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

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Date:   Thu, 17 Nov 2022 18:50:55 +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 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:
> > The code currently uses C90 standard extension based zero length arrays.
> > The zero length array member also happens to be the only member of the
> > structs. Such zero length array declarations are deprecated and the
> > new C99 standard extension of flexible array declarations are to be
> > used instead.
> >
> > The DECLARE_FLEX_ARRAY() helper allows for a flexible array member as
> > the only member in a structure. Refer to these links [1], [2] for
> > details.
> >
> > [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> > [2] https://lkml.kernel.org/r/YxKY6O2hmdwNh8r8@work
> >
> > Issue identified using Coccinelle.
> >
> > Signed-off-by: Deepak R Varma <drv@...lo.com>
> > ---
> >
> > Notes:
> >    1. Proposed change is compile tested only.
> >    2. Solution feedback from gustavoars@...nel.org
> >
> >
> >  drivers/staging/wlan-ng/hfa384x.h | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/staging/wlan-ng/hfa384x.h b/drivers/staging/wlan-ng/hfa384x.h
> > index 0611e37df6ac..3a1edcb43e07 100644
> > --- a/drivers/staging/wlan-ng/hfa384x.h
> > +++ b/drivers/staging/wlan-ng/hfa384x.h
> > @@ -960,15 +960,15 @@ struct hfa384x_pdr_nicid {
> >  } __packed;
> >
> >  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.

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.

If there is a way to confirm that the structures are indeed not needed, I can
revise the patch and send the cleanup accordingly. Please suggest.

Thank you,
./drv


>
> thanks,
>
> greg k-h