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

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Message-ID: <CAE9FiQU4mSzpLd9yTzMjAZCprC+FHYqn3Fw3xfgQK_Lnr8RtQw@mail.gmail.com>
Date:	Mon, 14 Jan 2013 17:04:25 -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 3:11 PM, Borislav Petkov <bp@...en8.de> wrote:
>> >> +       /*
>> >> +        * 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.
>
> Ok, but this needlessly mentiones some sort of allocation technique
> which the bootloader does and which we don't care about. What we do care
> about is the sentinel variable and what it means: if the bootloader
> copies it accidentally, we use *that* as a trigger. So let's revise it:
>
> "The sentinel variable is set by the linker script to 0xff. A bootloader
> is supposed to only take setup_header and put it into a clean
> boot_params buffer. If it turns out that it is clumsy or too generous
> with the buffer, it most probably will pick up the sentinel variable
> too. The fact that this variable then is still non-zero signals to
> us that that we should zero out certain portions of boot_params (see
> sanitize_real_mode()) because we assume that they contain garbage."


ok, I changed to :

        /*
         * The sentinel is set to 0xff via the linker script (setup.ld).
         * A bootloader is supposed to only take setup_header and put
         * it into a clean boot_params buffer. If it turns out that
         * it is clumsy or too generous with the buffer, it most
         * probably will pick up the sentinel variable too. The fact
         * that this variable then is still 0xff will let kernel
         * know that some variables in boot_params are invalid and
         * kernel should zero out certain portions of boot_params
         * (see sanitize_real_mode()).
         */
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