linux-kernel - Re: [PATCH][next] net: mlxfw: Replace zero-length array with flexible-array member

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Message-ID: <20200303132152.GI2178@nanopsycho>
Date:   Tue, 3 Mar 2020 14:21:52 +0100
From:   Jiri Pirko <jiri@...nulli.us>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     "mlxsw@...lanox.com David S. Miller" <davem@...emloft.net>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] net: mlxfw: Replace zero-length array with
 flexible-array member

Mon, Mar 02, 2020 at 10:04:37PM CET, 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>

Reviewed-by: Jiri Pirko <jiri@...lanox.com>