linux-kernel - Re: [RESEND] drm/msm/dpu: Delete a variable initialisation before a null pointer check in two functions

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Message-ID: <ce9599fb-a798-4f22-b51a-3341e690f8bc@stanley.mountain>
Date: Mon, 3 Mar 2025 11:24:26 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Markus Elfring <Markus.Elfring@....de>
Cc: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
	freedreno@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
	linux-arm-msm@...r.kernel.org, kernel-janitors@...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: [RESEND] drm/msm/dpu: Delete a variable initialisation before a
 null pointer check in two functions

On Mon, Mar 03, 2025 at 09:15:14AM +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.
> Will any software development concerns evolve further also according to
> undesirable null pointer dereferences?
> https://wiki.sei.cmu.edu/confluence/display/c/EXP34-C.+Do+not+dereference+null+pointers
> 

It's not a NULL pointer dereference.  It's just pointer math.  It was
a common way to implement offsetof() before we had a builtin for that.

samples/bpf/test_lru_dist.c
# define offsetof(TYPE, MEMBER) ((size_t)&((TYPE *)0)->MEMBER)

regards,
dan carpenter