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

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Message-Id: <DA9P6CSP6563.1OYPZXIP5S7N1@kernel.org>
Date: Fri, 30 May 2025 20:13:19 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>, "Miguel Ojeda" <ojeda@...nel.org>,
 "Andrew Morton" <akpm@...ux-foundation.org>, "Alexander Viro"
 <viro@...iv.linux.org.uk>, "Greg Kroah-Hartman"
 <gregkh@...uxfoundation.org>
Cc: "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

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.

> +#[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?

---
Cheers,
Benno

> +}