[PATCH 3/5] of: overlay: Master enable switch

[Pantelis Antoniou <pantelis.antoniou@xxxxxxxxxxxx>]

Implement a throw once master enable switch to protect against any
further overlay applications if the administrator desires so.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@xxxxxxxxxxxx>
---
 drivers/of/overlay.c | 66 +++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 65 insertions(+), 1 deletion(-)

diff --git a/drivers/of/overlay.c b/drivers/of/overlay.c
index f17f5ef..6688797 100644
--- a/drivers/of/overlay.c
+++ b/drivers/of/overlay.c
@@ -21,6 +21,7 @@
 #include <linux/err.h>
 #include <linux/idr.h>
 #include <linux/sysfs.h>
+#include <linux/atomic.h>
 
 #include "of_private.h"
 
@@ -55,6 +56,9 @@ struct of_overlay {
 	struct kobject kobj;
 };
 
+/* master enable switch; once set to 0 can't be re-enabled */
+static atomic_t ov_enable = ATOMIC_INIT(1);
+
 static int of_overlay_apply_one(struct of_overlay *ov,
 		struct device_node *target, const struct device_node *overlay);
 
@@ -345,6 +349,60 @@ static struct kobj_type of_overlay_ktype = {
 
 static struct kset *ov_kset;
 
+static ssize_t enable_read(struct file *filp, struct kobject *kobj,
+		struct bin_attribute *bin_attr, char *buf,
+		loff_t offset, size_t count)
+{
+	char tbuf[3];
+
+	if (offset < 0)
+		return -EINVAL;
+
+	if (offset >= sizeof(tbuf))
+		return 0;
+
+	if (count > sizeof(tbuf) - offset)
+		count = sizeof(tbuf) - offset;
+
+	/* fill in temp */
+	tbuf[0] = '0' + atomic_read(&ov_enable);
+	tbuf[1] = '\n';
+	tbuf[2] = '\0';
+
+	/* copy to buffer */
+	memcpy(buf, tbuf + offset, count);
+
+	return count;
+}
+
+static ssize_t enable_write(struct file *filp, struct kobject *kobj,
+		struct bin_attribute *bin_attr, char *buf,
+		loff_t off, size_t count)
+{
+	int new_enable;
+
+	if (off != 0 || (buf[0] != '0' && buf[1] != '1'))
+		return -EINVAL;
+
+	new_enable = buf[0] - '0';
+	if (new_enable != 0 && new_enable != 1)
+		return -EINVAL;
+
+	/* NOP for same value */
+	if (new_enable == atomic_read(&ov_enable))
+		return count;
+
+	/* if we've disabled it, no going back */
+	if (atomic_read(&ov_enable) == 0)
+		return -EPERM;
+
+	atomic_set(&ov_enable, new_enable);
+	return count;
+}
+
+/* just a single char + '\n' + '\0' */
+static BIN_ATTR_RW(enable, 3);
+
 /**
  * of_overlay_create() - Create and apply an overlay
  * @tree:	Device node containing all the overlays
@@ -360,6 +418,10 @@ int of_overlay_create(struct device_node *tree)
 	struct of_overlay *ov;
 	int err, id;
 
+	/* administratively disabled */
+	if (!atomic_read(&ov_enable))
+		return -EPERM;
+
 	/* allocate the overlay structure */
 	ov = kzalloc(sizeof(*ov), GFP_KERNEL);
 	if (ov == NULL)
@@ -596,5 +658,7 @@ int of_overlay_init(void)
 	if (!ov_kset)
 		return -ENOMEM;
 
-	return 0;
+	rc = sysfs_create_bin_file(&ov_kset->kobj, &bin_attr_enable);
+	WARN(rc, "%s: error adding enable attribute\n", __func__);
+	return rc;
 }
-- 
1.7.12

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