linux-kernel - Re: [RFC][PATCH] PM / Hibernate / x86: Change RESTORE_MAGIC on x86

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Message-ID: <4CA27A01.1080606@zytor.com>
Date:	Tue, 28 Sep 2010 16:28:01 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
CC:	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Nigel Cunningham <nigel@...onice.net>
Subject: Re: [RFC][PATCH] PM / Hibernate / x86: Change RESTORE_MAGIC on x86_64

On 09/28/2010 04:12 PM, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rjw@...k.pl>
> 
> On x86_64 the configuration and version of the kernel that
> hibernates and creates a system image may be different from the
> configuration and version of the boot kernel that loads the image.
> So long as both these kernels are built with the same value of
> RESTORE_MAGIC, the image created by one of them should be
> successfully loaded and restored by the other one.
> 
> It wasn't necessary to modify RESTORE_MAGIC in the past, but now
> that we are adding compression to the in-kernel hibernate code,
> change the value of RESTORE_MAGIC so that earlier kernels don't
> try to load compressed images they can't handle.
> 
> Signed-off-by: Rafael J. Wysocki <rjw@...k.pl>
> ---
>  arch/x86/power/hibernate_64.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: linux-2.6/arch/x86/power/hibernate_64.c
> ===================================================================
> --- linux-2.6.orig/arch/x86/power/hibernate_64.c
> +++ linux-2.6/arch/x86/power/hibernate_64.c
> @@ -137,7 +137,7 @@ struct restore_data_record {
>  	unsigned long magic;
>  };
>  
> -#define RESTORE_MAGIC	0x0123456789ABCDEFUL
> +#define RESTORE_MAGIC	0x0123456789ABCDF0UL
>  

Two issues with this:

a) shouldn't we only set this when the image is actually compressed?

b) using systematic magics like this is pessimal in terms of collision
avoidance.  It's much better to use a true random number.

Hence, I propose:

: anacreon 95 ; ranpwd -xc 16
0x4ddedd3236f1e6e1

	-hpa
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