Re: [PATCH 1/3] sysfs: Fix crash on empty group attributes array

[Dan Williams]

Dan Williams wrote:
> Lukas Wunner wrote:
> > On Sat, Apr 27, 2024 at 09:49:41AM -0700, Dan Williams wrote:
> > > Lukas Wunner wrote:
> > > > But I want to raise awareness that the inability to hide
> > > > empty attribute groups feels awkward.
> > > 
> > > That is fair, it was definitely some gymnastics to only change user
> > > visible behavior for new "invisible aware" attribute groups that opt-in
> > > while leaving all the legacy cases alone.
> > > 
> > > The concern is knowing when it is ok to call an is_visible() callback
> > > with a NULL @attr argument, or knowing when an empty array actually
> > > means "hide the group directory".
> > > 
> > > We could add a sentinel value to indicate "I am an empty attribute list
> > > *AND* I want my directory hidden by default". However, that's almost
> > > identical to requiring a placeholder attribute in the list just to make
> > > __first_visible() happy.
> > > 
> > > Other ideas?
> > 
> > Perhaps an optional ->is_group_visible() callback in struct attribute_group
> > which gets passed only the struct kobject pointer?
> > 
> > At least for PCI device authentication, that would be sufficient.
> > I could get from the kobject to the corresponding struct device,
> > then determine whether the device supports authentication or not.
> > 
> > Because it's a new, optional callback, there should be no compatibility
> > issues.  The SYSFS_GROUP_INVISIBLE return code from the ->is_visible()
> > call for individual attributes would not be needed then, at least in my
> > use case.
> 
> That's where I started with this, but decided it was overkill to
> increase the size of that data structure globally for a small number of
> use cases.

Perhaps could try something like this:

diff --git a/fs/sysfs/group.c b/fs/sysfs/group.c
index d22ad67a0f32..f4054cf08e58 100644
--- a/fs/sysfs/group.c
+++ b/fs/sysfs/group.c
@@ -33,11 +33,23 @@ static void remove_files(struct kernfs_node *parent,
 
 static umode_t __first_visible(const struct attribute_group *grp, struct kobject *kobj)
 {
-       if (grp->attrs && grp->attrs[0] && grp->is_visible)
-               return grp->is_visible(kobj, grp->attrs[0], 0);
+       if (grp->attrs && grp->is_visible) {
+               struct attribute *attr = grp->attrs[0];
+               struct attribute empty_attr = { 0 };
 
-       if (grp->bin_attrs && grp->bin_attrs[0] && grp->is_bin_visible)
-               return grp->is_bin_visible(kobj, grp->bin_attrs[0], 0);
+               if (!attr)
+                       attr = &empty_attr;
+               return grp->is_visible(kobj, attr, 0);
+       }
+
+       if (grp->bin_attrs && grp->is_bin_visible) {
+               struct bin_attribute *bin_attr = grp->bin_attrs[0];
+               struct bin_attribute empty_bin_attr = { 0 };
+
+               if (!bin_attr)
+                       bin_attr = &empty_bin_attr;
+               return grp->is_bin_visible(kobj, bin_attr, 0);
+       }
 
        return 0;
 }

..because it is highly likely that existing is_visible() callers will
return @attr->mode when they do not recognize the attribute. But this
could lead to some subtle bugs if something only checks the attribute
index value. For example:

lbr_is_visible(struct kobject *kobj, struct attribute *attr, int i)
{
        /* branches */
        if (i == 0)
                return x86_pmu.lbr_nr ? attr->mode : 0;

        return (x86_pmu.flags & PMU_FL_BR_CNTR) ? attr->mode : 0;
}

..but in this case we get lucky because it would return attr->mode
which is 0.