Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 22 Sep 2022 15:18:01 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Will Deacon <will@kernel.org>
Cc:     linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        Sami Tolvanen <samitolvanen@google.com>,
        Mark Rutland <mark.rutland@arm.com>,
        "Mohan Rao .vanimina" <mailtoc.mohanrao@gmail.com>,
        Nathan Chancellor <nathan@kernel.org>
Subject: Re: [PATCH] vmlinux.lds.h: CFI: Reduce alignment of jump-table to
 function alignment
Message-ID: <202209221514.B2B02622@keescook>
References: <20220922215715.13345-1-will@kernel.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <20220922215715.13345-1-will@kernel.org>
Precedence: bulk

On Thu, Sep 22, 2022 at 10:57:15PM +0100, Will Deacon wrote:
> Due to undocumented, hysterical raisins on x86, the CFI jump-table
> sections in .text are needlessly aligned to PMD_SIZE in the vmlinux
> linker script. When compiling a CFI-enabled arm64 kernel with a 64KiB
> page-size, a PMD maps 512MiB of virtual memory and so the .text section
> increases to a whopping 940MiB and blows the final Image up to 960MiB.
> Others report a link failure.

Heh. "That's not an image ... THIS is an image."[1]

> Since the CFI jump-table requires only instruction alignment, reduce the
> alignment directives to function alignment for parity with other parts
> of the .text section. This reduces the size of the .text section for the
> aforementioned 64KiB page size arm64 kernel to 19MiB for a much more
> reasonable total Image size of 39MiB.

❤️

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

[1] https://www.youtube.com/watch?v=rne8pOxGuwM#t=27

-- 
Kees Cook