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Date: Fri, 7 Apr 2017 15:39:56 +0800
From: Dave Young <dyoung@redhat.com>
To: David Howells <dhowells@redhat.com>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        gnomes@lxorguk.ukuu.org.uk, linux-efi@vger.kernel.org,
        Josh Boyer <jwboyer@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
        linux-security-module@vger.kernel.org, keyrings@vger.kernel.org,
        matthew.garrett@nebula.com
Subject: Re: [PATCH 17/24] acpi: Ignore acpi_rsdp kernel param when the
 kernel has been locked down
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On 04/07/17 at 08:05am, David Howells wrote:
> Dave Young <dyoung@redhat.com> wrote:
> 
> > > > This option allows userspace to pass the RSDP address to the kernel, which
> > > > makes it possible for a user to circumvent any restrictions imposed on
> > > > loading modules.  Ignore the option when the kernel is locked down.
> > > 
> > > I'm not really sure here.
> > > 
> > > What exactly is the mechanism?
> > 
> > Actually this acpi_rsdp param is created for EFI kexec reboot in old
> > days when we had not supported persistent efi vm space across kexec
> > reboot. At that time kexec reboot runs as noefi mode, it can not find
> > the acpi root table thus kernel will hang early.
> > 
> > Now kexec can support EFI boot so this param is not necessary for most
> > user unless they still use efi=old_map.
> 
> Is this patch now unnecessary?

I think it is still necessary because the acpi_rsdp kernel param is still
a valid paramater and one can still pass a pointer to be recognized as acpi
root pointer.

Maybe "imposed on loading modules" is not clear which can be dropped.

Thanks
Dave