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

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Message-ID: <20200219164311.GA348671@splinter>
Date:   Wed, 19 Feb 2020 18:43:11 +0200
From:   Ido Schimmel <idosch@...lanox.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Jiri Pirko <jiri@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] mlxsw: Replace zero-length array with
 flexible-array member

On Tue, Feb 18, 2020 at 02:57:05PM -0600, 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://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> 
> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>

Thanks, Gustavo. Looks good to me. Ran a few tests with a debug config
and nothing exploded.

I was just about to submit some patches myself, but they will conflict
with this patch, so I will wait :)

Reviewed-by: Ido Schimmel <idosch@...lanox.com>
Tested-by: Ido Schimmel <idosch@...lanox.com>