[PATCH 3/5] rust: sync: support using bool with READ_ONCE

[Peter Zijlstra]

On Wed, Dec 31, 2025 at 12:22:27PM +0000, Alice Ryhl wrote:
> Normally it is undefined behavior for a bool to take any value other
> than 0 or 1. However, in the case of READ_ONCE(some_bool) is used, this
> UB seems dangerous and unnecessary. I can easily imagine some Rust code
> that looks like this:
> 
> 	if READ_ONCE(&raw const (*my_c_struct).my_bool_field) {
> 	    ...
> 	}
> 
> And by making an analogy to what the equivalent C code is, anyone
> writing this probably just meant to treat any non-zero value as true.
> 
> For WRITE_ONCE no special logic is required.
> 
> Signed-off-by: Alice Ryhl <aliceryhl at google.com>
> ---
>  rust/kernel/sync/rwonce.rs | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/rust/kernel/sync/rwonce.rs b/rust/kernel/sync/rwonce.rs
> index a1660e43c9ef94011812d1816713cf031a73de1d..73477f53131926996614df573b2d50fff98e624f 100644
> --- a/rust/kernel/sync/rwonce.rs
> +++ b/rust/kernel/sync/rwonce.rs
> @@ -163,6 +163,7 @@ unsafe fn write_once(ptr: *mut Self, val: Self) {
>  // sizes, so picking the wrong helper should lead to a build error.
>  
>  impl_rw_once_type! {
> +    bool, read_once_bool, write_once_1;
>      u8,   read_once_1, write_once_1;
>      i8,   read_once_1, write_once_1;
>      u16,  read_once_2, write_once_2;
> @@ -186,3 +187,21 @@ unsafe fn write_once(ptr: *mut Self, val: Self) {
>      usize, read_once_8, write_once_8;
>      isize, read_once_8, write_once_8;
>  }
> +
> +/// Read an integer as a boolean once.
> +///
> +/// Returns `true` if the value behind the pointer is non-zero. Otherwise returns `false`.
> +///
> +/// # Safety
> +///
> +/// It must be safe to `READ_ONCE` the `ptr` with type `u8`.
> +#[inline(always)]
> +#[track_caller]
> +unsafe fn read_once_bool(ptr: *const bool) -> bool {
> +    // Implement `read_once_bool` in terms of `read_once_1`. The arch-specific logic is inside
> +    // of `read_once_1`.
> +    //
> +    // SAFETY: It is safe to `READ_ONCE` the `ptr` with type `u8`.
> +    let byte = unsafe { read_once_1(ptr.cast::<u8>()) };
> +    byte != 0u8
> +}

Does this hardcode that sizeof(_Bool) == 1? There are ABIs where this is
not the case.