Re: [PATCH v4 2/2] uaccess: rust: add UserSliceReader::strcpy_into_buf

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

On Tue May 27, 2025 at 2:34 PM CEST, Alice Ryhl wrote:
> This patch adds a more convenient method for reading C strings from
> userspace. Logic is added to NUL-terminate the buffer when necessary so
> that a &CStr can be returned.
>
> Note that we treat attempts to read past `self.length` as a fault, so
> this returns EFAULT if that limit is exceeded before `buf.len()` is
> reached.
>
> Reviewed-by: Danilo Krummrich <dakr@kernel.org>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/kernel/uaccess.rs | 56 +++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 55 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index 9b1e4016fca2c25a44a8417c7e35e0fcf08aa959..e6534b52a1920254d61f8349426d4cdb38286089 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -293,6 +293,61 @@ pub fn read_all<A: Allocator>(mut self, buf: &mut Vec<u8, A>, flags: Flags) -> R
>          unsafe { buf.set_len(buf.len() + len) };
>          Ok(())
>      }
> +
> +    /// Read a NUL-terminated string from userspace and return it.
> +    ///
> +    /// The string is read into `buf` and a NUL-terminator is added if the end of `buf` is reached.
> +    /// Since there must be space to add a NUL-terminator, the buffer must not be empty. The
> +    /// returned `&CStr` points into `buf`.
> +    ///
> +    /// Fails with [`EFAULT`] if the read happens on a bad address (some data may have been
> +    /// copied).
> +    #[doc(alias = "strncpy_from_user")]
> +    pub fn strcpy_into_buf<'buf>(self, buf: &'buf mut [u8]) -> Result<&'buf CStr> {
> +        if buf.is_empty() {
> +            return Err(EINVAL);
> +        }
> +
> +        // SAFETY: The types are compatible and `strncpy_from_user` doesn't write uninitialized
> +        // bytes to `buf`.
> +        let mut dst = unsafe { &mut *(buf as *mut [u8] as *mut [MaybeUninit<u8>]) };
> +
> +        // We never read more than `self.length` bytes.
> +        if dst.len() > self.length {
> +            dst = &mut dst[..self.length];
> +        }
> +
> +        let mut len = raw_strncpy_from_user(dst, self.ptr)?;
> +        if len < dst.len() {
> +            // Add one to include the NUL-terminator.
> +            len += 1;
> +        } else if len < buf.len() {
> +            // This implies that `len == dst.len() < buf.len()`.
> +            //
> +            // This means that we could not fill the entire buffer, but we had to stop reading
> +            // because we hit the `self.length` limit of this `UserSliceReader`. Since we did not
> +            // fill the buffer, we treat this case as if we tried to read past the `self.length`
> +            // limit and received a page fault, which is consistent with other `UserSliceReader`
> +            // methods that also return page faults when you exceed `self.length`.
> +            return Err(EFAULT);
> +        } else {
> +            // This implies that len == buf.len().
> +            //
> +            // This means that we filled the buffer exactly. In this case, we add a NUL-terminator
> +            // and return it. Unlike the `len < dst.len()` branch, don't modify `len` because it
> +            // already represents the length including the NUL-terminator.
> +            //
> +            // SAFETY: Due to the check at the beginning, the buffer is not empty.
> +            unsafe { *buf.last_mut().unwrap_unchecked() = 0 };

In this case you're overwriting the last character read. Should we give
`raw_strncpy_from_user` access to one less byte and then write NUL into
that?

---
Cheers,
Benno

> +        }
> +
> +        // SAFETY: There are two cases:
> +        // * If we hit the `len < dst.len()` case, then `raw_strncpy_from_user` guarantees that
> +        //   this slice contains exactly one NUL byte at the end of the string.
> +        // * Otherwise, `raw_strncpy_from_user` guarantees that the string contained no NUL bytes,
> +        //   and we have since added a NUL byte at the end.
> +        Ok(unsafe { CStr::from_bytes_with_nul_unchecked(&buf[..len]) })
> +    }
>  }
>  
>  /// A writer for [`UserSlice`].
> @@ -383,7 +438,6 @@ pub fn write<T: AsBytes>(&mut self, value: &T) -> Result {
>  /// initialized and non-zero. Furthermore, if `len < dst.len()`, then `dst[len]` is a NUL byte.
>  /// Unsafe code may rely on these guarantees.
>  #[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;