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

[Benno Lossin]

On 3/19/24 06:28, Laine Taffin Altman wrote:
> On Mar 18, 2024, at 9:39 PM, Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
>> On Mon, Mar 18, 2024 at 08:17:07PM -0700, Laine Taffin Altman wrote:
>>> On Mar 18, 2024, at 10:25 AM, Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
>>>> On Wed, Mar 13, 2024 at 11:09:37PM +0000, Benno Lossin wrote:
>>>>> From: Laine Taffin Altman <alexanderaltman@xxxxxx>
>>>>>
>>>>> 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@xxxxxxxxxxxxxxx
>>>>> 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@xxxxxx>
>>>>> Signed-off-by: Benno Lossin <benno.lossin@xxxxxxxxx>
>>>>
>>>> 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.
>>>
>>
>> Yeah, this works for me. Thanks!
> 
> Great!  Should it be re-sent or can the new wording be incorporated upon merge?

I can re-send it for you again, or do you want to send it yourself?
I think it is also a good idea to add a link to [1] in the code, since
the above explanation is rather long and fits better in the commit
message.

-- 
Cheers,
Benno

[1]: https://doc.rust-lang.org/nomicon/exotic-sizes.html#empty-types