On Fri, Apr 26, 2024 at 09:18:15PM +0200, Lukas Wunner wrote: > On Fri, Apr 26, 2024 at 10:59:06AM -0700, Dan Williams wrote: > > Lukas Wunner wrote: > > > > --- a/fs/sysfs/group.c > > > > +++ b/fs/sysfs/group.c > > > > @@ -33,10 +33,10 @@ 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->is_visible) > > > > + if (grp->attrs && grp->attrs[0] && grp->is_visible) > > > > return grp->is_visible(kobj, grp->attrs[0], 0); > > > > > > > > - if (grp->bin_attrs && grp->is_bin_visible) > > > > + if (grp->bin_attrs && grp->bin_attrs[0] && grp->is_bin_visible) > > > > return grp->is_bin_visible(kobj, grp->bin_attrs[0], 0); > > > > > > > > return 0; > > > > > > I'm wondering why 0 is returned by default and not SYSFS_GROUP_INVISIBLE. > > > > > > An empty attribute list (containing just the NULL sentinel) will now > > > result in the attribute group being visible as an empty directory. > > > > > > I thought the whole point was to hide such empty directories. > > > > > > Was it a conscious decision to return 0? > > > Did you expect breakage if SYSFS_GROUP_INVISIBLE is returned? > > > > Yes, the history is here: > > > > https://lore.kernel.org/all/YwZCPdPl2T+ndzjU@xxxxxxxxx/ > > > > ...where an initial attempt to hide empty group directories resulted in > > boot failures. The concern is that there might be user tooling that > > depends on that empty directory. So the SYSFS_GROUP_INVISIBLE behavior > > can only be enabled by explicit result from an is_visible() handler. > > > > That way there is no regression potential for legacy cases where the > > empty directory might matter. > > The problem is that no ->is_visible() or ->is_bin_visible() callback > is ever invoked for an empty attribute group. So there is nothing > that could return SYSFS_GROUP_INVISIBLE. > > It is thus impossible to hide them. > > Even though an attribute group may be declared empty, attributes may > dynamically be added it to it using sysfs_add_file_to_group(). > > Case in point: I'm declaring an empty attribute group named > "spdm_signatures_group" in this patch, to which attributes are > dynamically added: > > https://github.com/l1k/linux/commit/ca420b22af05 > > Because it is impossible to hide the group, every PCI device exposes > it as an empty directory in sysfs, even if it doesn't support CMA > (PCI device authentication). > > Fortunately the next patch in the series adds a single bin_attribute > "next_requester_nonce" to the attribute group. Now I can suddenly > hide the group on devices incapable of CMA, because an > ->is_bin_visible() callback is executed: > > https://github.com/l1k/linux/commit/8248bc34630e > > So in this case I'm able to dodge the bullet because the empty > signatures/ directory for CMA-incapable devices is only briefly > visible in the series. Nobody will notice unless they apply > only a subset of the series. > > But I want to raise awareness that the inability to hide > empty attribute groups feels awkward. It does, but that's because we can't break existing systems :) Documenting this to be more obvious would be great, I'll glady take changes for that as I agree, the implementation is "tricky" and took me a long time to review/understand it as well, as it is complex to deal with (and I thank Dan for getting it all working properly, I had tried and failed...) thanks, greg k-h