linux-kernel - Re: [PATCH v4 1/2] uaccess: rust: add strncpy_from

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Message-ID: <CAH5fLgjoX5AEO8NvDdj+05HUtpoVUPpOa7ri5UG6Djp98vQDxQ@mail.gmail.com>
Date: Sat, 31 May 2025 15:27:41 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Benno Lossin <lossin@...nel.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/2] uaccess: rust: add strncpy_from_user

On Fri, May 30, 2025 at 8:13 PM Benno Lossin <lossin@...nel.org> wrote:
>
> 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.

I do not mind that.

> > +#[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?

I can see the idea behind such comments, but I do not believe we've
used them so far.

Alice