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From: jim.cromie@gmail.com
Date: Tue, 21 May 2024 08:41:45 -0600
Message-ID: <CAJfuBxyera+QsjgH95Qm1bAzUY0PivwvO7Pg1-oUQ5esj+dZ=A@mail.gmail.com>
Subject: Re: [PATCH v8-RESEND 04/33] dyndbg: make ddebug_class_param union
 members same size
To: =?UTF-8?Q?=C5=81ukasz_Bartosik?= <ukaszb@chromium.org>
Cc: jbaron@akamai.com, gregkh@linuxfoundation.org, 
	linux-kernel@vger.kernel.org, linux@rasmusvillemoes.dk, joe@perches.com, 
	mcgrof@kernel.org, daniel.vetter@ffwll.ch, tvrtko.ursulin@linux.intel.com, 
	jani.nikula@intel.com, ville.syrjala@linux.intel.com, seanpaul@chromium.org, 
	robdclark@gmail.com, groeck@google.com, yanivt@google.com, bleung@google.com
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On Tue, May 21, 2024 at 5:42=E2=80=AFAM =C5=81ukasz Bartosik <ukaszb@chromi=
um.org> wrote:
>
> On Thu, May 16, 2024 at 7:44=E2=80=AFPM Jim Cromie <jim.cromie@gmail.com>=
 wrote:
> >
> > struct ddebug_class_param keeps a ref to the state-storage of the
> > param; make both class-types use the same unsigned long storage type.
> > ISTM this is simpler and safer.
>
> Why is it safer for members of a union to have the same size ?
>

Its mostly extra paranoia.
but if 2 users somehow get the class-type mixed up,
at least theres no alignment issue added into it.

And a problem like this would naturally happen (murphys law)
when the PARAM_REF macro is used, which uses/shares the existing storage,
so that existing code can use that storage.
For example, DRM uses PARAM_REF so it can continue to use __drm_debug
for a few spots in code that still examine the value during normal operatio=
ns.

I'll reword the commit msg to say that more clearly (than this?)