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

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Message-ID: <ed9e65a7-6cdf-4d93-83a8-464b47a4bc1d@stanley.mountain>
Date: Wed, 5 Mar 2025 17:30:28 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: David Laight <david.laight.linux@...il.com>
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, Mar 05, 2025 at 02:17:32PM +0000, David Laight wrote:
> 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.)

Kernel style guidelines don't even allow if (p == NULL) so we would be
screwed.  :P

> 
> I know Clang has started warning about arithmetic on NULL.

Huh.  You're right.

$ clang -Weverything test.c
test.c:13:22: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
   13 |         printf("%p\n", NULL + 1);
      |                        ~~~~ ^
test.c:13:22: warning: arithmetic on a pointer to void is a GNU extension [-Wgnu-pointer-arith]
   13 |         printf("%p\n", NULL + 1);
      |                        ~~~~ ^
test.c:11:14: warning: unused parameter 'argc' [-Wunused-parameter]
   11 | int main(int argc, char *argv[])
      |              ^
test.c:11:26: warning: unused parameter 'argv' [-Wunused-parameter]
   11 | int main(int argc, char *argv[])
      |                          ^
test.c:13:17: warning: unsafe pointer arithmetic [-Wunsafe-buffer-usage]
   13 |         printf("%p\n", NULL + 1);
      |                        ^~~~
/usr/lib/llvm-19/lib/clang/19/include/__stddef_null.h:26:14: note: expanded from macro 'NULL'
   26 | #define NULL ((void*)0)
      |              ^~~~~~~~~~
5 warnings generated.

Well, that's stupid.

regards,
dan carpenter