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Date: Fri, 4 Aug 2017 19:40:05 +0800
From: Baoquan He <bhe@redhat.com>
To: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
        x86@kernel.org, keescook@chromium.org, tglx@linutronix.de,
        hpa@zytor.com, izumi.taku@jp.fujitsu.com, fanc.fnst@cn.fujitsu.com,
        thgarnie@google.com, n-horiguchi@ah.jp.nec.com,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>, linux-efi@vger.kernel.org
Subject: Re: [PATCH v6 RESEND] x86/boot/KASLR: Restrict kernel to be
 randomized in mirror regions
Message-ID: <20170804114005.GC29975@x1>
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On 08/04/17 at 12:23pm, Matt Fleming wrote:
> On Fri, 28 Jul, at 07:26:03PM, Baoquan He wrote:
> > Hi Matt,
> > 
> > On 07/28/17 at 11:55am, Ingo Molnar wrote:
> > > 
> > > * Matt Fleming <matt@codeblueprint.co.uk> wrote:
> > > 
> > > > On Fri, 21 Jul, at 09:19:56PM, Baoquan He wrote:
> > > > >
> > > > > There are places where the efi map is getting and used like this. E.g
> > > > > in efi_high_alloc() of drivers/firmware/efi/libstub/efi-stub-helper.c.
> > > > > EFI developers worry the size of efi_memory_desc_t could not be the same
> > > > > as e->efi_memdesc_size?
> > > > > 
> > > > > Hi Matt,
> > > > > 
> > > > > Could you help have a look at this?
> > > > 
> > > > You're exactly right. The code guards against the size of the
> > > > efi_memory_desc_t struct changing. The UEFI spec says to traverse the
> > > > memory map this way.
> > > 
> > > This is not obvious and looks pretty ugly as well, and open coded in several 
> > > places.
> > > 
> > > At minimum we should have an efi_memdesc_ptr(efi, i) wrapper inline (or so) that 
> > > gives us the entry pointer, plus a comment that points out that ->memdesc_size 
> > > might not be equal to sizeof(efi_memory_memdesc_t).
> > 
> > I can make a efi_memdesc_ptr(efi, i) wrapper as Ingo suggested and use
> > it here if you agree. Seems it might be not good to add another
> > for_each_efi_memory_desc_xxxx wrapper since there are different memmap
> > data structures in x86 boot and in general efi libstub. Or any other
> > idea?
> 
> I think adding a wrapper is fine, but I'd suggest including the word
> "early" (or something similar) to explain that it should only be used
> during bootup -- we want everyone else to use the
> for_each_efi_memory_*() API.

Thanks, Matt. I can do that. Do you think below helper definition is OK
to you? If yes, I can upstate with it and post v9.

#define efi_early_memdesc_ptr(map, desc_size, n)				\
	(efi_memory_desc_t *)((void *)(map) + ((n) * (desc_size)))