[PATCH v3] 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 writing 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>
---
v3:
Measure the binary length via wc -c instead of hard-coding "size + 4".

v2:
Update comment about 64K pages.
---
 .../ftrace/test.d/00basic/trace_marker_raw.tc      | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 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..f985ff391463 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
@@ -36,15 +36,23 @@ make_str() {
 
 	data=`printf -- 'X%.0s' $(seq $cnt)`
 
-	printf "${val}${data}"
+	# Return escape-sequence text (e.g. "\003\000..."); the caller
+	# converts to binary. Shell command substitution strips NUL bytes,
+	# so the binary form cannot survive being captured into a variable.
+	printf '%s' "${val}${data}"
 }
 
 write_buffer() {
 	id=$1
 	size=$2
 
-	# write the string into the raw marker
-	make_str $id $size > trace_marker_raw
+	str=`make_str $id $size`
+	len=`printf "$str" | wc -c`
+	# 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.
+	printf "$str" | dd of=trace_marker_raw bs=$len iflag=fullblock
 }
 
 
-- 
2.39.3