[PATCH] acpi: button: Provide option for power button to directly signal init

[Josh Triplett <josh@joshtriplett.org>]

Virtual machines and containers often use an ACPI power button event to
tell the machine to shut down gracefully.

Provide an optional, extremely lightweight way to handle this event by
signaling init directly, rather than running a separate daemon (such as
acpid or systemd-logind) that adds to startup time and VM image
complexity.

By default, the power button will continue to notify userspace through
the input layer. With the button.power_signal parameter set, the
power button will instead send the configured signal to init. (For
instance, sending SIGINT will make the power button simulate
ctrl-alt-del.)

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
---
 Documentation/admin-guide/kernel-parameters.txt |  6 ++++++
 drivers/acpi/button.c                           | 11 +++++++++++
 2 files changed, 17 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index ade4e6ec23e0..bbb598e148f4 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -450,6 +450,12 @@
 			firmware feature for flushing multiple hpte entries
 			at a time.
 
+	button.power_signal=
+			[ACPI] When the power button is pressed, send this
+			signal number to the init process. If set to 0
+			(default), do not send a signal.
+			Format: integer
+
 	c101=		[NET] Moxa C101 synchronous serial card
 
 	cachesize=	[BUGS=X86-32] Override level 2 CPU cache size detection.
diff --git a/drivers/acpi/button.c b/drivers/acpi/button.c
index b758b45737f5..923259f132d6 100644
--- a/drivers/acpi/button.c
+++ b/drivers/acpi/button.c
@@ -14,6 +14,7 @@
 #include <linux/init.h>
 #include <linux/types.h>
 #include <linux/proc_fs.h>
+#include <linux/sched/signal.h>
 #include <linux/seq_file.h>
 #include <linux/input.h>
 #include <linux/slab.h>
@@ -167,6 +168,10 @@ static unsigned long lid_report_interval __read_mostly = 500;
 module_param(lid_report_interval, ulong, 0644);
 MODULE_PARM_DESC(lid_report_interval, "Interval (ms) between lid key events");
 
+static int power_signal __read_mostly = 0;
+module_param(power_signal, int, 0644);
+MODULE_PARM_DESC(power_signal, "Power button sends this signal to init");
+
 /* --------------------------------------------------------------------------
                               FS Interface (/proc)
    -------------------------------------------------------------------------- */
@@ -426,6 +431,12 @@ static void acpi_button_notify(struct acpi_device *device, u32 event)
 			if (button->suspended)
 				break;
 
+			if (power_signal
+			    && button->type == ACPI_BUTTON_TYPE_POWER) {
+				kill_cad_pid(power_signal, 1);
+				break;
+			}
+
 			keycode = test_bit(KEY_SLEEP, input->keybit) ?
 						KEY_SLEEP : KEY_POWER;
 			input_report_key(input, keycode, 1);
-- 
2.25.0.rc2