Re: [PATCH v2 1/3] firmware: Add boot information to sysfs

Machines often have firmware that perform some action before Linux is
loaded. It's useful to know how this firmware is configured, so create a
sysfs directory and some properties that a system can choose to expose
to describe how the system was started.

Currently the intended use describes five files, relating to hardware
root of trust configuration.

These properties are populated by platform code at startup. Using fixed
values is suitable as the state that the system booted in will not
change after firmware has handed over.

Signed-off-by: Joel Stanley <joel@jms.id.au>
---
v2:
 - Rewrite so properties are present in common code and are exposed based
   on the is_visible callback.
 - Use sysfs_emit
---
 .../ABI/testing/sysfs-firmware-bootinfo       | 43 +++++++++
 drivers/base/firmware.c                       | 90 +++++++++++++++++++
 include/linux/firmware_bootinfo.h             | 22 +++++
 3 files changed, 155 insertions(+)
 create mode 100644 Documentation/ABI/testing/sysfs-firmware-bootinfo
 create mode 100644 include/linux/firmware_bootinfo.h

diff --git a/Documentation/ABI/testing/sysfs-firmware-bootinfo b/Documentation/ABI/testing/sysfs-firmware-bootinfo
new file mode 100644
index 000000000000..cd6c42316345
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-firmware-bootinfo

@@ -0,0 +1,43 @@
+What:		/sys/firmware/bootinfo/*
+Date:		Jan 2022
+Description:
+		A system can expose information about how it was started in
+		this directory.
+
+		This information is agnostic as to the firmware implementation.
+
+		A system may expose a subset of these properties as applicable.
+
+
+What:		/sys/firmware/bootinfo/secure_boot
+Date:		Jan 2022
+Description:
+		Indicates the system was started with secure boot enabled in
+		the firmware.
+
+
+What:		/sys/firmware/bootinfo/abr_image
+Date:		Jan 2022
+Description:
+		Indicates the system was started from the alternate image
+		loaded from an Alternate Boot Region. Often this is a result of
+		the primary firmware image failing to start the system.
+
+
+What:		/sys/firmware/bootinfo/low_security_key
+Date:		Jan 2022
+Description:
+		Indicates the system's secure boot was verified with a low
+		security or development key.
+
+What:		/sys/firmware/bootinfo/otp_protected
+Date:		Jan 2022
+Description:
+		Indicates the system's boot configuration region is write
+		protected and cannot be modified.
+
+What:		/sys/firmware/bootinfo/uart_boot
+Date:		Jan 2022
+Description:
+		Indicates the system firmware was loaded from a UART instead of
+		an internal boot device.

diff --git a/drivers/base/firmware.c b/drivers/base/firmware.c
index 8dff940e0db9..24b931232eb2 100644
--- a/drivers/base/firmware.c
+++ b/drivers/base/firmware.c

@@ -11,6 +11,7 @@
 #include <linux/module.h>
 #include <linux/init.h>
 #include <linux/device.h>
+#include <linux/firmware_bootinfo.h>
 
 #include "base.h"
 

@@ -24,3 +25,92 @@ int __init firmware_init(void)
 		return -ENOMEM;
 	return 0;
 }
+
+/*
+ * Exposes attributes documented in Documentation/ABI/testing/sysfs-firmware-bootinfo
+ */
+static struct bootinfo bootinfo;
+
+static ssize_t abr_image_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%d\n", bootinfo.abr_image.val);
+}
+static DEVICE_ATTR_RO(abr_image);
+
+static ssize_t low_security_key_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%d\n", bootinfo.low_security_key.val);
+}
+static DEVICE_ATTR_RO(low_security_key);
+
+static ssize_t otp_protected_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%d\n", bootinfo.otp_protected.val);
+}
+static DEVICE_ATTR_RO(otp_protected);
+
+static ssize_t secure_boot_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%d\n", bootinfo.secure_boot.val);
+}
+static DEVICE_ATTR_RO(secure_boot);
+
+static ssize_t uart_boot_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%d\n", bootinfo.uart_boot.val);
+}
+static DEVICE_ATTR_RO(uart_boot);
+
+#define ATTR_ENABLED(a) ((attr == &dev_attr_##a.attr) && bootinfo.a.en)
+
+static umode_t bootinfo_attr_mode(struct kobject *kobj, struct attribute *attr, int index)
+{
+	if (ATTR_ENABLED(abr_image))
+		return 0444;
+
+	if (ATTR_ENABLED(otp_protected))
+		return 0444;
+
+	if (ATTR_ENABLED(low_security_key))
+		return 0444;
+
+	if (ATTR_ENABLED(otp_protected))
+		return 0444;
+
+	if (ATTR_ENABLED(low_security_key))
+		return 0444;
+
+	if (ATTR_ENABLED(secure_boot))
+		return 0444;
+
+	if (ATTR_ENABLED(uart_boot))
+		return 0444;
+
+	return 0;
+}
+
+static struct attribute *bootinfo_attrs[] = {
+	&dev_attr_abr_image.attr,
+	&dev_attr_low_security_key.attr,
+	&dev_attr_otp_protected.attr,
+	&dev_attr_secure_boot.attr,
+	&dev_attr_uart_boot.attr,
+	NULL,
+};
+
+static const struct attribute_group bootinfo_attr_group = {
+	.attrs = bootinfo_attrs,
+	.is_visible = bootinfo_attr_mode,
+};
+
+int __init firmware_bootinfo_init(struct bootinfo *bootinfo_init)
+{
+	struct kobject *kobj = kobject_create_and_add("bootinfo", firmware_kobj);
+	if (!kobj)
+		return -ENOMEM;
+
+	memcpy(&bootinfo, bootinfo_init, sizeof(bootinfo));
+
+	return sysfs_create_group(kobj, &bootinfo_attr_group);
+}
+EXPORT_SYMBOL_GPL(firmware_bootinfo_init);

diff --git a/include/linux/firmware_bootinfo.h b/include/linux/firmware_bootinfo.h
new file mode 100644
index 000000000000..3fe630b061b9
--- /dev/null
+++ b/include/linux/firmware_bootinfo.h

@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */