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Date:   Fri, 28 Feb 2020 12:09:14 -0800 (PST)
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Subject: Re: [PATCH][next] xdp: Replace zero-length array with
 flexible-array member
From:   David Miller <davem@davemloft.net>
In-Reply-To: <20200228131907.GA17911@embeddedor>
References: <20200228131907.GA17911@embeddedor>
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From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Date: Fri, 28 Feb 2020 07:19:07 -0600

> 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@embeddedor.com>

Applied.