linux-kernel - Re: [PATCH] crypto: s5p-sss - Replace zero-length array with flexible-array member

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Message-ID: <CAJKOXPeCF7be5xNO3uMmrJ1VC4MK61RxxhuAve6zi56rR1TZ=A@mail.gmail.com>
Date:   Fri, 14 Feb 2020 09:22:40 +0100
From:   Krzysztof Kozlowski <krzk@...nel.org>
To:     Kamil Konieczny <k.konieczny@...sung.com>
Cc:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Vladimir Zapolskiy <vz@...ia.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        linux-crypto@...r.kernel.org,
        "linux-samsung-soc@...r.kernel.org" 
        <linux-samsung-soc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH] crypto: s5p-sss - Replace zero-length array with
 flexible-array member

On Fri, 14 Feb 2020 at 08:09, Kamil Konieczny <k.konieczny@...sung.com> wrote:
>
> On 13.02.2020 18:21, 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]
> >
> > This issue was found with the help of Coccinelle.
> >
> > [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> > [2] https://protect2.fireeye.com/url?k=7fbec6f4-22720d30-7fbf4dbb-0cc47a314e9a-2a4d03985644c7ed&u=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>
> > ---
> >  drivers/crypto/s5p-sss.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/crypto/s5p-sss.c b/drivers/crypto/s5p-sss.c
> > index d66e20a2f54c..2a16800d2579 100644
> > --- a/drivers/crypto/s5p-sss.c
> > +++ b/drivers/crypto/s5p-sss.c
> > @@ -369,7 +369,7 @@ struct s5p_hash_reqctx {
> >       bool                    error;
> >
> >       u32                     bufcnt;
> > -     u8                      buffer[0];
> > +     u8                      buffer[];
> >  };
> >
> >  /**
> >
>
> Looks good to me.
>
> Acked-by: Kamil Konieczny <k.konieczny@...sung.com>

Why not making it simple/obvious u8 *buffer? Or fixed length (BUFLEN length)?

Best regards,
Krzysztof