[PATCH] selftests/dma: fix invalid array access in printf

[PATCH] selftests/dma: fix invalid array access in printf

[Zhang Chujun]

The printf statement attempts to print the DMA direction string using
the syntax 'dir[directions]', which is an invalid array access. The
variable 'dir' is an integer, and 'directions' is a char pointer array.
This incorrect syntax should be 'directions[dir]', using 'dir' as the
index into the 'directions' array. Fix this by correcting the array
access from 'dir[directions]' to 'directions[dir]'.

Signed-off-by: Zhang Chujun <zhangchujun@cmss.chinamobile.com>

diff --git a/tools/testing/selftests/dma/dma_map_benchmark.c b/tools/testing/selftests/dma/dma_map_benchmark.c
index b12f1f9babf8..b925756373ce 100644
--- a/tools/testing/selftests/dma/dma_map_benchmark.c
+++ b/tools/testing/selftests/dma/dma_map_benchmark.c
@@ -118,7 +118,7 @@ int main(int argc, char **argv)
 	}
 
 	printf("dma mapping benchmark: threads:%d seconds:%d node:%d dir:%s granule: %d\n",
-			threads, seconds, node, dir[directions], granule);
+			threads, seconds, node, directions[dir], granule);
 	printf("average map latency(us):%.1f standard deviation:%.1f\n",
 			map.avg_map_100ns/10.0, map.map_stddev/10.0);
 	printf("average unmap latency(us):%.1f standard deviation:%.1f\n",
-- 
2.50.1.windows.1

Re: [PATCH] selftests/dma: fix invalid array access in printf

[Shuah Khan]

On 11/5/25 20:30, Zhang Chujun wrote:
> The printf statement attempts to print the DMA direction string using
> the syntax 'dir[directions]', which is an invalid array access. The
> variable 'dir' is an integer, and 'directions' is a char pointer array.
> This incorrect syntax should be 'directions[dir]', using 'dir' as the
> index into the 'directions' array. Fix this by correcting the array
> access from 'dir[directions]' to 'directions[dir]'.
> 
> Signed-off-by: Zhang Chujun <zhangchujun@cmss.chinamobile.com>
> 
> diff --git a/tools/testing/selftests/dma/dma_map_benchmark.c b/tools/testing/selftests/dma/dma_map_benchmark.c
> index b12f1f9babf8..b925756373ce 100644
> --- a/tools/testing/selftests/dma/dma_map_benchmark.c
> +++ b/tools/testing/selftests/dma/dma_map_benchmark.c
> @@ -118,7 +118,7 @@ int main(int argc, char **argv)
>   	}
>   
>   	printf("dma mapping benchmark: threads:%d seconds:%d node:%d dir:%s granule: %d\n",
> -			threads, seconds, node, dir[directions], granule);
> +			threads, seconds, node, directions[dir], granule);
>   	printf("average map latency(us):%.1f standard deviation:%.1f\n",
>   			map.avg_map_100ns/10.0, map.map_stddev/10.0);
>   	printf("average unmap latency(us):%.1f standard deviation:%.1f\n",

Looks like you sent the same patch again. I replied to your previous
patch asking you how you found this problem. It looks like a needed
change. Compiler doesn't complain about it which is strange.

thanks,
-- Shuah

Re: [PATCH] dma_map_benchmark: fix incorrect array access in printf

[Zhang Chujun]

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[-- Attachment #1: Type: text/plain; charset=y, Size: 1384 bytes --]

On Tue, Nov 05, 2025 at 08:30:00PM +0800, Zhang Chujun wrote:
> The printf statement attempts to print the DMA direction string using
> the syntax 'dir[directions]', which is an invalid array access. The
> variable 'dir' is an integer, and 'directions' is a char pointer array.
> This incorrect syntax should be 'directions[dir]', using 'dir' as the
> index into the 'directions' array. Fix this by correcting the array
> access from 'dir[directions]' to 'directions[dir]'.

Hi Shuah,

Thanks for your patience.

I found this issue while carefully reading the code in
`dma_map_benchmark.c`. The expression `dir[directions]` stood out because
`dir` is an integer (enum or int), while `directions` is a string array —
so using `dir` as the index into `directions` is the correct form.
Although C allows `a[b]` and `b[a]` syntactically due to pointer arithmetic,
in this context `dir[directions]` is logically wrong and would print an
unexpected (likely garbage) string, since it treats the address of the
string array as an array indexed by a small integer.

The compiler doesn’t warn because `dir[directions]` is technically valid
C (equivalent to `*(directions + dir)`), but semantically it’s backwards
and breaks the intended output.

Sorry for sending the patch twice, I’ll make sure to follow the proper
mailing list guidelines going forward.

