Index: user/pho/stress2/tools/flip.c
===================================================================
--- user/pho/stress2/tools/flip.c	(nonexistent)
+++ user/pho/stress2/tools/flip.c	(revision 367363)
@@ -0,0 +1,121 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ *
+ * Copyright (c) 2020 Peter Holm <pho@FreeBSD.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+/*
+ * Flip one or more bits in a file.
+ */
+
+#include <sys/types.h>
+#include <sys/stat.h>
+
+#include <err.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+extern char *optarg;
+extern int optind;
+
+static long
+random_long(long mi, long ma)
+{
+        return (arc4random()  % (ma - mi + 1) + mi);
+}
+
+static void
+flip(void *ap, size_t len)
+{
+	unsigned char *cp;
+	int byte;
+	unsigned char bit, buf, mask, old;
+
+	cp = (unsigned char *)ap;
+	byte = random_long(0, len);
+	bit = random_long(0,7);
+	mask = ~(1 << bit);
+	buf = cp[byte];
+	old = cp[byte];
+	buf = (buf & mask) | (~buf & ~mask);
+	cp[byte] = buf;
+#if defined(DEBUG)
+	printf("Change %2x to %2x at %d by flipping bit %d\n",
+	    old, buf, byte, bit);
+#endif
+}
+
+int
+main(int argc, char *argv[])
+{
+	struct stat st;
+	off_t pos;
+	int fd, i, times;
+	char c;
+
+	times = 1;
+	while ((c = getopt(argc, argv, "n:")) != -1) {
+		switch (c) {
+			case 'n':
+				times = atoi(optarg);
+				break;
+			case '?':
+			default:
+				fprintf(stderr,
+				    "Usage: %s [ -n <num> <file>]\n",
+				    argv[0]);
+				exit(1);
+		}
+	}
+	argc -= optind;
+	argv += optind;
+
+	if (argc != 1) {
+		fprintf(stderr, "Missing file name\n");
+		exit(1);
+	}
+
+	if ((fd = open(argv[0], O_RDWR)) == -1)
+		err(1, "open(%s)", argv[0]);
+
+	if (fstat(fd, &st) == -1)
+		err(1, "stat %s", argv[0]);
+
+	for (i = 0; i < times; i++) {
+		pos = arc4random() % st.st_size;
+		if (lseek(fd, pos, SEEK_SET) == -1)
+			err(1, "lseek()");
+		read(fd, &c, 1);
+		flip(&c, 1);
+		if (lseek(fd, pos, SEEK_SET) == -1)
+			err(1, "lseek()");
+		write(fd, &c, 1);
+	}
+
+	return (0);
+}