linux-kernel - Re: [PATCH v7u1 22/31] x86, boot: add fields to support load bzImage and ramdisk above 4G

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Date:	Mon, 14 Jan 2013 12:14:18 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	Borislav Petkov <bp@...en8.de>, Yinghai Lu <yinghai@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kiszka <jan.kiszka@....de>,
	Jason Wessel <jason.wessel@...driver.com>,
	linux-kernel@...r.kernel.org, Rob Landley <rob@...dley.net>,
	Matt Fleming <matt.fleming@...el.com>,
	Gokul Caushik <caushik1@...il.com>,
	Josh Triplett <josh@...htriplett.org>,
	Joe Millenbach <jmillenbach@...il.com>
Subject: Re: [PATCH v7u1 22/31] x86, boot: add fields to support load bzImage
 and ramdisk above 4G

On Mon, Jan 14, 2013 at 12:05 PM, Borislav Petkov <bp@...en8.de> wrote:
> On Mon, Jan 14, 2013 at 10:57:08AM -0800, Yinghai Lu wrote:
>> please check if following is enough?
>>
>> +       /*
>> +        * kernel have sentinel to set as 0xff in setup link scripts,
>> +        * so if bootloader just copy whole page from kernel image to
>> +        * get setup_header instead of clearing boot_param buffer and
>> +        * copying setup_header only, will leave sentinel as 0xff.
>> +        * With that, we can tell some fields in boot_param have
>> +        * invalid values, and we need to zero them in kernel.
>> +        */
>> +       __u8  sentinel;                                 /* 0x1ef */
>
> "The sentinel variable is set by the linker script to 0xff. If a
> bootloader doesn't know about this variable and just copies the
> setup_header portion and doesn't clear the boot_params buffer as it is
> supposed to, it will leave the sentinel to its initial value of 0xff.

no, no, no.

bootloader does not need to know sentinel, and they only need to do:
    clearing boot_param buffer and copying setup_header only

even new bootloader is not supposed to know sentinel ...


>
> This tells the kernel that some fields in boot_params have invalid
> values and we have to zero them out in the kernel."
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