linux-kernel - Re: [PATCH v6 04/18] rust: str: introduce `NullTerminatedFormatter`

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Message-Id: <2C9D54CC-F931-41C9-967C-14EA375F9F2A@collabora.com>
Date: Wed, 27 Aug 2025 10:02:38 -0300
From: Daniel Almeida <daniel.almeida@...labora.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>,
 Alice Ryhl <aliceryhl@...gle.com>,
 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 04/18] rust: str: introduce `NullTerminatedFormatter`



> On 22 Aug 2025, at 09:14, Andreas Hindborg <a.hindborg@...nel.org> wrote:
> 
> Add `NullTerminatedFormatter`, a formatter that writes a null terminated
> string to an array or slice buffer. Because this type needs to manage the
> trailing null marker, the existing formatters cannot be used to implement
> this type.
> 
> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
> ---
> rust/kernel/str.rs | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 49 insertions(+)
> 
> diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
> index 46cdc85dad63..d8326f7bc9c1 100644
> --- a/rust/kernel/str.rs
> +++ b/rust/kernel/str.rs
> @@ -871,6 +871,55 @@ fn write_str(&mut self, s: &str) -> fmt::Result {
>     }
> }
> 
> +/// A mutable reference to a byte buffer where a string can be written into.
> +///
> +/// The buffer will be automatically null terminated after the last written character.
> +///
> +/// # Invariants
> +///
> +/// * The first byte of `buffer` is always zero.
> +/// * The length of `buffer` is at least 1.
> +pub(crate) struct NullTerminatedFormatter<'a> {
> +    buffer: &'a mut [u8],
> +}
> +
> +impl<'a> NullTerminatedFormatter<'a> {
> +    /// Create a new [`Self`] instance.
> +    #[expect(dead_code)]
> +    pub(crate) fn new(buffer: &'a mut [u8]) -> Option<NullTerminatedFormatter<'a>> {
> +        *(buffer.first_mut()?) = 0;
> +
> +        // INVARIANT:
> +        //  - We wrote zero to the first byte above.
> +        //  - If buffer was not at least length 1, `buffer.first_mut()` would return None.
> +        Some(Self { buffer })
> +    }
> +}
> +
> +impl Write for NullTerminatedFormatter<'_> {
> +    fn write_str(&mut self, s: &str) -> fmt::Result {
> +        let bytes = s.as_bytes();
> +        let len = bytes.len();
> +
> +        // We want space for a zero. By type invariant, buffer length is always at least 1, so no
> +        // underflow.
> +        if len > self.buffer.len() - 1 {
> +            return Err(fmt::Error);
> +        }
> +
> +        let buffer = core::mem::take(&mut self.buffer);
> +        // We break the zero start invariant for a short while.
> +        buffer[..len].copy_from_slice(bytes);
> +        // INVARIANT: We checked above that buffer will have size at least 1 after this assignment.
> +        self.buffer = &mut buffer[len..];
> +
> +        // INVARIANT: We write zero to the first byte of the buffer.
> +        self.buffer[0] = 0;
> +
> +        Ok(())
> +    }
> +}
> +
> /// An owned string that is guaranteed to have exactly one `NUL` byte, which is at the end.
> ///
> /// Used for interoperability with kernel APIs that take C strings.
> 
> -- 
> 2.47.2
> 
> 
> 


Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>