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

    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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