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

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Date: Sat, 27 Apr 2024 09:49:41 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: Lukas Wunner <lukas@...ner.de>, 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

Lukas Wunner wrote:
[..]
> 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.

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? I expect this issue to come up again because dynamically
populated attribute arrays of a statically defined group is a useful
mechanism. I might need this in the PCI TSM enabling... but having at
least one attribute there is likely not a problem.