linux-kernel - Re: [RFC] Clarification for “undefined behaviour”?

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Message-ID: <20250305141732.26b91742@pumpkin>
Date: Wed, 5 Mar 2025 14:17:32 +0000
From: David Laight <david.laight.linux@...il.com>
To: Dan Carpenter <dan.carpenter@...aro.org>
Cc: Markus Elfring <Markus.Elfring@....de>, Dmitry Baryshkov
 <dmitry.baryshkov@...aro.org>, kernel-janitors@...r.kernel.org,
 freedreno@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
 linux-arm-msm@...r.kernel.org, Abhinav Kumar <quic_abhinavk@...cinc.com>,
 Archit Taneja <architt@...eaurora.org>, Daniel Vetter <daniel@...ll.ch>,
 David Airlie <airlied@...il.com>, Jeykumar Sankaran
 <jsanka@...eaurora.org>, Jordan Crouse <jordan@...micpenguin.net>, Marijn
 Suijten <marijn.suijten@...ainline.org>, Rob Clark <robdclark@...il.com>,
 Sean Paul <sean@...rly.run>, Simona Vetter <simona@...ll.ch>, Vinod Koul
 <vkoul@...nel.org>, cocci@...ia.fr, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Clarification for “undefined behaviour”?

On Wed, 5 Mar 2025 11:51:59 +0300
Dan Carpenter <dan.carpenter@...aro.org> wrote:

> On Wed, Mar 05, 2025 at 09:40:43AM +0100, Markus Elfring wrote:
> > >>> The address of a data structure member was determined before
> > >>> a corresponding null pointer check in the implementation of
> > >>> the functions “dpu_hw_pp_enable_te” and “dpu_hw_pp_get_vsync_info”.
> > >>>
> > >>> Thus avoid the risk for undefined behaviour by removing extra
> > >>> initialisations for the variable “c” (also because it was already
> > >>> reassigned with the same value behind this pointer check).  
> > > There is no undefined behavior here.  
> > 
> > Is there a need to improve the wording precision?
> > 
> > There are words which denote a special meaning according to aspects of
> > the programming language “C”.
> > https://en.cppreference.com/w/c/language/behavior
> > 
> > Dereferences of null pointers are treated in special ways.  
> 
> This not a dereference.  It's just pointer math.

And the 'fun' starts because NULL isn't required to use the all-zero
bit pattern.
Regardless of the bit-pattern, things like (void *)(1 - 1) are valid
NULL pointers.

Of course, while C allows this, I doubt NULL has ever been other than 0.
(It was 0 on a system I used many years ago where the O/S invalid pointer
was ~0.)

I know Clang has started warning about arithmetic on NULL.
I wonder when it is going to start warning about memset(p, 0, sz)
for anything that contains a pointer - equally invalid.

	David