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

[Greg KH <greg@kroah.com>]

On Mon, Apr 27, 2015 at 09:13:56PM +0300, Pantelis Antoniou wrote:
> Hi Greg,
> 
> > On Apr 24, 2015, at 23:29 , Greg KH <greg@kroah.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@konsulko.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.

Ick, no, never attach attributes to a kobject that you don't create
yourself, otherwise it's a race that you lost.

> >> +
> >> 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 :)

You can't load this as a module?  Where exactly in sysfs is this
kobject, and who creates it?

thanks,

greg k-h