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

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Date:   Tue, 3 Mar 2020 11:45:40 -0500
From:   "J. Bruce Fields" <bfields@...ldses.org>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Chuck Lever <chuck.lever@...cle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, linux-nfs@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] svcrdma: Replace zero-length array with
 flexible-array member

Applying for 5.7, thanks.--b.

On Mon, Feb 17, 2020 at 02:05:00PM -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>
> ---
>  net/sunrpc/xprtrdma/svc_rdma_rw.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/sunrpc/xprtrdma/svc_rdma_rw.c b/net/sunrpc/xprtrdma/svc_rdma_rw.c
> index 48fe3b16b0d9..003610ce00bc 100644
> --- a/net/sunrpc/xprtrdma/svc_rdma_rw.c
> +++ b/net/sunrpc/xprtrdma/svc_rdma_rw.c
> @@ -41,7 +41,7 @@ struct svc_rdma_rw_ctxt {
>  	struct rdma_rw_ctx	rw_ctx;
>  	int			rw_nents;
>  	struct sg_table		rw_sg_table;
> -	struct scatterlist	rw_first_sgl[0];
> +	struct scatterlist	rw_first_sgl[];
>  };
>  
>  static inline struct svc_rdma_rw_ctxt *
> -- 
> 2.25.0