linux-kernel - Re: [PATCH] staging: wlan-ng: Replace zero-length arrays with DECLARE_FLEX

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Date:   Mon, 28 Nov 2022 13:15:43 +0530
From:   Deepak R Varma <drv@...lo.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org,
        gustavoars@...nel.org
Subject: Re: [PATCH] staging: wlan-ng: Replace zero-length arrays with
 DECLARE_FLEX_ARRAY() helper

On Sat, Nov 19, 2022 at 08:08:15PM +0530, Deepak R Varma wrote:
> On Thu, Nov 17, 2022 at 07:03:21PM +0100, Greg Kroah-Hartman wrote:
> > On Thu, Nov 17, 2022 at 06:50:55PM +0530, Deepak R Varma wrote:
> > > On Thu, Nov 17, 2022 at 01:54:49PM +0100, Greg Kroah-Hartman wrote:
> > > > On Thu, Nov 17, 2022 at 03:48:45PM +0530, Deepak R Varma wrote:
> > > > >
> > > > >  struct hfa384x_pdr_refdac_measurements {
> > > > > -	u16 value[0];
> > > > > +	DECLARE_FLEX_ARRAY(u16, value);
> > > > >  } __packed;
> > > >
> > > > Why?  This structure is never used anywhere, right?  So why is this
> > > > needed to be changed and not just removed entirely?  Same for the other
> > > > structures in this patch.
> > >
> > > Hello Greg,
> > > I am unable to confirm that these structures are truly not needed in the absence
> > > if a real device based testing. I could only validate that using the compile
> > > build and driver loading.
> >
> > Think this through, if no one is actually using this structure, and it
> > is of 0 size, then do you think it is being used?
>
> Hello Greg,
> I did not find any memory allocation for these zero length array structures.
> Also, the union or its enclosing structure do not appear to access the members.
> Hence I am leaning towards concluding that these zero length array structures do
> not appear to be necessary.
>
> There are a few other structures that are part of the same union, however, they
> too do not appear to be used for accessing the memory assigned to the union [or
> its enclosing structure]. I think most of the members of these unions can be
> replaced by one max size structure of this union [e.g. struct
> hfa384x_pdr_mkk_measurements].
>
> Could you please comment if I am reading the code right?
>
> For your quick reference, the zero length structure declaration are online 963
> whereas the union is on line number 1080 of the file drivers/staging/wlan-ng/hfa384x.h

Hello Greg,
can you please suggest how should I approach this clean-up/correction?

Thank you,
./drv

>
>
> Thank you,
> ./drv
>
>
> >
> > > This change that I am proposing in the interim would enable the compiler to
> > > protect the structure from addition of a new member below the zero length array.
> >
> > Why would you want to add a new member below this?  That's not what is
> > happening here at all.
>
> I came across this one old commit where such an accident happened. This is from
> a recent LWN article:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e48f129c2f20
>
> I understand the C99 now protects from such an attempt at the compile time
> itself.
>
> Thank you,
> ./drv
>
>
> >
> > Please think this through a bit more.
> >
> > good luck!
> >
> > greg k-h
> >