Re: [PATCH v3 2/4] of: overlay: global sysfs enable attribute

Hi Greg,

On Apr 24, 2015, at 23:29 , Greg KH [off-list ref] wrote:

On Fri, Apr 24, 2015 at 12:45:42PM +0300, Pantelis Antoniou wrote:

quoted

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-OWPKS81ov/FWk0Htik3J/w@public.gmane.org>
---
drivers/of/overlay.c | 45 ++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 44 insertions(+), 1 deletion(-)

diff --git a/drivers/of/overlay.c b/drivers/of/overlay.c
index f17f5ef..c335809 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"

@@ -55,8 +56,12 @@ 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);
+static int overlay_removal_is_ok(struct of_overlay *ov);

static int of_overlay_apply_single_property(struct of_overlay *ov,
		struct device_node *target, struct property *prop)

@@ -339,6 +344,37 @@ void of_overlay_release(struct kobject *kobj)

	kfree(ov);
}

+static ssize_t enable_show(struct kobject *kobj,
+		struct kobj_attribute *attr, char *buf)
+{
+	return snprintf(buf, PAGE_SIZE, "%d\n", atomic_read(&ov_enable));
+}
+
+static ssize_t enable_store(struct kobject *kobj,
+		struct kobj_attribute *attr, const char *buf, size_t count)
+{
+	int ret;
+	long new_enable;
+
+	ret = kstrtol(buf, 10, &new_enable);
+	if (ret != 0)
+		return ret;
+	if ((unsigned long)new_enable > 1)
+		return -EINVAL;
+	/* if we've disabled it, no going back */
+	if (atomic_read(&ov_enable) == 0)
+		return -EPERM;
+	atomic_set(&ov_enable, (int)new_enable);
+	return count;
+}
+
+static struct kobj_attribute enable_attr = __ATTR_RW(enable);
+
+static const struct attribute *overlay_global_attrs[] = {
+	&enable_attr.attr,
+	NULL
+};

Why not make this an attribute group and then attach it to the kobj_type
to create the files in a race-free manner?

Err, these are the global attributes. They are attached to the
parent of the overlay objects which is a kset object itself.

There’s no way which I can attach the attribute to it as far
as I can tell.

quoted

+
static struct kobj_type of_overlay_ktype = {
	.release = of_overlay_release,
};

@@ -360,6 +396,10 @@ 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 +636,8 @@ int of_overlay_init(void)

	if (!ov_kset)
		return -ENOMEM;

-	return 0;
+	rc = sysfs_create_files(&ov_kset->kobj, overlay_global_attrs);
+	WARN(rc, "%s: error adding global attributes\n", __func__);

What can a user do with this warning message?  If nothing, then don't
print it out, right?

You are creating sysfs files _after_ the kobject has been announced to
userspace, causing nasty race conditions.  Please don't do that.

This is at overlay_init() time, which is way way before userspace ever
has a chance to start. If there’s a different way to attach a attribute
group to a kset object I’d like to find out how :)

thanks,

greg k-h

Regards

— Pantelis--
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