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

[Boqun Feng]

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!

Regards,
Boqun

> Thanks,
> Laine
> 
> > 
> > Reviewed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> > 
> > 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
> >> 
> >> 
> >> 
>