linux-kernel - Re: [PATCH v2] rust: init: remove impl Zeroable for Infallible

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Date: Mon, 1 Apr 2024 18:56:23 -0700
From: Laine Taffin Altman <alexanderaltman@...com>
To: Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Wedson Almeida Filho <wedsonaf@...il.com>,
 Boqun Feng <boqun.feng@...il.com>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <benno.lossin@...ton.me>,
 Andreas Hindborg <a.hindborg@...sung.com>,
 Alice Ryhl <aliceryhl@...gle.com>,
 Martin Rodriguez Reboredo <yakoyoku@...il.com>
Cc: stable@...r.kernel.org,
 rust-for-linux@...r.kernel.org,
 lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] rust: init: remove impl Zeroable for Infallible

Oops!  Formatting issue; ignore this one.

Thanks,
Laine

> On Apr 1, 2024, at 6:53 PM, Laine Taffin Altman <alexanderaltman@...com> wrote:
> 
> 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.
> 
> The current kernel code allows this UB to be triggered, for example by code like:
> `pr_info!("{}”, Box::<core::convert::Infallible>::init(kernel::init::zeroed())?);`
> 
> Thus, remove the implementation of `Zeroable` for `Infallible`, thereby avoiding the UB.
> 
> Cc: stable@...r.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@...com>
> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> Reviewed-by: Boqun Feng <boqun.feng@...il.com>
> —
> V1 -> V2: Added more documentation to the comment, with links; also added more details to the commit message.
> 
> rust/kernel/init.rs | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
> 
> 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,
> 
> -    // 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.html
> +    {<T: ?Sized>} PhantomData<T>, core::marker::PhantomPinned, (),
> 
>     // SAFETY: Type is allowed to take any value, including all zeros.
>     {<T>} MaybeUninit<T>,
> -- 
> 2.44.0