linux-kernel - Re: [PATCH] staging: greybus: Replace zero-length array with flexible-array

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Date:   Wed, 13 May 2020 17:39:18 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Alex Elder <elder@...nel.org>, greybus-dev@...ts.linaro.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: greybus: Replace zero-length array with
 flexible-array

On Wed, May 13, 2020 at 05:03:43PM +0200, Johan Hovold wrote:
> On Thu, May 07, 2020 at 01:53:18PM -0500, Gustavo A. R. Silva wrote:
> > The current codebase makes use of the zero-length array language
> > extension to the C90 standard, but the preferred mechanism to declare
> > variable-length types such as these ones is a flexible array member[1][2],
> > introduced in C99:
> > 
> > struct foo {
> >         int stuff;
> >         struct boo array[];
> > };
> > 
> > By making use of the mechanism above, we will get a compiler warning
> > in case the flexible array does not occur last in the structure, which
> > will help us prevent some kind of undefined behavior bugs from being
> > inadvertently introduced[3] to the codebase from now on.
> > 
> > Also, notice that, dynamic memory allocations won't be affected by
> > this change:
> > 
> > "Flexible array members have incomplete type, and so the sizeof operator
> > may not be applied. As a quirk of the original implementation of
> > zero-length arrays, sizeof evaluates to zero."[1]
> > 
> > sizeof(flexible-array-member) triggers a warning because flexible array
> > members have incomplete type[1]. There are some instances of code in
> > which the sizeof operator is being incorrectly/erroneously applied to
> > zero-length arrays and the result is zero. Such instances may be hiding
> > some bugs. So, this work (flexible-array member conversions) will also
> > help to get completely rid of those sorts of issues.
> > 
> > This issue was found with the help of Coccinelle.
> > 
> > [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> > [2] https://github.com/KSPP/linux/issues/21
> > [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> > 
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> > ---
> >  drivers/greybus/arpc.h                    |    2 -
> >  include/linux/greybus/greybus_protocols.h |   44 +++++++++++++++---------------
> 
> I noticed Greg just applied this one to his -testing branch, but do we
> really want this in greybus_protocols.h, which is meant to be shared
> with the firmware side? Perhaps not an issue, just figured I'd point
> this out.

Why not, it should be the same thing, right?  No logic has changed that
I see.

thanks,

greg k-h