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

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Date:	Mon, 27 Apr 2015 21:13:56 +0300
From:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>
To:	Greg KH <greg@...ah.com>
Cc:	Rob Herring <robherring2@...il.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matt Porter <mporter@...sulko.com>,
	Koen Kooi <koen@...inion.thruhere.net>,
	Guenter Roeck <linux@...ck-us.net>, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v3 2/4] of: overlay: global sysfs enable attribute

Hi Greg,

> On Apr 24, 2015, at 23:29 , Greg KH <greg@...ah.com> wrote:
> 
> On Fri, Apr 24, 2015 at 12:45:42PM +0300, Pantelis Antoniou wrote:
>> 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@...sulko.com>
>> ---
>> 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.

>> +
>> 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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