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Date: Fri, 23 Feb 2024 15:52:00 -0800
From: Kees Cook <keescook@chromium.org>
To: Jann Horn <jannh@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,
	"H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] x86/boot: avoid recompiling kaslr.c for incremental
 rebuilds
Message-ID: <202402231550.80B97C8E@keescook>
References: <20240220192144.2050167-1-jannh@google.com>
 <20240220192144.2050167-4-jannh@google.com>
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In-Reply-To: <20240220192144.2050167-4-jannh@google.com>

On Tue, Feb 20, 2024 at 08:21:44PM +0100, Jann Horn wrote:
> Currently, every kernel rebuild needs to compile kaslr.c again because
> UTS_VERSION changes on every rebuild.
> Move the build string into a separate object file to speed things up.
> 
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
>  arch/x86/boot/compressed/dynamic_vars.c |  8 ++++++++
>  arch/x86/boot/compressed/dynamic_vars.h |  3 +++
>  arch/x86/boot/compressed/kaslr.c        | 10 ++--------
>  3 files changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/dynamic_vars.c b/arch/x86/boot/compressed/dynamic_vars.c
> index cda64ff4b6da..15a57fbb05e3 100644
> --- a/arch/x86/boot/compressed/dynamic_vars.c
> +++ b/arch/x86/boot/compressed/dynamic_vars.c
> @@ -1,9 +1,17 @@
>  // SPDX-License-Identifier: GPL-2.0
>  #include <linux/const.h>
>  #include "dynamic_vars.h"
> +#include <generated/compile.h>
> +#include <generated/utsrelease.h>
> +#include <generated/utsversion.h>
>  #include "../voffset.h"
>  
>  const unsigned long vo__text = VO__text;
>  const unsigned long vo___bss_start = VO___bss_start;
>  const unsigned long vo__end = VO__end;
>  const unsigned long kernel_total_size = VO__end - VO__text;
> +
> +/* Simplified build-specific string for starting entropy. */
> +const char build_str[] = UTS_RELEASE " (" LINUX_COMPILE_BY "@"
> +		LINUX_COMPILE_HOST ") (" LINUX_COMPILER ") " UTS_VERSION;
> +unsigned long build_str_len = sizeof(build_str)-1;

This can be const too, yes? (Also, you didn't want to include the
trailing NUL in the xor?

Otherwise, yeah, I like this whole series.

-- 
Kees Cook