Re: [PATCH v2] selftests/ftrace: Fix trace_marker_raw test on 64K page kernels

[Tianchen Ding <dtcccc@linux.alibaba.com>]

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.

>>   }
>>   
>>