Re: [PATCH v4 1/2] uaccess: rust: add strncpy_from_user

["Benno Lossin" <lossin@kernel.org>]

This patch's title should be adjusted, as it's adding
`raw_strncpy_from_user` and not `strncpy_from_user`.

On Tue May 27, 2025 at 2:34 PM CEST, Alice Ryhl wrote:
> +/// Reads a nul-terminated string into `dst` and returns the length.
> +///
> +/// This reads from userspace until a NUL byte is encountered, or until `dst.len()` bytes have been
> +/// read. Fails with [`EFAULT`] if a read happens on a bad address (some data may have been
> +/// copied). When the end of the buffer is encountered, no NUL byte is added, so the string is
> +/// *not* guaranteed to be NUL-terminated when `Ok(dst.len())` is returned.
> +///
> +/// # Guarantees
> +///
> +/// When this function returns `Ok(len)`, it is guaranteed that the first `len` bytes of `dst` are
> +/// initialized and non-zero. Furthermore, if `len < dst.len()`, then `dst[len]` is a NUL byte.
> +/// Unsafe code may rely on these guarantees.

I would remove the last sentence, it already is implied.

> +#[inline]
> +#[expect(dead_code)]
> +fn raw_strncpy_from_user(dst: &mut [MaybeUninit<u8>], src: UserPtr) -> Result<usize> {
> +    // CAST: Slice lengths are guaranteed to be `<= isize::MAX`.
> +    let len = dst.len() as isize;
> +
> +    // SAFETY: `dst` is valid for writing `dst.len()` bytes.
> +    let res = unsafe {
> +        bindings::strncpy_from_user(dst.as_mut_ptr().cast::<c_char>(), src as *const c_char, len)
> +    };
> +
> +    if res < 0 {
> +        return Err(Error::from_errno(res as i32));
> +    }
> +
> +    #[cfg(CONFIG_RUST_OVERFLOW_CHECKS)]
> +    assert!(res <= len);
> +
> +    Ok(res as usize)

We probably should add a `GUARANTEES` comment here, no?

---
Cheers,
Benno

> +}