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 <dtc...@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.

  }