linux-kernel - Re: [PATCH] driver core: faux: fix Undefined Behavior in faux_device

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Message-ID: <d478fbca-e98d-4a16-9298-01dcaa8bb008@suswa.mountain>
Date: Thu, 19 Jun 2025 07:02:25 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Marc Herbert <marc.herbert@...ux.intel.com>
Cc: Dan Williams <dan.j.williams@...el.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Miguel Ojeda <ojeda@...nel.org>, Benjamin.Cheatham@....com,
	Jonathan.Cameron@...wei.com, dakr@...nel.org,
	linux-acpi@...r.kernel.org, linux-cxl@...r.kernel.org,
	linux-kernel@...r.kernel.org, rafael.j.wysocki@...el.com,
	rafael@...nel.org, sudeep.holla@....com,
	Kees Cook <kees@...nel.org>
Subject: Re: [PATCH] driver core: faux: fix Undefined Behavior in
 faux_device_destroy()

On Wed, Jun 18, 2025 at 08:33:27PM -0700, Marc Herbert wrote:
> > But, again, this is a totally different thing from what the patch does.
> > The faux_device_destroy() code is not doing a dereference, it's doing
> > pointer math.
> 
> pointer math is what we _want_ the code to do. But if that relies on
> some undefined behavior(s) then the bets are off again. Check
> https://stackoverflow.com/questions/26906621/does-struct-name-null-b-cause-undefined-behaviour-in-c11
> where offsetof() is a suggested alternative.

The answers talk about "But the value of a null pointer constant is not
defined as 0." which is some trivia that had heard before.  Probably
I heard it in the context of someone saying that we should check
"if (p == NULL)" instead of "if (!p)"...

regards,
dan carpenter