linux-kernel - Re: [PATCH 1/3] rust: add userspace pointers

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Message-ID: <CAH5fLghRiVGWr--9hqwr4Cg4anaKWVqZA0D65DgJzLPPOAvNgg@mail.gmail.com>
Date: Thu, 8 Feb 2024 13:20:28 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Valentin Obst <kernel@...entinobst.de>
Cc: a.hindborg@...sung.com, akpm@...ux-foundation.org, alex.gaynor@...il.com, 
	arnd@...db.de, arve@...roid.com, benno.lossin@...ton.me, 
	bjorn3_gh@...tonmail.com, boqun.feng@...il.com, brauner@...nel.org, 
	cmllamas@...gle.com, gary@...yguo.net, gregkh@...uxfoundation.org, 
	joel@...lfernandes.org, keescook@...omium.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, maco@...roid.com, ojeda@...nel.org, 
	rust-for-linux@...r.kernel.org, surenb@...gle.com, tkjos@...roid.com, 
	viro@...iv.linux.org.uk, wedsonaf@...il.com
Subject: Re: [PATCH 1/3] rust: add userspace pointers

On Thu, Jan 25, 2024 at 12:13 AM Valentin Obst <kernel@...entinobstde> wrote:
>
> > +//! User pointers.
> > +//!
> > +//! C header: [`include/linux/uaccess.h`](../../../../include/linux/uaccess.h)
> > +
>
> nit: could this be using srctree-relative links?
>
> > +/// The maximum length of a operation using `copy_[from|to]_user`.
>
> nit: 'a' -> 'an'
>
> > +///
> > +/// If a usize is not greater than this constant, then casting it to `c_ulong`
> > +/// is guaranteed to be lossless.
>
> nit: could this be `usize` or [`usize`]. Maybe would also be clearer to
> say "... a value of type [`usize`] is smaller than ..."
>
> > +///
> > +/// These APIs are designed to make it difficult to accidentally write TOCTOU
> > +/// bugs. Every time you read from a memory location, the pointer is advanced by
>
> Maybe makes sense to also introduce the abbreviation TOCTOU in the type
> documentation when it is first used.
>
> > +    /// Reads the entirety of the user slice.
> > +    ///
> > +    /// Returns `EFAULT` if the address does not currently point to
> > +    /// mapped, readable memory.
> > +    pub fn read_all(self) -> Result<Vec<u8>> {
> > +        self.reader().read_all()
> > +    }
>
> If I understand it correctly, the function will return `EFAULT` if _any_
> address in the interval `[self.0, self.0 + self.1)` does not point to
> mapped, readable memory. Maybe the docs could be more explicit.
>
> > +        // Since this is not a pointer to a valid object in our program,
> > +        // we cannot use `add`, which has C-style rules for defined
> > +        // behavior.
> > +        self.0 = self.0.wrapping_add(len);
>
> If I understand it correctly, you are using 'valid object' to refer to
> an 'allocated object' [1] as this is what the `add` method's docs
> refer to [2]. In that case it might be better to use the latter term as
> it has a defined meaning. Also see [3] and [4] which are about making it
> more precise.
>
> [1]: https://doc.rust-lang.org/core/ptr/index.html#allocated-object
> [2]: https://doc.rust-lang.org/core/primitive.pointer.html#method.add
> [3]: https://github.com/rust-lang/rust/pull/116675
> [4]: https://github.com/rust-lang/unsafe-code-guidelines/issues/465

Thanks. I'll include all of your suggestions in my next version.

Alice