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Subject: Re: [PATCH v2] rust: init: remove impl Zeroable for Infallible
From: Laine Taffin Altman <alexanderaltman@me.com>
In-Reply-To: <9D0C95D2-6239-4A3B-B9DD-66299B9911EF@me.com>
Date: Mon, 1 Apr 2024 18:56:23 -0700
Cc: stable@vger.kernel.org,
 rust-for-linux@vger.kernel.org,
 lkml <linux-kernel@vger.kernel.org>
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Message-Id: <08AD88C6-0054-4DF9-96BE-8213655FAAB1@me.com>
References: <9D0C95D2-6239-4A3B-B9DD-66299B9911EF@me.com>
To: Miguel Ojeda <ojeda@kernel.org>,
 Alex Gaynor <alex.gaynor@gmail.com>,
 Wedson Almeida Filho <wedsonaf@gmail.com>,
 Boqun Feng <boqun.feng@gmail.com>,
 =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
 Benno Lossin <benno.lossin@proton.me>,
 Andreas Hindborg <a.hindborg@samsung.com>,
 Alice Ryhl <aliceryhl@google.com>,
 Martin Rodriguez Reboredo <yakoyoku@gmail.com>

Oops!  Formatting issue; ignore this one.

Thanks,
Laine

> On Apr 1, 2024, at 6:53=E2=80=AFPM, Laine Taffin Altman =
<alexanderaltman@me.com> wrote:
>=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.
>=20
> The current kernel code allows this UB to be triggered, for example by =
code like:
> `pr_info!("{}=E2=80=9D, =
Box::<core::convert::Infallible>::init(kernel::init::zeroed())?);`
>=20
> Thus, remove the implementation of `Zeroable` for `Infallible`, =
thereby avoiding the UB.
>=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>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> =E2=80=94
> V1 -> V2: Added more documentation to the comment, with links; also =
added more details to the commit message.
>=20
> rust/kernel/init.rs | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>=20
> diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
> index 424257284d16..9353c9919fd4 100644
> --- a/rust/kernel/init.rs
> +++ b/rust/kernel/init.rs
> @@ -1292,8 +1292,11 @@ 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.
> +    // Note: do not add uninhabited types (such as ! or Infallible) =
to this list; creating an instance of an uninhabited type is immediate =
undefined behavior.
> +    // For more on uninhabited/empty types, consult The Rustonomicon: =
https://doc.rust-lang.org/stable/nomicon/exotic-sizes.html#empty-types
> +    // The Rust Reference also has information on undefined behavior: =
https://doc.rust-lang.org/stable/reference/behavior-considered-undefined.h=
tml
> +    {<T: ?Sized>} PhantomData<T>, core::marker::PhantomPinned, (),
>=20
>     // SAFETY: Type is allowed to take any value, including all zeros.
>     {<T>} MaybeUninit<T>,
> --=20
> 2.44.0