linux-kernel - [PATCH v2] kernel/reboot: Explicitly notify if halt occurred instead of power off

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Message-Id: <20231104113320.72440-1-ldmldm05@gmail.com>
Date:   Sat,  4 Nov 2023 20:33:20 +0900
From:   Dongmin Lee <ldmldm05@...il.com>
To:     akpm@...ux-foundation.org, lkp@...el.com
Cc:     ldmldm05@...il.com, linux-kernel@...r.kernel.org,
        oe-kbuild-all@...ts.linux.dev
Subject: [PATCH v2] kernel/reboot: Explicitly notify if halt occurred instead of power off

When kernel_can_power_off() returns false, and reboot has called with
LINUX_REBOOT_CMD_POWER_OFF, kernel_halt() will be initiated instead of
actual power off function.

However, in this situation, Kernel never explicitly notifies user that
system halted instead of requested power off.

Since halt and power off perform different behavior, and user initiated
reboot call with power off command, not halt, This could be unintended
behavior to user, like this:

~ # poweroff -f
[    3.581482] reboot: System halted

Therefore, this explicitly notifies user that poweroff is not available,
and halting has been occured as an alternative behavior instead:

~ # poweroff -f
[    4.123668] reboot: Power off not available: System halted instead

Signed-off-by: Dongmin Lee <ldmldm05@...il.com>
---
v1->v2:
 - changed new variable poweroff_fallback_to_halt to static bool as per
   kernel test robot's sparse warning notification.
 - fixed minor coding style issue

 kernel/reboot.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/reboot.c b/kernel/reboot.c
index 395a0ea3c7a8..34ea519e3d94 100644
--- a/kernel/reboot.c
+++ b/kernel/reboot.c
@@ -58,6 +58,13 @@ struct sys_off_handler {
 	struct device *dev;
 };
 
+/*
+ * This variable is used to indicate if a halt initiated instead when
+ * reboot call is invoked with LINUX_REBOOT_CMD_POWER_OFF, but system
+ * cannot be powered off. This allowes kernel_halt() to notify that.
+ */
+static bool poweroff_fallback_to_halt;
+
 /*
  * Temporary stub that prevents linkage failure while we're in process
  * of removing all uses of legacy pm_power_off() around the kernel.
@@ -297,7 +304,10 @@ void kernel_halt(void)
 	kernel_shutdown_prepare(SYSTEM_HALT);
 	migrate_to_reboot_cpu();
 	syscore_shutdown();
-	pr_emerg("System halted\n");
+	if (poweroff_fallback_to_halt)
+		pr_emerg("Power off not available: System halted instead\n");
+	else
+		pr_emerg("System halted\n");
 	kmsg_dump(KMSG_DUMP_SHUTDOWN);
 	machine_halt();
 }
@@ -732,8 +742,10 @@ SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
 	/* Instead of trying to make the power_off code look like
 	 * halt when pm_power_off is not set do it the easy way.
 	 */
-	if ((cmd == LINUX_REBOOT_CMD_POWER_OFF) && !kernel_can_power_off())
+	if ((cmd == LINUX_REBOOT_CMD_POWER_OFF) && !kernel_can_power_off()) {
+		poweroff_fallback_to_halt = true;
 		cmd = LINUX_REBOOT_CMD_HALT;
+	}
 
 	mutex_lock(&system_transition_mutex);
 	switch (cmd) {

base-commit: bc3012f4e3a9765de81f454cb8f9bb16aafc6ff5
-- 
2.39.3 (Apple Git-145)