On Tue, Jul 08, 2025 at 09:44:58PM +0200, Andreas Hindborg wrote:
> Add `NullBorrowFormatter`, 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@xxxxxxxxxx>
> ---
> rust/kernel/str.rs | 59 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 59 insertions(+)
>
> diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
> index 78b2f95eb3171..05d79cf40c201 100644
> --- a/rust/kernel/str.rs
> +++ b/rust/kernel/str.rs
> @@ -860,6 +860,65 @@ fn deref_mut(&mut self) -> &mut Self::Target {
> }
> }
>
> +/// 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.
> +pub(crate) struct NullBorrowFormatter<'a> {
> + buffer: &'a mut [u8],
> + pos: usize,
> +}
Do you need `pos`? Often I see this kind of code subslice `buffer`
instead.
> +impl<'a> NullBorrowFormatter<'a> {
> + /// Create a new [`Self`] instance.
> + pub(crate) fn new(buffer: &'a mut [u8]) -> Result<NullBorrowFormatter<'a>> {
> + *(buffer.first_mut().ok_or(EINVAL)?) = 0;
> +
> + // INVARIANT: We null terminated the buffer above.
> + Ok(Self { buffer, pos: 0 })
> + }
I would probably just use an Option for this constructor.
> + #[expect(dead_code)]
> + pub(crate) fn from_array<const N: usize>(
> + a: &'a mut [crate::ffi::c_char; N],
> + ) -> Result<NullBorrowFormatter<'a>> {
> + Self::new(
> + // SAFETY: the buffer of `a` is valid for read and write as `u8` for
> + // at least `N` bytes.
> + unsafe { core::slice::from_raw_parts_mut(a.as_mut_ptr().cast::<u8>(), N) },
> + )
> + }
Arrays automatically coerce to slices, so I don't think this is
necessary. You can just call `new`.
> + /// Return the position of the write pointer in the underlying buffer.
> + #[expect(dead_code)]
> + pub(crate) fn pos(&self) -> usize {
> + self.pos
> + }
You delete this function in one of the later patches, so it makes more
sense not to add it.
> +}
> +
> +impl Write for NullBorrowFormatter<'_> {
> + 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
> + if self.pos + len > self.buffer.len() - 1 {
Integer overflow?
> + return Err(fmt::Error);
> + }
> +
> + self.buffer[self.pos..self.pos + len].copy_from_slice(bytes);
> + self.pos += len;
> +
> + // INVARIANT: The buffer is null terminated.
> + self.buffer[self.pos] = 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
>
>
