linux-kernel - Re: [PATCH v2] efi: Introduce EFI bootloader control driver

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Date:	Fri, 18 Mar 2016 19:18:17 +0000
From:	"Stanacar, Stefan" <stefan.stanacar@...el.com>
To:	"Baluta, Daniel" <daniel.baluta@...el.com>,
	"matt@...eblueprint.co.uk" <matt@...eblueprint.co.uk>
CC:	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Abbas, Mohamed" <mohamed.abbas@...el.com>,
	"Gumbel, Matthew K" <matthew.k.gumbel@...el.com>,
	"Musca, Constantin" <constantin.musca@...el.com>,
	"Purdila, Octavian" <octavian.purdila@...el.com>
Subject: Re: [PATCH v2] efi: Introduce EFI bootloader control driver

On Fri, 2016-03-18 at 16:15 +0000, Matt Fleming wrote:
> On Fri, 18 Mar, at 12:11:50PM, Daniel Baluta wrote:
> > 
> > From: Matt Gumbel <matthew.k.gumbel@...el.com>
> > 
> > This driver intercepts system reboot requests and populates the
> > LoaderEntryOneShot EFI variable with the user-supplied reboot
> > argument. EFI bootloaders such as Gummiboot will consume this
> > variable and use it to control which OS is booted next.
> > 
> > We use this with Android where reboot() tells the kernel that
> > we want to boot into recovery or other non-default OS environment.
> > 
> > It is the bootloader's job to guard against this variable being
> > uninitialzed or containing invalid data, and just boot normally
> > if that is the case.
> > 
> > Signed-off-by: Matt Gumbel <matthew.k.gumbel@...el.com>
> > Signed-off-by: Mohamed Abbas <mohamed.abbas@...el.com>
> > Signed-off-by: Constantin Musca <constantin.musca@...el.com>
> > Signed-off-by: Daniel Baluta <daniel.baluta@...el.com>
> > ---
> > Changes since v1:
> > 	* updated Makefile after changing source name from efibc.c ->
> > efi-bc.c
> > 	to comply with naming rules in drivers/firmware/efi/
> > 
> >  drivers/firmware/efi/Kconfig  |  11 ++
> >  drivers/firmware/efi/Makefile |   1 +
> >  drivers/firmware/efi/efi-bc.c | 251
> > ++++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 263 insertions(+)
> >  create mode 100644 drivers/firmware/efi/efi-bc.c
> Why does this require a driver? Why is it not possible to solve this
> problem by creating the variable in userspace before invoking
> reboot(2)?


Hi Matt,

It is possible, but that means modifying those userspace apps :)
There are reboot implementations that do "reboot <reason>", such as
Android's reboot command [1] and Upstart's reboot replacement [2], which
pass the reason as an argument to the reboot syscall. 
Probably your first question will be - "Why don't you modify those
apps?" Well, I don't see platform-agnostic way how those could be
modified to pass the reason to the bootloader, regardless of platform or
bootloader.

Other non-EFI platforms use a driver as well:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/dri
vers/soc/tegra/pmc.c#n382



[1] https://android.googlesource.com/platform/system/core/+/master/libcu
tils/android_reboot.c#228

[2] https://bazaar.launchpad.net/~upstart-devel/upstart/trunk/view/head:
/util/reboot.c#L259


Cheers,
Stefan