On ARM64 kernels with 64K pages, the trace_marker_raw test fails because
bash's printf builtin uses stdio buffering which splits output into
multiple small write() calls to the tracefs file. Since each individual
write is within TRACE_MARKER_MAX_SIZE (4096), they all succeed, causing
the "too big" write test to incorrectly pass.

Fix by piping make_str output through dd with iflag=fullblock to
guarantee a single atomic write() syscall to trace_marker_raw.

Fixes: 37f46601383a ("selftests/tracing: Add basic test for trace_marker_raw file")
Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
---
v2:
Update comment about 64K pages.
---
 .../selftests/ftrace/test.d/00basic/trace_marker_raw.tc    | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc b/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
index 8e905d4fe6dd..f68f1901f65f 100644
--- a/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
+++ b/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
@@ -43,8 +43,11 @@ write_buffer() {
 	id=$1
 	size=$2
 
-	# write the string into the raw marker
-	make_str $id $size > trace_marker_raw
+	# Pipe through dd to ensure a single atomic write() syscall
+	# on architectures with 64K pages, where shell's printf builtin
+	# uses stdio buffering which may split the output into multiple
+	# writes.
+	make_str $id $size | dd of=trace_marker_raw bs=`expr $size + 4` iflag=fullblock
 }
 
 
-- 
2.39.3

On Thu, 28 May 2026 10:24:17 +0800
Tianchen Ding <dtcccc@linux.alibaba.com> wrote:

> 
> diff --git a/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc b/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
> index 8e905d4fe6dd..f68f1901f65f 100644
> --- a/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
> +++ b/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
> @@ -43,8 +43,11 @@ write_buffer() {
>  	id=$1
>  	size=$2
>  
> -	# write the string into the raw marker
> -	make_str $id $size > trace_marker_raw
> +	# Pipe through dd to ensure a single atomic write() syscall
> +	# on architectures with 64K pages, where shell's printf builtin
> +	# uses stdio buffering which may split the output into multiple
> +	# writes.
> +	make_str $id $size | dd of=trace_marker_raw bs=`expr $size + 4` iflag=fullblock

I was looking at this more, and I'm not comfortable with the hard coded
4 above. I rather use the length of the string. Something like:

	str=`make_str $id $size`
	len=${#str}
	echo "$str" | dd of=trace_marker_raw bs=$len iflag=fullblock

-- Steve

>  }
>  
>

On 5/28/26 9:13 PM, Steven Rostedt wrote:
> On Thu, 28 May 2026 10:24:17 +0800
> Tianchen Ding <dtcccc@linux.alibaba.com> wrote:
> 
>>
>> diff --git a/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc b/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
>> index 8e905d4fe6dd..f68f1901f65f 100644
>> --- a/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
>> +++ b/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
>> @@ -43,8 +43,11 @@ write_buffer() {
>>   	id=$1
>>   	size=$2
>>   
>> -	# write the string into the raw marker
>> -	make_str $id $size > trace_marker_raw
>> +	# Pipe through dd to ensure a single atomic write() syscall
>> +	# on architectures with 64K pages, where shell's printf builtin
>> +	# uses stdio buffering which may split the output into multiple
>> +	# writes.
>> +	make_str $id $size | dd of=trace_marker_raw bs=`expr $size + 4` iflag=fullblock
> 
> I was looking at this more, and I'm not comfortable with the hard coded
> 4 above. I rather use the length of the string. Something like:
> 
> 	str=`make_str $id $size`
> 	len=${#str}
> 	echo "$str" | dd of=trace_marker_raw bs=$len iflag=fullblock
> 
> -- Steve
> 

Capturing make_str output into a shell variable doesn't work because make_str 
outputs raw binary that may contain NUL bytes, and shell command substitution 
silently strips them.

However, the val variable inside make_str doesn't hold actual NUL bytes — it 
holds the text of escape sequences (e.g., the literal characters 
\003\000\000\000). The binary conversion only happens at the final printf 
"${val}${data}".

We can take advantage of this by having make_str return the escape-sequence text 
instead of binary, and letting write_buffer handle the conversion:

   make_str() {
         ...
         printf '%s' "${val}${data}"
   }

   write_buffer() {
         id=$1
         size=$2

         str=`make_str $id $size`
         len=$(printf "$str" | wc -c)
         printf "$str" | dd of=trace_marker_raw bs=$len iflag=fullblock
   }

This way str holds only printable escape-sequence text (no NUL), printf "$str" 
converts it to real binary through the pipe, and wc -c measures the true binary 
length.

>>   }
>>   
>>   

On Fri, 29 May 2026 10:59:34 +0800
Tianchen Ding <dtcccc@linux.alibaba.com> wrote:

> We can take advantage of this by having make_str return the escape-sequence text 
> instead of binary, and letting write_buffer handle the conversion:
> 
>    make_str() {
>          ...
>          printf '%s' "${val}${data}"
>    }
> 
>    write_buffer() {
>          id=$1
>          size=$2
> 
>          str=`make_str $id $size`
>          len=$(printf "$str" | wc -c)
>          printf "$str" | dd of=trace_marker_raw bs=$len iflag=fullblock
>    }
> 
> This way str holds only printable escape-sequence text (no NUL), printf "$str" 
> converts it to real binary through the pipe, and wc -c measures the true binary 
> length.

This is quite hacky, but at least it removes the hardcoded assumptions.

OK, you can send a v3 that does that.

Thanks,

-- Steve