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

[Alice Ryhl <aliceryhl@google.com>]

On Fri, Jul 11, 2025 at 01:43:06PM +0200, Andreas Hindborg 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@kernel.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.
> +///
> +/// # Invariants
> +///
> +/// `buffer` is always null terminated.

Since you modify the buffer range, the actual invariant is that the
first byte of `buffer` is zero.

> +pub(crate) struct NullTerminatedFormatter<'a> {

Isn't it called "nul" rather than "null"? My understanding is that
"null" is for the pointer case, and "nul" is the name of the ascii
character at codepoint zero.

> +    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>> {

Can't you just call `::new` where you use this method?

> +        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.

overflow -> underflow

> +        if len > self.buffer.len() - 1 {

this is just `len >= self.buffer.len()`.

> +            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..];
> +
> +        // 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
> 
>