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

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Date: Sat, 27 Apr 2024 23:14:28 +0200
From: Lukas Wunner <lukas@...ner.de>
To: Dan Williams <dan.j.williams@...el.com>
Cc: gregkh@...uxfoundation.org,
	Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
	Marc Herbert <marc.herbert@...el.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-coco@...ts.linux.dev, alsa-devel@...a-project.org
Subject: Re: [PATCH 1/3] sysfs: Fix crash on empty group attributes array

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.

Thanks,

Lukas