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Date:   Tue, 10 Oct 2023 10:12:20 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Matthew Maurer <mmaurer@google.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?iso-8859-1?Q?Bj=F6rn?= Roy Baron <bjorn3_gh@protonmail.com>,
        Benno Lossin <benno.lossin@proton.me>,
        Andreas Hindborg <a.hindborg@samsung.com>,
        Alice Ryhl <aliceryhl@google.com>,
        linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org
Subject: Re: [PATCH] x86: Enable IBT in Rust if enabled in C
Message-ID: <20231010081220.GD377@noisy.programming.kicks-ass.net>
References: <20231009224347.2076221-1-mmaurer@google.com>
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On Mon, Oct 09, 2023 at 10:42:54PM +0000, Matthew Maurer wrote:
> These flags are not made conditional on compiler support because at the
> moment exactly one version of rustc supported, and that one supports
> these flags.
> 
> Building without these additional flags will manifest as objtool
> printing a large number of errors about missing ENDBR and if CFI is
> enabled (not currently possible) will result in incorrectly structured
> function prefixes.

Well, I would also imagine running it on actual IBT enabled hardware
will get you a non-booting kernel.

> Signed-off-by: Matthew Maurer <mmaurer@google.com>
> ---
> 
> Split out the IBT additions as per
> https://lkml.kernel.org/linux-fsdevel/CANiq72kK6ppBE7j=z7uua1cJMKaLoR5U3NUAZXT5MrNEs9ZhfQ@mail.gmail.com/
> 
> arch/x86/Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 5bfe5caaa444..941f7abf6dbf 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -81,6 +81,7 @@ ifeq ($(CONFIG_X86_KERNEL_IBT),y)
>  #   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104816
>  #
>  KBUILD_CFLAGS += $(call cc-option,-fcf-protection=branch -fno-jump-tables)
> +KBUILD_RUSTFLAGS += -Zcf-protection=branch -Zno-jump-tables

One question, -Zcf-protection=branch, will that ever emit NOTRACK
prefix? The kernel very explicitly does not support (enable) NOTRACK.

>  else
>  KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
>  endif
> -- 
> 2.42.0.609.gbb76f46606-goog
>