Messages in this thread | | | From | "Gustavo A. R. Silva" <> | Subject | Re: [PATCH][next] nvdimm: nd.h: Replace zero-length array with flexible-array member | Date | Thu, 19 Mar 2020 19:03:22 -0500 |
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On 3/19/20 6:47 PM, Ira Weiny wrote: > 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? >
Yep. They won't:
"As a quirk of the original implementation of zero-length arrays, sizeof evaluates to zero."
> My reading of [1] indicates that this change will break the allocation inha
The allocation will remain the same as with *flush_wpg[0]. See this example: https://godbolt.org/z/oohwmB
> nd_region_activate() because sizeof() can no longer be used on the base > structure? >
In this case the sizeof operator cannot be applied to flush_wpg, not the base structure, which is nd_region_data.
> What am I missing? >
I think you are confusing the base structure with the actual flexible-array member.
Thanks -- Gustavo
> 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@embeddedor.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 >>
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