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.

[[email protected]: fix variable assignments in type_store]
  Link: https://lkml.kernel.org/r/[email protected]
  Link: https://github.com/ClangBuiltLinux/linux/issues/1197

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Matteo Croce <[email protected]>
Signed-off-by: Nathan Chancellor <[email protected]>
Reviewed-by: Petr Mladek <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Guenter Roeck <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Pavel Tatashin <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Tyler Hicks <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions