linux-kernel - Re: [PATCH 2/3] rust: add typed accessors for userspace pointers

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Message-ID: <CAH5fLgi8D18ufma0X49nWhXpnz47t-C=OAtM+wwnYu78hEnwhA@mail.gmail.com>
Date: Thu, 25 Jan 2024 13:37:07 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Arnd Bergmann <arnd@...db.de>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Wedson Almeida Filho <wedsonaf@...il.com>, 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@...sung.com>, 
	Kees Cook <keescook@...omium.org>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Andrew Morton <akpm@...ux-foundation.org>, 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>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH 2/3] rust: add typed accessors for userspace pointers

On Thu, Jan 25, 2024 at 1:27 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Wed, Jan 24, 2024, at 12:20, Alice Ryhl wrote:
> > +unsigned long
> > rust_helper_copy_from_user_unsafe_skip_check_object_size(void *to,
> > const void __user *from, unsigned long n)
> > +{
> > +     unsigned long res;
> > +
> > +     might_fault();
> > +     instrument_copy_from_user_before(to, from, n);
> > +     if (should_fail_usercopy())
> > +             return n;
> > +     res = raw_copy_from_user(to, from, n);
> > +     instrument_copy_from_user_after(to, from, n, res);
> > +     return res;
> > +}
> > +EXPORT_SYMBOL_GPL(rust_helper_copy_from_user_unsafe_skip_check_object_size);
> > +
> > +unsigned long
> > rust_helper_copy_to_user_unsafe_skip_check_object_size(void __user *to,
> > const void *from, unsigned long n)
> > +{
> > +     might_fault();
> > +     if (should_fail_usercopy())
> > +             return n;
> > +     instrument_copy_to_user(to, from, n);
> > +     return raw_copy_to_user(to, from, n);
> > +}
> > +EXPORT_SYMBOL_GPL(rust_helper_copy_to_user_unsafe_skip_check_object_size);
>
> These functions are almost identical to the ones in
> lib/usercopy.c for !defined(INLINE_COPY_TO_USER).
>
> That version has an extra memset() after a partial
> copy_from_user(), and you probably want to have the
> same thing here for consistency.
>
> I think ideally we should only have one out-of-line copy
> of these two functions and have that one shared between
> rust and architectures that want the C version out of line
> as well.

I had a bit of trouble figuring out all of the copy_[to/from]_user
methods that are available. I was hoping that a better solution would
be available, and it sounds like one is. Is _copy_from_user always
available as an exported symbol? If it's always available and skips
the check, then I can just use that. I don't think the memset matters
for my case.

Otherwise, I can add a helper in rust/helpers.c that wraps
_copy_from_user only when INLINE_COPY_FROM_USER is defined, and call
the helper in those cases, and otherwise call the exported symbol
directly. (I need an exported symbol to call into C from Rust.)

Would that make sense?

Alice