linux-kernel - Re: [PATCH v3 05/16] rust: str: introduce `NullTerminatedFormatter`

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Message-Id: <D9A4DD7C-D9C2-4D91-B6C3-684BA1C100C0@collabora.com>
Date: Wed, 6 Aug 2025 10:15:57 -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>,
 linux-block@...r.kernel.org,
 rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 05/16] rust: str: introduce `NullTerminatedFormatter`



> On 11 Jul 2025, at 08:43, 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.
> 
> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
> ---
> rust/kernel/str.rs | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 50 insertions(+)
> 
> diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
> index b1bc584803b0..c58925438c6e 100644
> --- a/rust/kernel/str.rs
> +++ b/rust/kernel/str.rs
> @@ -838,6 +838,56 @@ 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.

Hmm, I suppose you want this to be the only null? See below.

> +///
> +/// # Invariants
> +///
> +/// `buffer` is always null terminated.
> +pub(crate) struct NullTerminatedFormatter<'a> {
> +    buffer: &'a mut [u8],
> +}
> +
> +impl<'a> NullTerminatedFormatter<'a> {
> +    /// Create a new [`Self`] instance.
> +    pub(crate) fn new(buffer: &'a mut [u8]) -> Option<NullTerminatedFormatter<'a>> {
> +        *(buffer.first_mut()?) = 0;
> +
> +        // INVARIANT: We null terminated the buffer above.
> +        Some(Self { buffer })
> +    }
> +
> +    #[expect(dead_code)]
> +    pub(crate) fn from_array<const N: usize>(
> +        buffer: &'a mut [crate::ffi::c_char; N],
> +    ) -> Option<NullTerminatedFormatter<'a>> {
> +        Self::new(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 null terminator. Buffer length is always at least 1, so no overflow.

Perhaps this should be a type invariant? I know this is a logical conclusion
from saying “buffer is always NULL terminated”, but it’s always
nice to be even more explicit.

> +        if len > self.buffer.len() - 1 {
> +            return Err(fmt::Error);
> +        }
> +
> +        let buffer = core::mem::take(&mut self.buffer);
> +        // We break the null termination invariant for a short while.
> +        buffer[..len].copy_from_slice(bytes);
> +        self.buffer = &mut buffer[len..];

As I said in my first comment, if you want this to be the only null, I
don’t think the copy above enforces it?

> +
> +        // INVARIANT: We null terminate 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
> 
> 
> 

— Daniel