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

[Tianchen Ding]

On 5/28/26 9:13 PM, Steven Rostedt wrote:
> On Thu, 28 May 2026 10:24:17 +0800
> Tianchen Ding <dtcccc@xxxxxxxxxxxxxxxxx> 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.

> >   }
> >   
> >