The padding field in the structure was previously reserved to
maintain a stable interface for potential new fields, ensuring
compatibility with user-space shared data structures.
However,it was accidentally removed by tiantao in a prior commit,
which may lead to incompatibility between user space and the kernel.

This patch reinstates the padding to restore the original structure
layout and preserve compatibility.

Fixes: 8ddde07a3d28 ("dma-mapping: benchmark: extract a common header file for map_benchmark definition")
Cc: stable@vger.kernel.org
Acked-by: Barry Song <baohua@kernel.org>
Signed-off-by: Qinxin Xia <xiaqinxin@huawei.com>
---
 include/linux/map_benchmark.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/linux/map_benchmark.h b/include/linux/map_benchmark.h
index 62674c83bde4..2ac2fe52f248 100644
--- a/include/linux/map_benchmark.h
+++ b/include/linux/map_benchmark.h
@@ -27,5 +27,6 @@ struct map_benchmark {
 	__u32 dma_dir; /* DMA data direction */
 	__u32 dma_trans_ns; /* time for DMA transmission in ns */
 	__u32 granule;  /* how many PAGE_SIZE will do map/unmap once a time */
+	__u8 expansion[76];     /* For future use */
 };
 #endif /* _KERNEL_DMA_BENCHMARK_H */
-- 
2.33.0

On Sat, 14 Jun 2025 22:34:51 +0800
Qinxin Xia <xiaqinxin@huawei.com> wrote:

> The padding field in the structure was previously reserved to
> maintain a stable interface for potential new fields, ensuring
> compatibility with user-space shared data structures.
> However,it was accidentally removed by tiantao in a prior commit,
> which may lead to incompatibility between user space and the kernel.
> 
> This patch reinstates the padding to restore the original structure
> layout and preserve compatibility.
> 
> Fixes: 8ddde07a3d28 ("dma-mapping: benchmark: extract a common header file for map_benchmark definition")
> Cc: stable@vger.kernel.org
> Acked-by: Barry Song <baohua@kernel.org>
> Signed-off-by: Qinxin Xia <xiaqinxin@huawei.com>

FWIW I checked the patch above indeed accidentally dropped the padding and the structure
is copied to userspace so this fix is correct.  Given it's not in a uapi header this
only really affects the selftest I think this is mostly a case of there possibly being
out of tree tools with a local copy of this structure definition.

Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>

On Mon, Jun 16, 2025 at 9:53 PM Jonathan Cameron
<Jonathan.Cameron@huawei.com> wrote:
>
> On Sat, 14 Jun 2025 22:34:51 +0800
> Qinxin Xia <xiaqinxin@huawei.com> wrote:
>
> > The padding field in the structure was previously reserved to
> > maintain a stable interface for potential new fields, ensuring
> > compatibility with user-space shared data structures.
> > However,it was accidentally removed by tiantao in a prior commit,
> > which may lead to incompatibility between user space and the kernel.
> >
> > This patch reinstates the padding to restore the original structure
> > layout and preserve compatibility.
> >
> > Fixes: 8ddde07a3d28 ("dma-mapping: benchmark: extract a common header file for map_benchmark definition")
> > Cc: stable@vger.kernel.org
> > Acked-by: Barry Song <baohua@kernel.org>
> > Signed-off-by: Qinxin Xia <xiaqinxin@huawei.com>
>
> FWIW I checked the patch above indeed accidentally dropped the padding and the structure
> is copied to userspace so this fix is correct.  Given it's not in a uapi header this
> only really affects the selftest I think this is mostly a case of there possibly being
> out of tree tools with a local copy of this structure definition.

Somehow, I feel we have placed
tools/testing/selftests/dma/dma_map_benchmark.c in the wrong location.
As a selftest, it should have a mechanism to check kernel dependencies,
start properly and automatically, and report pass or fail.

dma_map_benchmark.c seems more like a tool that belongs in tools/dma,
rather than a test.

>
> Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>

Thanks
Barry

On 16.06.2025 12:40, Barry Song wrote:
> On Mon, Jun 16, 2025 at 9:53 PM Jonathan Cameron
> <Jonathan.Cameron@huawei.com> wrote:
>> On Sat, 14 Jun 2025 22:34:51 +0800
>> Qinxin Xia <xiaqinxin@huawei.com> wrote:
>>> The padding field in the structure was previously reserved to
>>> maintain a stable interface for potential new fields, ensuring
>>> compatibility with user-space shared data structures.
>>> However,it was accidentally removed by tiantao in a prior commit,
>>> which may lead to incompatibility between user space and the kernel.
>>>
>>> This patch reinstates the padding to restore the original structure
>>> layout and preserve compatibility.
>>>
>>> Fixes: 8ddde07a3d28 ("dma-mapping: benchmark: extract a common header file for map_benchmark definition")
>>> Cc: stable@vger.kernel.org
>>> Acked-by: Barry Song <baohua@kernel.org>
>>> Signed-off-by: Qinxin Xia <xiaqinxin@huawei.com>
>> FWIW I checked the patch above indeed accidentally dropped the padding and the structure
>> is copied to userspace so this fix is correct.  Given it's not in a uapi header this
>> only really affects the selftest I think this is mostly a case of there possibly being
>> out of tree tools with a local copy of this structure definition.
> Somehow, I feel we have placed
> tools/testing/selftests/dma/dma_map_benchmark.c in the wrong location.
> As a selftest, it should have a mechanism to check kernel dependencies,
> start properly and automatically, and report pass or fail.
>
> dma_map_benchmark.c seems more like a tool that belongs in tools/dma,
> rather than a test.

Indeed imho it would be better to move it out of selftests directory.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland