Re: [PATCH v2 20/28] rust: str: add `Formatter` type

["Dr. David Alan Gilbert" <dave@treblig.org>]

* ojeda@kernel.org (ojeda@kernel.org) wrote:
> From: Wedson Almeida Filho <wedsonaf@gmail.com>
> 
> Add the `Formatter` type, which leverages `RawFormatter`,
> but fails if callers attempt to write more than will fit
> in the buffer.
> 
> In order to so, implement the `RawFormatter::from_buffer()`
> constructor as well.
> 
> Co-developed-by: Adam Bratschi-Kaye <ark.email@gmail.com>
> Signed-off-by: Adam Bratschi-Kaye <ark.email@gmail.com>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> Reviewed-by: Gary Guo <gary@garyguo.net>
> [Reworded, adapted for upstream and applied latest changes]
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
> ---
>  rust/kernel/str.rs | 57 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 57 insertions(+)
> 
> diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
> index a995db36486f..ce207d1b3d2a 100644
> --- a/rust/kernel/str.rs
> +++ b/rust/kernel/str.rs
> @@ -406,6 +406,23 @@ impl RawFormatter {
>          }
>      }
>  
> +    /// Creates a new instance of [`RawFormatter`] with the given buffer.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The memory region starting at `buf` and extending for `len` bytes must be valid for writes
> +    /// for the lifetime of the returned [`RawFormatter`].
> +    pub(crate) unsafe fn from_buffer(buf: *mut u8, len: usize) -> Self {
> +        let pos = buf as usize;
> +        // INVARIANT: We ensure that `end` is never less then `buf`, and the safety requirements
> +        // guarantees that the memory region is valid for writes.
> +        Self {
> +            pos,
> +            beg: pos,
> +            end: pos.saturating_add(len),
> +        }
> +    }
> +
>      /// Returns the current insert position.
>      ///
>      /// N.B. It may point to invalid memory.
> @@ -439,3 +456,43 @@ impl fmt::Write for RawFormatter {
>          Ok(())
>      }
>  }
> +
> +/// Allows formatting of [`fmt::Arguments`] into a raw buffer.
> +///
> +/// Fails if callers attempt to write more than will fit in the buffer.
> +pub(crate) struct Formatter(RawFormatter);
> +
> +impl Formatter {
> +    /// Creates a new instance of [`Formatter`] with the given buffer.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The memory region starting at `buf` and extending for `len` bytes must be valid for writes
> +    /// for the lifetime of the returned [`Formatter`].
> +    #[allow(dead_code)]
> +    pub(crate) unsafe fn from_buffer(buf: *mut u8, len: usize) -> Self {
> +        // SAFETY: The safety requirements of this function satisfy those of the callee.
> +        Self(unsafe { RawFormatter::from_buffer(buf, len) })
> +    }
> +}
> +
> +impl Deref for Formatter {
> +    type Target = RawFormatter;
> +
> +    fn deref(&self) -> &Self::Target {
> +        &self.0
> +    }
> +}
> +
> +impl fmt::Write for Formatter {
> +    fn write_str(&mut self, s: &str) -> fmt::Result {
> +        self.0.write_str(s)?;
> +
> +        // Fail the request if we go past the end of the buffer.

Reading this for the first time, I'm surprised by this, perhaps a
bit more comment is needed?  I was expecting that nothing would
let pos pass end.

Dave

> +        if self.0.pos > self.0.end {
> +            Err(fmt::Error)
> +        } else {
> +            Ok(())
> +        }
> +    }
> +}
> -- 
> 2.38.1
> 
-- 
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