reboot: allow to specify reboot mode via sysfs

The kernel cmdline reboot= option offers some sort of control on how the
reboot is issued.

We don't always know in advance what type of reboot to perform.

Sometimes a warm reboot is preferred to persist certain memory regions
across the reboot.  Others a cold one is needed to apply a future system
update that makes a memory memory model change, like changing the base
page size or resizing a persistent memory region.

Or simply we want to enable reboot_force because we noticed that
something bad happened.

Add handles in sysfs to allow setting these reboot options, so they can
be changed when the system is booted, other than at boot time.

The handlers are under <sysfs>/kernel/reboot, can be read to get the
current configuration and written to alter it.

	# cd /sys/kernel/reboot/

	# grep . *
	cpu:0
	force:0
	mode:cold
	type:acpi

	# echo 2 >cpu
	# echo yes >force
	# echo soft >mode
	# echo bios >type

	# grep . *
	cpu:2
	force:1
	mode:soft
	type:bios

Before setting anything, check for CAP_SYS_BOOT capability, so it's
possible to allow an unpriviledged process to change these settings simply
by relaxing the handles permissions, without opening them to the world.

[natechancellor@gmail.com: fix variable assignments in type_store]
  Link: https://lkml.kernel.org/r/20201112035023.974748-1-natechancellor@gmail.com
  Link: https://github.com/ClangBuiltLinux/linux/issues/1197

Link: https://lkml.kernel.org/r/20201110202746.9690-1-mcroce@linux.microsoft.com
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Tyler Hicks <tyhicks@linux.microsoft.com>
Cc: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 32 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-kernel-reboot b/Documentation/ABI/testing/sysfs-kernel-reboot
new file mode 100644
index 000000000000..837330fb2511
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-kernel-reboot
@@ -0,0 +1,32 @@
+What:		/sys/kernel/reboot
+Date:		November 2020
+KernelVersion:	5.11
+Contact:	Matteo Croce <mcroce@microsoft.com>
+Description:	Interface to set the kernel reboot behavior, similarly to
+		what can be done via the reboot= cmdline option.
+		(see Documentation/admin-guide/kernel-parameters.txt)
+
+What:		/sys/kernel/reboot/mode
+Date:		November 2020
+KernelVersion:	5.11
+Contact:	Matteo Croce <mcroce@microsoft.com>
+Description:	Reboot mode. Valid values are: cold warm hard soft gpio
+
+What:		/sys/kernel/reboot/type
+Date:		November 2020
+KernelVersion:	5.11
+Contact:	Matteo Croce <mcroce@microsoft.com>
+Description:	Reboot type. Valid values are: bios acpi kbd triple efi pci
+
+What:		/sys/kernel/reboot/cpu
+Date:		November 2020
+KernelVersion:	5.11
+Contact:	Matteo Croce <mcroce@microsoft.com>
+Description:	CPU number to use to reboot.
+
+What:		/sys/kernel/reboot/force
+Date:		November 2020
+KernelVersion:	5.11
+Contact:	Matteo Croce <mcroce@microsoft.com>
+Description:	Don't wait for any other CPUs on reboot and
+		avoid anything that could hang.