linux-kernel - Re: [PATCH v4] rust: init: remove impl Zeroable for Infallible

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Date: Thu, 04 Apr 2024 09:01:26 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Laine Taffin Altman <alexanderaltman@...com>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, rust-for-linux@...r.kernel.org, Wedson Almeida Filho <wedsonaf@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...sung.com>, Alice Ryhl <aliceryhl@...gle.com>, Martin Rodriguez Reboredo <yakoyoku@...il.com>, lkml <linux-kernel@...r.kernel.org>
Cc: stable@...r.kernel.org
Subject: Re: [PATCH v4] rust: init: remove impl Zeroable for Infallible

On 03.04.24 23:06, Laine Taffin Altman wrote:
> 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 `Box::<core::convert::Infallible>::init(kernel::init::zeroed())`.
> 
> Thus, remove the implementation of `Zeroable` for `Infallible`, thereby
> avoiding the unsoundness (potential for future 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>

Reviewed-by: Benno Lossin <benno.lossin@...ton.me>

> ---
> V3 -> V4: Address review nits; run checkpatch properly.
> V2 -> V3: Email formatting correction.
> V1 -> V2: Added more documentation to the comment, with links; also added more details to the commit message.
> 
>  rust/kernel/init.rs | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
> index 424257284d16..3859c7ff81b7 100644
> --- a/rust/kernel/init.rs
> +++ b/rust/kernel/init.rs
> @@ -1292,8 +1292,15 @@ 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, (),
> +    // Note: do not add uninhabited types (such as `!` or `core::convert::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
> +    //
> +    // SAFETY: These are inhabited ZSTs; there is nothing to zero and a valid value exists.
> +    {<T: ?Sized>} PhantomData<T>, core::marker::PhantomPinned, (),
> 
>      // SAFETY: Type is allowed to take any value, including all zeros.
>      {<T>} MaybeUninit<T>,
> 
> base-commit: c85af715cac0a951eea97393378e84bb49384734

I don't see this commit in the kernel tree, what did you specify as
`--base` when running `git format`?

-- 
Cheers,
Benno