rust: init: remove impl Zeroable for Infallible

commit 49ceae68a0df9a92617a61e9ce8a0efcf6419585 upstream.

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@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>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/CA160A4E-561E-4918-837E-3DCEBA74F808@me.com
[ Reformatted the comment slightly. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
index 65be9ae57b80b53b4860774e2b2a2487e7611e5d..cf9575f156a639288ff15f8a8965d350b2d8dbdf 100644 (file)
--- a/rust/kernel/init.rs
+++ b/rust/kernel/init.rs
@@ -1292,8 +1292,15 @@ 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>,