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Date: Sat, 30 Mar 2024 12:03:31 +0000
To: Laine Taffin Altman <alexanderaltman@me.com>
From: Benno Lossin <benno.lossin@proton.me>
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>
Subject: Re: [PATCH] rust: init: remove impl Zeroable for Infallible
Message-ID: <2dd2c052-0d1f-4a77-9fff-1d6db80310e1@proton.me>
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On 21.03.24 05:53, Laine Taffin Altman wrote:
> 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.co=
m> wrote:
>>>>>> On Wed, Mar 13, 2024 at 11:09:37PM +0000, Benno Lossin wrote:
>>>>>>> From: Laine Taffin Altman <alexanderaltman@me.com>
>>>>>>>
>>>>>>> It is not enough for a type to be a ZST to guarantee that zeroed me=
mory
>>>>>>> is a valid value for it; it must also be inhabited. Creating a valu=
e of
>>>>>>> an uninhabited type, ZST or no, is immediate UB.
>>>>>>> Thus remove the implementation of `Zeroable` for `Infallible`, sinc=
e
>>>>>>> that type is not inhabited.
>>>>>>>
>>>>>>> Cc: stable@vger.kernel.org
>>>>>>> Fixes: 38cde0bd7b67 ("rust: init: add `Zeroable` trait and `init::z=
eroed` 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>
>>>>>>
>>>>>> I think either in the commit log or in the code comment, there bette=
r be
>>>>>> a link or explanation on "(un)inhabited type". The rest looks good t=
o
>>>>>> me.
>>>>>
>>>>> Would the following be okay for that purpose?
>>>>>
>>>>> 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.
>>>>>
>>>>> In Rust, producing an invalid value of any type is immediate undefine=
d
>>>>> 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.
>>>>>
>>>>> 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.
>>>>>
>>>>
>>>> Yeah, this works for me. Thanks!
>>>
>>> Great!  Should it be re-sent or can the new wording be incorporated upo=
n merge?
>>
>> 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?

Do you still want to send it yourself? If you don't have the time, no
problem, I can send it again.

--=20
Cheers,
Benno