Best regards,
Zhang Chujun

Re: [PATCH] dma_map_benchmark: fix incorrect array access in printf

[Shuah Khan]

On 11/6/25 18:55, Zhang Chujun wrote:
> On Tue, Nov 05, 2025 at 08:30:00PM +0800, Zhang Chujun wrote:
>> The printf statement attempts to print the DMA direction string using
>> the syntax 'dir[directions]', which is an invalid array access. The
>> variable 'dir' is an integer, and 'directions' is a char pointer array.
>> This incorrect syntax should be 'directions[dir]', using 'dir' as the
>> index into the 'directions' array. Fix this by correcting the array
>> access from 'dir[directions]' to 'directions[dir]'.
> 
> Hi Shuah,
> 
> Thanks for your patience.
> 
> I found this issue while carefully reading the code in
> `dma_map_benchmark.c`. The expression `dir[directions]` stood out because
> `dir` is an integer (enum or int), while `directions` is a string array —
> so using `dir` as the index into `directions` is the correct form.
> Although C allows `a[b]` and `b[a]` syntactically due to pointer arithmetic,
> in this context `dir[directions]` is logically wrong and would print an
> unexpected (likely garbage) string, since it treats the address of the
> string array as an array indexed by a small integer.
> 
> The compiler doesn’t warn because `dir[directions]` is technically valid
> C (equivalent to `*(directions + dir)`), but semantically it’s backwards
> and breaks the intended output.
> 

Applied to linux-kselftest next for Linux 6.19-rc1

thanks,
-- Shuah

[PATCH] selftests/dma: fix invalid array access in printf

[Zhang Chujun]

The printf statement attempts to print the DMA direction string using
the syntax 'dir[directions]', which is an invalid array access. The
variable 'dir' is an integer, and 'directions' is a char pointer array.
This incorrect syntax should be 'directions[dir]', using 'dir' as the
index into the 'directions' array. Fix this by correcting the array
access from 'dir[directions]' to 'directions[dir]'.

Signed-off-by: Zhang Chujun <zhangchujun@cmss.chinamobile.com>

diff --git a/tools/testing/selftests/dma/dma_map_benchmark.c b/tools/testing/selftests/dma/dma_map_benchmark.c
index b12f1f9babf8..b925756373ce 100644
--- a/tools/testing/selftests/dma/dma_map_benchmark.c
+++ b/tools/testing/selftests/dma/dma_map_benchmark.c
@@ -118,7 +118,7 @@ int main(int argc, char **argv)
 	}
 
 	printf("dma mapping benchmark: threads:%d seconds:%d node:%d dir:%s granule: %d\n",
-			threads, seconds, node, dir[directions], granule);
+			threads, seconds, node, directions[dir], granule);
 	printf("average map latency(us):%.1f standard deviation:%.1f\n",
 			map.avg_map_100ns/10.0, map.map_stddev/10.0);
 	printf("average unmap latency(us):%.1f standard deviation:%.1f\n",
-- 
2.50.1.windows.1

Re: [PATCH] selftests/dma: fix invalid array access in printf

[Shuah Khan]

On 11/3/25 19:52, Zhang Chujun wrote:
> The printf statement attempts to print the DMA direction string using
> the syntax 'dir[directions]', which is an invalid array access. The
> variable 'dir' is an integer, and 'directions' is a char pointer array.
> This incorrect syntax should be 'directions[dir]', using 'dir' as the
> index into the 'directions' array. Fix this by correcting the array
> access from 'dir[directions]' to 'directions[dir]'.

How did you find this problem? The patch looks good to me, but
I do want to know how you found this because compiler doesn't
complain and I can build this fine.> 
> Signed-off-by: Zhang Chujun <zhangchujun@cmss.chinamobile.com>
> 
> diff --git a/tools/testing/selftests/dma/dma_map_benchmark.c b/tools/testing/selftests/dma/dma_map_benchmark.c
> index b12f1f9babf8..b925756373ce 100644
> --- a/tools/testing/selftests/dma/dma_map_benchmark.c
> +++ b/tools/testing/selftests/dma/dma_map_benchmark.c
> @@ -118,7 +118,7 @@ int main(int argc, char **argv)
>   	}
>   
>   	printf("dma mapping benchmark: threads:%d seconds:%d node:%d dir:%s granule: %d\n",
> -			threads, seconds, node, dir[directions], granule);
> +			threads, seconds, node, directions[dir], granule);
>   	printf("average map latency(us):%.1f standard deviation:%.1f\n",
>   			map.avg_map_100ns/10.0, map.map_stddev/10.0);
>   	printf("average unmap latency(us):%.1f standard deviation:%.1f\n",
thanks,
-- Shuah