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

[Alice Ryhl <aliceryhl@google.com>]

On Fri, May 30, 2025 at 01:32:44PM +0200, Benno Lossin wrote:
> On Tue May 27, 2025 at 2:34 PM CEST, Alice Ryhl wrote:
> > This patch adds a direct wrapper around the C function of the same name.
> > It's not really intended for direct use by Rust code since
> > strncpy_from_user has a somewhat unfortunate API where it only
> > nul-terminates the buffer if there's space for the nul-terminator. This
> > means that a direct Rust wrapper around it could not return a &CStr
> > since the buffer may not be a cstring. However, we still add the method
> > to build more convenient APIs on top of it, which will happen in
> > subsequent patches.
> >
> > Reviewed-by: Danilo Krummrich <dakr@kernel.org>
> > Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> 
> Reviewed-by: Benno Lossin <lossin@kernel.org>

Thanks!

> > +/// 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.
> > +#[inline]
> > +#[expect(dead_code)]
> > +fn raw_strncpy_from_user(dst: &mut [MaybeUninit<u8>], src: UserPtr) -> Result<usize> {
> 
> We could also return `&[u8]` here instead of the size. Would that
> improve the users of this API?

Beyond what Greg says, convenience of use is not a goal *at all* of this
function. It's purpose is to faithfully wrap the C function and match
its semantics exactly. Ease of use is taken care of by patch 2 of this
series.

Alice