriguez Reboredo <yakoyoku@gmail.com>,
 stable@vger.kernel.org,
 rust-for-linux@vger.kernel.org,
 lkml <linux-kernel@vger.kernel.org>
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To: Benno Lossin <benno.lossin@proton.me>

On Mar 19, 2024, at 3:34=E2=80=AFAM, Benno Lossin =
<benno.lossin@proton.me> wrote:
> On 3/19/24 06:28, Laine Taffin Altman wrote:
>> On Mar 18, 2024, at 9:39=E2=80=AFPM, Boqun Feng =
<boqun.feng@gmail.com> wrote:
>>> On Mon, Mar 18, 2024 at 08:17:07PM -0700, Laine Taffin Altman wrote:
>>>> On Mar 18, 2024, at 10:25=E2=80=AFAM, Boqun Feng =
<boqun.feng@gmail.com> wrote:
>>>>> On Wed, Mar 13, 2024 at 11:09:37PM +0000, Benno Lossin wrote:
>>>>>> From: Laine Taffin Altman <alexanderaltman@me.com>
>>>>>>=20
>>>>>> It is not enough for a type to be a ZST to guarantee that zeroed =
memory
>>>>>> is a valid value for it; it must also be inhabited. Creating a =
value of
>>>>>> an uninhabited type, ZST or no, is immediate UB.
>>>>>> Thus remove the implementation of `Zeroable` for `Infallible`, =
since
>>>>>> that type is not inhabited.
>>>>>>=20
>>>>>> Cc: stable@vger.kernel.org
>>>>>> Fixes: 38cde0bd7b67 ("rust: init: add `Zeroable` trait and =
`init::zeroed` function")
>>>>>> Closes: https://github.com/Rust-for-Linux/pinned-init/pull/13
>>>>>> Signed-off-by: Laine Taffin Altman <alexanderaltman@me.com>
>>>>>> Signed-off-by: Benno Lossin <benno.lossin@proton.me>
>>>>>=20
>>>>> I think either in the commit log or in the code comment, there =
better be
>>>>> a link or explanation on "(un)inhabited type". The rest looks good =
to
>>>>> me.
>>>>=20
>>>> Would the following be okay for that purpose?
>>>>=20
>>>> A type is inhabited if at least one valid value of that type =
exists; a
>>>> type is uninhabited if no valid values of that type exist.  The =
terms
>>>> "inhabited" and "uninhabited" in this sense originate in type =
theory,
>>>> a branch of mathematics.
>>>>=20
>>>> In Rust, producing an invalid value of any type is immediate =
undefined
>>>> behavior (UB); this includes via zeroing memory.  Therefore, since =
an
>>>> uninhabited type has no valid values, producing any values at all =
for
>>>> it is UB.
>>>>=20
>>>> The Rust standard library type `core::convert::Infallible` is
>>>> uninhabited, by virtue of having been declared as an enum with no
>>>> cases, which always produces uninhabited types in Rust.  Thus, =
remove
>>>> the implementation of `Zeroable` for `Infallible`, thereby avoiding
>>>> the UB.
>>>>=20
>>>=20
>>> Yeah, this works for me. Thanks!
>>=20
>> Great!  Should it be re-sent or can the new wording be incorporated =
upon merge?
>=20
> I can re-send it for you again, or do you want to send it yourself?
> I think it is also a good idea to add a link to [1] in the code, since
> the above explanation is rather long and fits better in the commit
> message.
>=20

I=E2=80=99ll try and do it myself; thank you for sending the first round =
for me and illustrating procedures!  What =
Reviewed-By=E2=80=99s/Signed-Off-By's should I retain?

Thanks,
Laine

> --=20
> Cheers,
> Benno
>=20
> [1]: https://doc.rust-lang.org/nomicon/exotic-sizes.html#empty-types