linux-kernel - Re: [Fastboot] [PATCH] x86_64: Reload CS when startup

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Message-ID: <m1lkphqfz0.fsf@ebiederm.dsl.xmission.com>
Date:	Tue, 22 Aug 2006 03:25:55 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Magnus Damm" <magnus.damm@...il.com>
Cc:	"Andi Kleen" <ak@...e.de>, "Magnus Damm" <magnus@...inux.co.jp>,
	fastboot@...ts.osdl.org, linux-kernel@...r.kernel.org
Subject: Re: [Fastboot] [PATCH] x86_64: Reload CS when startup_64 is used.

"Magnus Damm" <magnus.damm@...il.com> writes:

> Hi Eric,
>
> On 8/22/06, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>
>> In long mode the %cs is largely a relic.  However there are a few cases
>> like lret where it matters that we have a valid value.  Without this
>> patch it is possible to enter the kernel in startup_64 without setting
>> %cs to a valid value.  With this patch we don't care what %cs value
>> we enter the kernel with, so long as the cs shadow register indicates
>> it is a privileged code segment.
>>
>> Thanks to Magnus Damm for finding this problem and posting the
>> first workable patch.  I have moved the jump to set %cs down a
>> few instructions so we don't need to take an extra jump.  Which
>> keeps the code simpler.
>>
>> Signed-of-by: Eric W. Biederman <ebiederm@...ssion.com>
>
> While at it, could you please fix up the purgatory code in kexec-tools
> to include this fix so we can boot older versions of the kernel too?

I guess I'm not opposed to a patch but I have some serious reservations
about a solution that limits my address to 32bits in 64bit mode, and
appears to break the gdt used for entering the 32bit kernel.

I'm up way to late tonight.  I just wanted to resolve the situation
when it was clear what was going on.

Eric
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