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
> >>
> >>
> >>
>
