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Subject: Re: [PATCH] rust: init: remove impl Zeroable for Infallible
From: Laine Taffin Altman <alexanderaltman@me.com>
In-Reply-To: <Zfh5DYkxNAm-mY_9@boqun-archlinux>
Date: Mon, 18 Mar 2024 20:17:07 -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>
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Message-Id: <93FD9491-7E2D-4324-8443-0884B7CFC6EF@me.com>
References: <20240313230713.987124-1-benno.lossin@proton.me>
 <Zfh5DYkxNAm-mY_9@boqun-archlinux>
To: Boqun Feng <boqun.feng@gmail.com>

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.

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 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.

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.

Thanks,
Laine

>=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