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

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

On Mar 19, 2024, at 3:34 AM, Benno Lossin <benno.lossin@proton.me> wrote:
> On 3/19/24 06:28, Laine Taffin Altman wrote:
>> On Mar 18, 2024, at 9:39 PM, Boqun Feng <boqun.feng@gmail.com> 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@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.
>>>> 
>>> 
>>> 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.
> 

I’ll try and do it myself; thank you for sending the first round for me and illustrating procedures!  What Reviewed-By’s/Signed-Off-By's should I retain?

Thanks,
Laine

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