Re: [PATCH v2 2/2] uaccess: rust: add UserSliceReader::strcpy_into_buf

[Danilo Krummrich <dakr@kernel.org>]

On Tue, Apr 29, 2025 at 09:02:23AM +0000, 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.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/kernel/uaccess.rs | 35 +++++++++++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
> 
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index acb703f074a30e60d42a222dd26aed80d8bdb76a..7cec1b62bd8b816f523c8be12cb29905740789fc 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -293,6 +293,41 @@ 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 append it to `dst`.
> +    ///
> +    /// Fails with [`EFAULT`] if the read happens on a bad address.
> +    pub fn strcpy_into_buf<'buf>(&mut 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(self.ptr, dst)?;
> +        if len < dst.len() {
> +            // Add one to include the NUL-terminator.
> +            len += 1;
> +        } else if len < buf.len() {
> +            // We hit the `self.length` limit before `buf.len()`.
> +            return Err(EFAULT);

So, this one we can only ever hit if `len == dst.len()`, which means that the
string (incl. the NULL terminator) is longer than dst. If at the same time
`len < buf.len()`, we know that dst has been shortened because
`buf.len() > self.length`, which means that the string spans across the
self.length boundary.

That seems a bit subtle to me. Maybe we should check for `dst.len() < buf.len()`
instead and add a comment explaining the logic a bit more in detail.

> +        } else {
> +            // SAFETY: Due to the check at the beginning, the buffer is not empty.
> +            unsafe { *buf.last_mut().unwrap_unchecked() = 0 };
> +        }
> +        self.skip(len)?;
> +
> +        // SAFETY: `raw_strncpy_from_user` guarantees that this range of bytes represents a
> +        // NUL-terminated string with the only NUL byte being at the end.
> +        Ok(unsafe { CStr::from_bytes_with_nul_unchecked(&buf[..len]) })
> +    }
>  }
>  
>  /// A writer for [`UserSlice`].
> 
> -- 
> 2.49.0.901.g37484f566f-goog
>