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

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

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