linux-kernel - Re: [PATCH v7 2/4] uaccess: always export _copy_[from|to]_user with CONFIG

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Message-ID: <Zu_CeRfMKyyt4E5O@sashalap>
Date: Sun, 22 Sep 2024 03:08:41 -0400
From: Sasha Levin <sashal@...nel.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>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 v7 2/4] uaccess: always export _copy_[from|to]_user with
 CONFIG_RUST

On Tue, May 28, 2024 at 02:58:03PM +0000, Alice Ryhl 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.

Hi folks,

I've noticed a build failure using GCC 9.5.0 on arm64 allmodconfig
builds:

In file included from ./arch/arm64/include/asm/preempt.h:6,
                  from ./include/linux/preempt.h:79,
                  from ./include/linux/alloc_tag.h:11,
                  from ./include/linux/percpu.h:5,
                  from ./include/linux/context_tracking_state.h:5,
                  from ./include/linux/hardirq.h:5,
                  from drivers/net/ethernet/mellanox/mlx4/cq.c:37:
In function 'check_copy_size',
     inlined from 'mlx4_init_user_cqes' at ./include/linux/uaccess.h:203:7:
./include/linux/thread_info.h:244:4: error: call to '__bad_copy_from' declared with attribute error: copy source size is too small
   244 |    __bad_copy_from();
       |    ^~~~~~~~~~~~~~~~~
make[7]: *** [scripts/Makefile.build:244: drivers/net/ethernet/mellanox/mlx4/cq.o] Error 1

I do not have CONFIG_RUST enabled in those builds.

I've bisected the issue (twice!) and bisection points to this patch
which landed upstream as 1f9a8286bc0c ("uaccess: always export
_copy_[from|to]_user with CONFIG_RUST").

Reverting said commit on top of Linus's tree fixes the build breakage.

-- 
Thanks,
Sasha