linux-kernel - Re: [PATCH][next] nvdimm: nd.h: Replace zero-length array with flexible-array member

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Date:   Thu, 19 Mar 2020 16:47:44 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>, linux-nvdimm@...ts.01.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] nvdimm: nd.h: Replace zero-length array with
 flexible-array member

On Thu, Mar 19, 2020 at 06:09:37PM -0500, 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:

"won't" be affected?

My reading of [1] indicates that this change will break the allocation in
nd_region_activate() because sizeof() can no longer be used on the base
structure?

What am I missing?

Ira

> 
> "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>
> ---
>  drivers/nvdimm/nd.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/nvdimm/nd.h b/drivers/nvdimm/nd.h
> index c4d69c1cce55..85dbb2a322b9 100644
> --- a/drivers/nvdimm/nd.h
> +++ b/drivers/nvdimm/nd.h
> @@ -39,7 +39,7 @@ struct nd_region_data {
>  	int ns_count;
>  	int ns_active;
>  	unsigned int hints_shift;
> -	void __iomem *flush_wpq[0];
> +	void __iomem *flush_wpq[];
>  };
>  
>  static inline void __iomem *ndrd_get_flush_wpq(struct nd_region_data *ndrd,
> @@ -157,7 +157,7 @@ struct nd_region {
>  	struct nd_interleave_set *nd_set;
>  	struct nd_percpu_lane __percpu *lane;
>  	int (*flush)(struct nd_region *nd_region, struct bio *bio);
> -	struct nd_mapping mapping[0];
> +	struct nd_mapping mapping[];
>  };
>  
>  struct nd_blk_region {
> -- 
> 2.23.0
>