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

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Message-ID: <CAH5fLgg2aPtnWDqoZ8wi1OGge4PBB_Y0LSM9TSviowquEQXCVw@mail.gmail.com>
Date: Mon, 16 Jun 2025 16:02:41 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: Marc Herbert <marc.herbert@...ux.intel.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Benjamin.Cheatham@....com, Jonathan.Cameron@...wei.com, dakr@...nel.org, 
	dan.j.williams@...el.com, 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, 
	Dan Carpenter <dan.carpenter@...aro.org>, Kees Cook <kees@...nel.org>
Subject: Re: [PATCH] driver core: faux: fix Undefined Behavior in faux_device_destroy()

On Mon, Jun 16, 2025 at 5:36 AM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Sat, Jun 14, 2025 at 07:53:34AM -0700, Marc Herbert wrote:
> > > the kernel relies on this not being "optimized away" by the compiler
> > > in many places.
> >
> > I think "undefined behavior" is the more general topic, more important
> > than null pointer checks specifically?
>
> Is this really "undefined behaviour"?  There are a lot of things that
> the kernel requires for a compiler to be able to build it, and this is
> one of those things, it can't do this type of "optimization" and expect
> the output to actually work properly.

My understanding is that -fno-delete-null-pointer-checks changes the
language semantics so that nullptr deref isn't UB anymore and instead
becomes a guaranteed crash.

Alice