Re: [PATCH v3 2/2] rust: uaccess: use NonEmpty in strcpy_into_buf()

["Alexandre Courbot" <acourbot@nvidia.com>]

On Wed Oct 14, 2026 at 5:30 PM JST, Lorenzo Delgado wrote:
<...>
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index 5f6c4d7a1a51..1cc5c7580d3a 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -11,6 +11,7 @@
>      error::Result,
>      ffi::{c_char, c_void},
>      fs::file,
> +    non_empty::NonEmpty,
>      prelude::*,
>      ptr::KnownSize,
>      transmute::{AsBytes, FromBytes},
> @@ -383,20 +384,16 @@ pub fn read_all<A: Allocator>(mut self, buf: &mut Vec<u8, A>, flags: Flags) -> R
>      /// 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
> +    /// Since there must be space to add a NUL-terminator, `buf` is a [`NonEmpty`] slice. 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);
> -        }
> -
> +    pub fn strcpy_into_buf<'buf>(self, buf: &'buf mut NonEmpty<[u8]>) -> Result<&'buf CStr> {

I understand this was done per the v1 feedback, but won't it make it
more tedious for users to call this method? (hard to tell as there are
no users at the moment :))

We could still benefit from `NonEmpty` if we replace the `buf.is_empty`
by an attempt to build it and return `EINVAL` if that fails (what v1 did
basically) and calling `last_mut` on it to remove the
`unwrap_unchecked`.

Of course if callers are expected to already work with a `NonEmpty` then
that's fine, but otherwise my concern is that every one of them would
have to manually build one to perform a check that this method can
easily do.