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From:   Jeff Moyer <jmoyer@redhat.com>
To:     "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Benjamin LaHaise <bcrl@kvack.org>,
        linux-fsdevel@vger.kernel.org, linux-aio@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH][next] aio: Replace zero-length array with flexible-array member
References: <20200306164446.GA21604@embeddedor>
Date:   Fri, 06 Mar 2020 14:20:49 -0500
In-Reply-To: <20200306164446.GA21604@embeddedor> (Gustavo A. R. Silva's
        message of "Fri, 6 Mar 2020 10:44:46 -0600")
Message-ID: <x49ftelnvce.fsf@segfault.boston.devel.redhat.com>
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"Gustavo A. R. Silva" <gustavo@embeddedor.com> writes:

> 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>
> ---
>  fs/aio.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/aio.c b/fs/aio.c
> index 94f2b9256c0c..13c4be7f00f0 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -68,7 +68,7 @@ struct aio_ring {
>  	unsigned	header_length;	/* size of aio_ring */
>  
>  
> -	struct io_event		io_events[0];
> +	struct io_event		io_events[];
>  }; /* 128 bytes + ring size */
>  
>  /*

Acked-by: Jeff Moyer <jmoyer@redhat.com>