linux-kernel - Re: [PATCH 1/2] cfi: add CONFIG_CFI_ICALL_NORMALIZE

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Message-ID: <20240730121336.GL33588@noisy.programming.kicks-ass.net>
Date: Tue, 30 Jul 2024 14:13:36 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc: Alice Ryhl <aliceryhl@...gle.com>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nicolas Schier <nicolas@...sle.eu>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Miguel Ojeda <ojeda@...nel.org>, Kees Cook <kees@...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>,
	Matthew Maurer <mmaurer@...gle.com>, linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 1/2] cfi: add CONFIG_CFI_ICALL_NORMALIZE_INTEGERS

On Tue, Jul 30, 2024 at 01:38:33PM +0200, Miguel Ojeda wrote:
> On Tue, Jul 30, 2024 at 11:40 AM Alice Ryhl <aliceryhl@...gle.com> wrote:
> >
> > Introduce a Kconfig option for enabling the experimental option to
> > normalize integer types. This ensures that integer types of the same
> > size and signedness are considered compatible by the Control Flow
> > Integrity sanitizer.
> >
> > This option exists for compatibility with Rust, as C and Rust do not
> > have the same set of integer types. There are cases where C has two
> > different integer types of the same size and alignment, but Rust only
> > has one integer type of that size and alignment. When Rust calls into
> > C functions using such types in their signature, this results in CFI
> > failures.
> >
> > This patch introduces a dedicated option for this because it is
> > undesirable to have CONFIG_RUST affect CC_FLAGS in this way.
> 
> Is there any case where we would want CFI_ICALL_NORMALIZE_INTEGERS
> when Rust is not enabled, then? If not, is the idea here to make this
> an explicit extra question in the config before enabling Rust? Or why
> wouldn't it be done automatically?

I suspect CFI_ICALL_NORMALIZE_INTEGERS breaks ABI, then again, Linux
doesn't promise or preserve ABI except for the SYSCALL layer. So yeah,
meh.