linux-kernel - Re: [PATCH v6 2/4] uaccess: always export _copy_[from|to]_user with CONFIG

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Date: Tue, 14 May 2024 10:27:57 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Matthew Wilcox <willy@...radead.org>,
 Al Viro <viro@...iv.linux.org.uk>, Kees Cook <keescook@...omium.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>,
 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>, Trevor
 Gross <tmgross@...ch.edu>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, Christian
 Brauner <brauner@...nel.org>
Subject: Re: [PATCH v6 2/4] uaccess: always export _copy_[from|to]_user with
 CONFIG_RUST

On Thu, 18 Apr 2024 08:59:18 +0000 Alice Ryhl <aliceryhl@...gle.com> wrote:

> From: Arnd Bergmann <arnd@...db.de>
> 
> Rust code needs to be able to access _copy_from_user and _copy_to_user
> so that it can skip the check_copy_size check in cases where the length
> is known at compile-time, mirroring the logic for when C code will skip
> check_copy_size. To do this, we ensure that exported versions of these
> methods are available when CONFIG_RUST is enabled.
> 
> Alice has verified that this patch passes the CONFIG_TEST_USER_COPY test
> on x86 using the Android cuttlefish emulator.
> 
>  ...
>
> -#ifdef INLINE_COPY_TO_USER
>  static inline __must_check unsigned long
> -_copy_to_user(void __user *to, const void *from, unsigned long n)
> +_inline_copy_to_user(void __user *to, const void *from, unsigned long n)
>  {

I think it would be helpful to have some comments in here describing
why we're doing this _inline_* thing.  What problem is it avoiding?