linux-kernel - Re: [PATCH v6 06/18] rust: str: add `bytes_to

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Message-ID: <CAH5fLgiEi4gZtb6qProd0tXBHU-ZEs2CKCduO25w9V4ZGqgF4g@mail.gmail.com>
Date: Mon, 1 Sep 2025 10:36:07 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Daniel Almeida <daniel.almeida@...labora.com>
Cc: Andreas Hindborg <a.hindborg@...nel.org>, 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 v6 06/18] rust: str: add `bytes_to_bool` helper function

On Wed, Aug 27, 2025 at 3:46 PM Daniel Almeida
<daniel.almeida@...labora.com> wrote:
>
>
>
> > On 22 Aug 2025, at 09:14, Andreas Hindborg <a.hindborg@...nel.org> 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 | 35 +++++++++++++++++++++++++++++------
> > 1 file changed, 29 insertions(+), 6 deletions(-)
> >
> > diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
> > index d070c0bd86c3..b185262b4851 100644
> > --- a/rust/kernel/str.rs
> > +++ b/rust/kernel/str.rs
> > @@ -921,6 +921,20 @@ fn write_str(&mut self, s: &str) -> fmt::Result {
> >     }
> > }
> >
> > +/// # Safety
> > +///
> > +/// - `string` must point to a null terminated string that is valid for read.
> > +unsafe fn kstrtobool_raw(string: *const u8) -> Result<bool> {
> > +    let mut result: bool = false;
> > +
> > +    // SAFETY:
> > +    // - By function safety requirement, `string` is a valid null-terminated string.
> > +    // - `result` is a valid `bool` that we own.
> > +    let ret = unsafe { bindings::kstrtobool(string, &mut result) };
> > +
> > +    kernel::error::to_result(ret).map(|()| result)
> > +}
> > +
> > /// Convert common user inputs into boolean values using the kernel's `kstrtobool` function.
> > ///
> > /// This routine returns `Ok(bool)` if the first character is one of 'YyTt1NnFf0', or
> > @@ -968,13 +982,22 @@ fn write_str(&mut self, s: &str) -> fmt::Result {
> > /// assert_eq!(kstrtobool(c_str!("2")), Err(EINVAL));
> > /// ```
> > pub fn kstrtobool(string: &CStr) -> Result<bool> {
> > -    let mut result: bool = false;
> > -
> > -    // SAFETY: `string` is a valid null-terminated C string, and `result` is a valid
> > -    // pointer to a bool that we own.
> > -    let ret = unsafe { bindings::kstrtobool(string.as_char_ptr(), &mut result) };
> > +    // SAFETY:
> > +    // - The pointer returned by `CStr::as_char_ptr` is guaranteed to be
> > +    //   null terminated.
> > +    // - `string` is live and thus the string is valid for read.
> > +    unsafe { kstrtobool_raw(string.as_char_ptr()) }
> > +}
> >
> > -    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 kstrtobool_bytes(bytes: &[u8]) -> Result<bool> {
> > +    // `ktostrbool` only considers the first two bytes of the input.
> > +    let stack_string = [*bytes.first().unwrap_or(&0), *bytes.get(1).unwrap_or(&0), 0];
>
> Can’t this be CStr::from_bytes_with_nul() ?
>
> This means that kstrtobool_raw could take a &CStr directly and thus not be unsafe IIUC?

That's what Andreas did in the previous version. It ended up being
pretty complex because CStr::from_bytes_with_nul() requires that we
compute the length of `stack_string`, which isn't really needed here.
I recommended having a _raw method like this to avoid that complexity.
I don't think this is meaningfully more unsafe the way it is in this
patch.

Alice