linux-kernel - Re: [PATCH 1/2] devcoredump: Remove devcoredump device if failing device is gone

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Date: Mon, 29 Jan 2024 18:48:12 +0100
From: Johannes Berg <johannes@...solutions.net>
To: Rodrigo Vivi <rodrigo.vivi@...el.com>, linux-kernel@...r.kernel.org
Cc: Jose Souza <jose.souza@...el.com>, Maarten Lankhorst
	 <maarten.lankhorst@...ux.intel.com>, Greg Kroah-Hartman
	 <gregkh@...uxfoundation.org>, "Rafael J . Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH 1/2] devcoredump: Remove devcoredump device if failing
 device is gone

On Fri, 2024-01-26 at 10:11 -0500, Rodrigo Vivi wrote:
> Make dev_coredumpm a real device managed helper, that not only
> frees the device after a scheduled delay (DEVCD_TIMEOUT), but
> also when the failing/crashed device is gone.
> 
> The module remove for the drivers using devcoredump are currently
> broken if attempted between the crash and the DEVCD_TIMEOUT, since
> the symbolic sysfs link won't be deleted.

Hmm, is it a problem to remove a whole dev when it still has some link
here? Maybe we could just make the link be managed/auto-removed?

Probably regardless of that you should change the comment in
devcd_dev_release() since it's no longer a concern?

> On top of that, for PCI devices, the unbind of the device will
> call the pci .remove void function, that cannot fail. At that
> time, our device is pretty much gone, but the read and free
> functions are alive trough the devcoredump device and they
                      ^ through, I guess

> can get some NULL dereferences or use after free.

Not sure I understand this part, how's this related to PCI's .remove?

> So, if the failing-device is gone let's also request for the
> devcoredump-device removal using the same mod_delayed_work
> as when writing anything through data. The flush cannot be
> used since it is synchronous and the devcd would be surely
> gone right before the mutex_unlock on the next line.

Can we just decouple it instead and remove the symlink? Which is kind of
what the comment in devcd_dev_release() says but at the time I wasn't
aware of all the devm mechanics etc.

I'm thinking this might be annoying in certain recovery cases, e.g.
iwlwifi uses this but may sometimes unbind/rebind itself to recover from
certain errors, and that'd make the FW dumps disappear.

johannes