linux-kernel - Re: [PATCH 3/5] rust: sync: support using bool with READ

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Message-ID: <20251231152534.14903719.gary@garyguo.net>
Date: Wed, 31 Dec 2025 15:25:34 +0000
From: Gary Guo <gary@...yguo.net>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Boqun Feng <boqun.feng@...il.com>, Will Deacon <will@...nel.org>, Peter
 Zijlstra <peterz@...radead.org>, Richard Henderson
 <richard.henderson@...aro.org>, Matt Turner <mattst88@...il.com>, Magnus
 Lindholm <linmag7@...il.com>, Catalin Marinas <catalin.marinas@....com>,
 Miguel Ojeda <ojeda@...nel.org>, "Björn Roy Baron"
 <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>, Andreas
 Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, Danilo
 Krummrich <dakr@...nel.org>, Mark Rutland <mark.rutland@....com>, FUJITA
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 <frederic@...nel.org>, Lyude Paul <lyude@...hat.com>, Thomas Gleixner
 <tglx@...utronix.de>, Anna-Maria Behnsen <anna-maria@...utronix.de>, John
 Stultz <jstultz@...gle.com>, Stephen Boyd <sboyd@...nel.org>, Alexander
 Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan
 Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
 linux-alpha@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 3/5] rust: sync: support using bool with READ_ONCE

On Wed, 31 Dec 2025 12:22:27 +0000
Alice Ryhl <aliceryhl@...gle.com> 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.

In C, bool can only hold value `false` and `true`, too, and putting
any other value there is going to be UB.

The C language provides automatic cast so when you write an integer to it,
non-zero values will cause `true` to be written. However, you're not
allowed to cast it into a char ptr and write other values into it.

So I think there shouldn't be any special treatment to boolean type in
this regard.

Best,
Gary

> 
> For WRITE_ONCE no special logic is required.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.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
> +}
>