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 <[email protected]>
> ---
> 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
>
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