Re: [PATCH] rust: init: remove impl Zeroable for Infallible

[Laine Taffin Altman <alexanderaltman@me.com>]

On Mar 18, 2024, at 10:25 AM, Boqun Feng <boqun.feng@gmail.com> wrote:
> On Wed, Mar 13, 2024 at 11:09:37PM +0000, Benno Lossin wrote:
>> From: Laine Taffin Altman <alexanderaltman@me.com>
>> 
>> It is not enough for a type to be a ZST to guarantee that zeroed memory
>> is a valid value for it; it must also be inhabited. Creating a value of
>> an uninhabited type, ZST or no, is immediate UB.
>> Thus remove the implementation of `Zeroable` for `Infallible`, since
>> that type is not inhabited.
>> 
>> 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>
>> Signed-off-by: Benno Lossin <benno.lossin@proton.me>
> 
> I think either in the commit log or in the code comment, there better be
> a link or explanation on "(un)inhabited type". The rest looks good to
> me.

Would the following be okay for that purpose?

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

Thanks,
Laine

> 
> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> 
> Regards,
> Boqun
> 
>> ---
>> rust/kernel/init.rs | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
>> index 424257284d16..538e03cfc84a 100644
>> --- a/rust/kernel/init.rs
>> +++ b/rust/kernel/init.rs
>> @@ -1292,8 +1292,8 @@ 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.
>> +    {<T: ?Sized>} PhantomData<T>, core::marker::PhantomPinned, (),
>> 
>>     // SAFETY: Type is allowed to take any value, including all zeros.
>>     {<T>} MaybeUninit<T>,
>> 
>> base-commit: 768409cff6cc89fe1194da880537a09857b6e4db
>> -- 
>> 2.42.0
>> 
>> 
>>