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Date: Mon, 27 Apr 2015 21:43:39 +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 27, 2015, at 21:39 , Greg KH <greg@...ah.com> wrote:
>
> 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@...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.
>
> Ick, no, never attach attributes to a kobject that you don't create
> yourself, otherwise it's a race that you lost.
>
Err, it is created by me. It’s a kset by the following call:
> ov_kset = kset_create_and_add("overlays", NULL, &of_kset->kobj);
>
What is the problem with this?
>>>> +
>>>> 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?
>
No, there’s no way to load this as a module, it’s in /sys/firmware/devicetree
And it’s created by the call to kset_create_and_add()
> thanks,
>
> greg k-h
Regards
— Pantelis
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