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

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Date: Mon, 18 Mar 2024 21:10:07 +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 8:33 PM Boqun Feng <boqun.feng@...il.com> wrote:
>
> On Mon, Mar 18, 2024 at 08:12:27PM +0100, Alice Ryhl wrote:
> > 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
>
> make it a `&mut [MayUninit<u8>]` then? If that works, then _raw version
> is not more powerful therefore no need to pub it.

Nobody actually has a need for that. Also, it doesn't even remove the
need for unsafe code in the caller, since the caller still needs to
assert that the call has initialized the memory.

> > 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),
>
> I might be too picky, but avoiding pub unsafe functions if not necessary
> could help us reduce unnecessary unsafe code ;-)
>
> Regards,
> Boqun
>
> > though, since it is used in `Page`.
> >
> > Alice