> On 22 Aug 2025, at 09:14, Andreas Hindborg <a.hindborg@kernel.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@google.com>
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.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@collabora.com>