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libnvdimm/region: Replace zero-length array with flexible-array member 

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 7649773293 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20200319230937.GA16648@embeddedor.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
This commit is contained in:
Gustavo A. R. Silva 2020-03-19 18:09:37 -05:00 committed by Dan Williams
parent 1e361632da
commit 9106137c6f
1 changed files with 2 additions and 2 deletions
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4
drivers/nvdimm/nd.h
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@ -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,
@ -156,7 +156,7 @@ 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 {