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From: Laine Taffin Altman <alexanderaltman@me.com>
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Subject: [PATCH v2] rust: init: remove impl Zeroable for Infallible
Message-Id: <9D0C95D2-6239-4A3B-B9DD-66299B9911EF@me.com>
Date: Mon, 1 Apr 2024 18:53:31 -0700
Cc: Laine Taffin Altman <alexanderaltman@me.com>,
 stable@vger.kernel.org,
 rust-for-linux@vger.kernel.org,
 lkml <linux-kernel@vger.kernel.org>
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>

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!("{}=E2=80=9D, =
Box::<core::convert::Infallible>::init(kernel::init::zeroed())?);`

Thus, remove the implementation of `Zeroable` for `Infallible`, thereby =
avoiding the UB.

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.

 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,
=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=