diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
index 0eef91daf289..a3778aff4fa9 100644
--- a/tools/include/nolibc/stdio.h
+++ b/tools/include/nolibc/stdio.h
@@ -21,6 +21,11 @@
 #define EOF (-1)
 #endif
 
+/* Buffering mode used by setvbuf.  */
+#define _IOFBF 0	/* Fully buffered. */
+#define _IOLBF 1	/* Line buffered. */
+#define _IONBF 2	/* No buffering. */
+
 /* just define FILE as a non-empty type. The value of the pointer gives
  * the FD: FILE=~fd for fd>=0 or NULL for fd<0. This way positive FILE
  * are immediately identified as abnormal entries (i.e. possible copies
@@ -350,6 +355,25 @@ void perror(const char *msg)
 	fprintf(stderr, "%s%serrno=%d\n", (msg && *msg) ? msg : "", (msg && *msg) ? ": " : "", errno);
 }
 
+static __attribute__((unused))
+int setvbuf(FILE *stream, char *buf, int mode, size_t size)
+{
+	/*
+	 * nolibc does not support buffering so this is a nop. Just check mode
+	 * is valid as required by the spec.
+	 */
+	switch (mode) {
+	case _IOFBF:
+	case _IOLBF:
+	case _IONBF:
+		break;
+	default:
+		return EOF;
+	}
+
+	return 0;
+}
+
 /* make sure to include all global symbols */
 #include "nolibc.h"
 
diff --git a/tools/testing/selftests/kselftest.h b/tools/testing/selftests/kselftest.h
index 829be379545a..529d29a35900 100644
--- a/tools/testing/selftests/kselftest.h
+++ b/tools/testing/selftests/kselftest.h
@@ -113,6 +113,15 @@ static inline int ksft_get_error_cnt(void) { return ksft_cnt.ksft_error; }
 
 static inline void ksft_print_header(void)
 {
+	/*
+	 * Force line buffering; If stdout is not connected to a terminal, it
+	 * will otherwise default to fully buffered, which can cause output
+	 * duplication if there is content in the buffer when fork()ing. If
+	 * there is a crash, line buffering also means the most recent output
+	 * line will be visible.
+	 */
+	setvbuf(stdout, NULL, _IOLBF, 0);
+
 	if (!(getenv("KSFT_TAP_LEVEL")))
 		printf("TAP version 13\n");
 }
diff --git a/tools/testing/selftests/kselftest/runner.sh b/tools/testing/selftests/kselftest/runner.sh
index 1c952d1401d4..261c73cab41b 100644
--- a/tools/testing/selftests/kselftest/runner.sh
+++ b/tools/testing/selftests/kselftest/runner.sh
@@ -105,15 +105,18 @@ run_one()
 		echo "# Warning: file $TEST is missing!"
 		echo "not ok $test_num $TEST_HDR_MSG"
 	else
+		if [ -x /usr/bin/stdbuf ]; then
+			stdbuf="/usr/bin/stdbuf --output=L "
+		fi
 		eval kselftest_cmd_args="\$${kselftest_cmd_args_ref:-}"
-		cmd="./$BASENAME_TEST $kselftest_cmd_args"
+		cmd="$stdbuf ./$BASENAME_TEST $kselftest_cmd_args"
 		if [ ! -x "$TEST" ]; then
 			echo "# Warning: file $TEST is not executable"
 
 			if [ $(head -n 1 "$TEST" | cut -c -2) = "#!" ]
 			then
 				interpreter=$(head -n 1 "$TEST" | cut -c 3-)
-				cmd="$interpreter ./$BASENAME_TEST"
+				cmd="$stdbuf $interpreter ./$BASENAME_TEST"
 			else
 				echo "not ok $test_num $TEST_HDR_MSG"
 				return