linux-kernel - Re: [PATCH 2/2] kexec: Provide arch_kexec_protect(unprotect)

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Date:	Mon, 28 Dec 2015 20:20:31 +0800
From:	Minfei Huang <mhuang@...hat.com>
To:	Xunlei Pang <xlpang@...hat.com>
Cc:	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Eric Biederman <ebiederm@...ssion.com>,
	akpm@...ux-foundation.org, Dave Young <dyoung@...hat.com>,
	vgoyal@...hat.com
Subject: Re: [PATCH 2/2] kexec: Provide
 arch_kexec_protect(unprotect)_crashkres()

On 12/28/15 at 08:14pm, Minfei Huang wrote:
> On 12/28/15 at 02:32pm, Xunlei Pang wrote:
> > On 12/24/2015 at 02:44 PM, Xunlei Pang wrote:
> > >>>>> +static void kexec_mark_crashkres(bool protect)
> > >>>>> +{
> > >>>>> +	unsigned long control;
> > >>>>> +
> > >>>>> +	kexec_mark_range(crashk_low_res.start, crashk_low_res.end, protect);
> > >>>>> +
> > >>>>> +	/* Don't touch the control code page used in crash_kexec().*/
> > >>>>> +	control = PFN_PHYS(page_to_pfn(kexec_crash_image->control_code_page));
> > >>>>> +	/* Control code page is located in the 2nd page. */
> > >>>>> +	control = control + PAGE_SIZE;
> > >> Though it works because the control code is less than 1 page, but use the macro
> > >> of KEXEC_CONTROL_PAGE_SIZE looks better..
> > 
> > The 1st page is pagetable, control code page locates at the 2nd page.
> > The following kexec_mark_range() wants to mark ro from crashk_res.start
> > to the 1st page(included), so here we must use PAGE_SIZE.
> > 
> > >>
> > >>>>> +	kexec_mark_range(crashk_res.start, control - 1, protect);
> > >>>>> +	kexec_mark_range(control + PAGE_SIZE, crashk_res.end, protect);
> > >>>> X86 kexec will copy the page while kexecing, could you check if we can move
> > >>>> that copying to earliyer while kexec loading, maybe machine_kexec_prepare so
> > >>>> that we can make a arch-independent implementation.
> > >>> For some arch, may use huge tlb directly to do the kernel mapping,
> > >>> in such cases, we can't implement this function. So I think it should
> > >>> be arch-dependent.
> > >> Ok, that's fine.
> > > At least moving the x86 control-copying code into arch-related
> > > machine_kexec_prepare() should work, and this can omit the
> > > special treatment of the control code page.
> > 
> > The "relocate_kernel" routine in "relocate_kernel_64.S" will use it as
> > a temp storage "for jumping back"(as its comment), so we can't mark
> > it readonly.
> 
> kexec will copy the relocate_kernel binary to control_code_page in
> function machine_kexec. This is a major reason to set the region
> control_code_page to control_code_page + PAGE_SIZE with mode read/write.

Fix it.
The region is from control_code_page + PAGE_SIZE to
control_code_page + PAGE_SIZE + PAGE_SIZE.

Thanks
Minfei
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