linux-kernel - Re: [PATCH][next] acpi: nfit.h: Replace zero-length array with flexible-array member

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Date:   Mon, 30 Mar 2020 12:54:57 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][next] acpi: nfit.h: Replace zero-length array with
 flexible-array member

On Wed, Mar 25, 2020 at 3:06 AM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Thu, Mar 19, 2020 at 9:15 PM Gustavo A. R. Silva
> <gustavo@...eddedor.com> 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]
> >
> > 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 <gustavo@...eddedor.com>
>
> Dan,
>
> I'm assuming that you will take care of this one or please let me know
> otherwise.

Yes, this one and the other 2 related libnvdimm fixups are in my queue.