netdev - Re: [PATCH] IB/mlx4: Replace zero-length array with flexible-array

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Message-ID: <20200513183749.GZ4814@unreal>
Date:   Wed, 13 May 2020 21:37:49 +0300
From:   Leon Romanovsky <leon@...nel.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Tariq Toukan <tariqt@...lanox.com>, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] IB/mlx4: Replace zero-length array with flexible-array

On Wed, May 13, 2020 at 03:33:35PM -0300, Jason Gunthorpe wrote:
> On Thu, May 07, 2020 at 01:59:21PM -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>
> > ---
> >  include/linux/mlx4/qp.h |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
>
> Applied to for-next, thanks

Jason,

Please be cautious here, Jakub already applied this patch.
https://lore.kernel.org/lkml/20200509205151.209bdc9d@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com/

>
> Jason