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From: Alice Ryhl <aliceryhl@google.com>
Date: Mon, 18 Mar 2024 21:10:07 +0100
Message-ID: <CAH5fLgg=7qyOiW6szF4+N=_oNfpqBBSmreeK4VT=HXq9qm_KAg@mail.gmail.com>
Subject: Re: [PATCH v3 1/4] rust: uaccess: add userspace pointers
To: Boqun Feng <boqun.feng@gmail.com>
Cc: Miguel Ojeda <ojeda@kernel.org>, Matthew Wilcox <willy@infradead.org>, 
	Al Viro <viro@zeniv.linux.org.uk>, Andrew Morton <akpm@linux-foundation.org>, 
	Kees Cook <keescook@chromium.org>, Alex Gaynor <alex.gaynor@gmail.com>, 
	Wedson Almeida Filho <wedsonaf@gmail.com>, Gary Guo <gary@garyguo.net>, 
	=?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, 
	Benno Lossin <benno.lossin@proton.me>, Andreas Hindborg <a.hindborg@samsung.com>, 
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>, =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= <arve@android.com>, 
	Todd Kjos <tkjos@android.com>, Martijn Coenen <maco@android.com>, 
	Joel Fernandes <joel@joelfernandes.org>, Carlos Llamas <cmllamas@google.com>, 
	Suren Baghdasaryan <surenb@google.com>, Arnd Bergmann <arnd@arndb.de>, linux-mm@kvack.org, 
	linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, 
	Christian Brauner <brauner@kernel.org>
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On Mon, Mar 18, 2024 at 8:33=E2=80=AFPM Boqun Feng <boqun.feng@gmail.com> w=
rote:
>
> On Mon, Mar 18, 2024 at 08:12:27PM +0100, Alice Ryhl wrote:
> > On Mon, Mar 18, 2024 at 7:59=E2=80=AFPM Boqun Feng <boqun.feng@gmail.co=
m> 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 buffe=
r.
> > > > +    ///
> > > > +    /// 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 functio=
n,
> > > 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 discouragem=
ent.
> >
> > 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