linux-kernel - Re: [PATCH v3 1/4] rust: uaccess: add userspace pointers

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Date: Mon, 18 Mar 2024 20:12:27 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Matthew Wilcox <willy@...radead.org>, 
	Al Viro <viro@...iv.linux.org.uk>, Andrew Morton <akpm@...ux-foundation.org>, 
	Kees Cook <keescook@...omium.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Wedson Almeida Filho <wedsonaf@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...sung.com>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Arve Hjønnevåg <arve@...roid.com>, 
	Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>, 
	Joel Fernandes <joel@...lfernandes.org>, Carlos Llamas <cmllamas@...gle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Arnd Bergmann <arnd@...db.de>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, 
	Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v3 1/4] rust: uaccess: add userspace pointers

On Mon, Mar 18, 2024 at 7:59 PM Boqun Feng <boqun.feng@...il.com> wrote:
>
> On Mon, Mar 11, 2024 at 10:47:13AM +0000, Alice Ryhl wrote:
> > +
> > +    /// Reads raw data from the user slice into a raw kernel buffer.
> > +    ///
> > +    /// Fails with `EFAULT` if the read encounters a page fault.
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// The `out` pointer must be valid for writing `len` bytes.
> > +    pub unsafe fn read_raw(&mut self, out: *mut u8, len: usize) -> Result {
>
> I don't think we want to promote the pub usage of this unsafe function,
> right? We can provide a safe version:
>
>         pub fn read_slice(&mut self, to: &[u8]) -> Result
>
> and all users can just use the safe version (with the help of
> slice::from_raw_parts_mut() if necessary).

Personally, I think having the function be unsafe is plenty discouragement.

Also, this method would need an &mut [u8], which opens the can of
worms related to uninitialized memory. The _raw version of this method
is strictly more powerful.

I don't think I actually use it directly in Binder, so I can make it
private if you think that's important. It needs to be pub(crate),
though, since it is used in `Page`.

Alice