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

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Message-Id: <DAAG7QOYXDFA.37VBAYSFJYJCF@kernel.org>
Date: Sat, 31 May 2025 17:24:38 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
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 Sat May 31, 2025 at 3:27 PM CEST, Alice Ryhl wrote:
> On Fri, May 30, 2025 at 8:13 PM Benno Lossin <lossin@...nel.org> wrote:
>> On Tue May 27, 2025 at 2:34 PM CEST, Alice Ryhl wrote:
>> > +#[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.

Yes, but we also haven't really used `# Guarantees` all that often.

---
Cheers,
Benno