Received-SPF: pass (google.com: domain of linux-kernel+bounces-107009-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
Content-Type: text/plain;
	charset=utf-8
Precedence: bulk
Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3774.500.171.1.1\))
Subject: Re: [PATCH] rust: init: remove impl Zeroable for Infallible
From: Laine Taffin Altman <alexanderaltman@me.com>
In-Reply-To: <ZfkW8rwpdRc_hJBU@Boquns-Mac-mini.home>
Date: Mon, 18 Mar 2024 22:28:35 -0700
Cc: Benno Lossin <benno.lossin@proton.me>,
 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>
Content-Transfer-Encoding: quoted-printable
Message-Id: <3FBC841A-968E-4AC5-83F0-E906C7EE85C3@me.com>
References: <20240313230713.987124-1-benno.lossin@proton.me>
 <Zfh5DYkxNAm-mY_9@boqun-archlinux>
 <93FD9491-7E2D-4324-8443-0884B7CFC6EF@me.com>
 <ZfkW8rwpdRc_hJBU@Boquns-Mac-mini.home>
To: Boqun Feng <boqun.feng@gmail.com>

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!

Great!  Should it be re-sent or can the new wording be incorporated upon =
merge?

Thank,
Laine

>=20
> Regards,
> Boqun
>=20
>> Thanks,
>> Laine
>>=20
>>>=20
>>> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
>>>=20
>>> Regards,
>>> Boqun
>>>=20
>>>> ---
>>>> rust/kernel/init.rs | 4 ++--
>>>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>>>=20
>>>> diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
>>>> index 424257284d16..538e03cfc84a 100644
>>>> --- a/rust/kernel/init.rs
>>>> +++ b/rust/kernel/init.rs
>>>> @@ -1292,8 +1292,8 @@ macro_rules! impl_zeroable {
>>>>    i8, i16, i32, i64, i128, isize,
>>>>    f32, f64,
>>>>=20
>>>> -    // SAFETY: These are ZSTs, there is nothing to zero.
>>>> -    {<T: ?Sized>} PhantomData<T>, core::marker::PhantomPinned, =
Infallible, (),
>>>> +    // SAFETY: These are inhabited ZSTs, there is nothing to zero =
and a valid value exists.
>>>> +    {<T: ?Sized>} PhantomData<T>, core::marker::PhantomPinned, (),
>>>>=20
>>>>    // SAFETY: Type is allowed to take any value, including all =
zeros.
>>>>    {<T>} MaybeUninit<T>,
>>>>=20
>>>> base-commit: 768409cff6cc89fe1194da880537a09857b6e4db
>>>> --=20
>>>> 2.42.0
>>>>=20
>>>>=20
>>>>=20
>>=20