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

[Benno Lossin]

On 21.03.24 05:53, Laine Taffin Altman wrote:
> On Mar 19, 2024, at 3:34 AM, Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
>> 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.
>>
> 
> 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?

Do you still want to send it yourself? If you don't have the time, no
problem, I can send it again.

-- 
Cheers,
Benno