linux-kernel - Re: [PATCH v5 06/18] rust: str: add `bytes_to

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Message-ID: <aJ7r_W0BzdSYMfT6@google.com>
Date: Fri, 15 Aug 2025 08:12:45 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, Jens Axboe <axboe@...nel.dk>, 
	Breno Leitao <leitao@...ian.org>, linux-block@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 06/18] rust: str: add `bytes_to_bool` helper function

On Fri, Aug 15, 2025 at 09:30:41AM +0200, Andreas Hindborg wrote:
> Add a convenience function to convert byte slices to boolean values by
> wrapping them in a null-terminated C string and delegating to the
> existing `kstrtobool` function. Only considers the first two bytes of
> the input slice, following the kernel's boolean parsing semantics.
> 
> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
> ---
>  rust/kernel/str.rs | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
> index 5611f7846dc0..ced1cb639efc 100644
> --- a/rust/kernel/str.rs
> +++ b/rust/kernel/str.rs
> @@ -978,6 +978,16 @@ pub fn kstrtobool(string: &CStr) -> Result<bool> {
>      kernel::error::to_result(ret).map(|()| result)
>  }
>  
> +/// Convert `&[u8]` to `bool` by deferring to [`kernel::str::kstrtobool`].
> +///
> +/// Only considers at most the first two bytes of `bytes`.
> +pub fn bytes_to_bool(bytes: &[u8]) -> Result<bool> {
> +    // `ktostrbool` only considers the first two bytes of the input.
> +    let nbuffer = [*bytes.first().unwrap_or(&0), *bytes.get(1).unwrap_or(&0), 0];
> +    let c_str = CStr::from_bytes_with_nul(nbuffer.split_inclusive(|c| *c == 0).next().unwrap())?;
> +    kstrtobool(c_str)
> +}

Ouch. That's unpleasant. I would probably suggest this instead to avoid
the length computation:

/// # Safety
/// `string` is a readable NUL-terminated string
unsafe fn kstrtobool_raw(string: *const c_char) -> Result<bool> {
    let mut result: bool = false;
    let ret = unsafe { bindings::kstrtobool(string, &raw mut result) };
    kernel::error::to_result(ret).map(|()| result)
}

pub fn kstrtobool(string: &CStr) -> Result<bool> {
    // SAFETY: Caller ensures that `string` is NUL-terminated.
    unsafe { kstrtobool_cstr(string.as_char_ptr()) }
}

pub fn kstrtobool_bytes(string: &[u8]) -> Result<bool> {
    let mut stack_string = [0u8; 3];

    if let Some(first) = string.get(0) {
        stack_string[0] = *first;
    }
    if let Some(second) = string.get(1) {
        stack_string[1] = *second;
    }

    // SAFETY: stack_string[2] is zero, so the string is NUL-terminated.
    unsafe { kstrtobool_cstr(stack_string.as_ptr()) }
}

Alice