[09/17] xfs_db: introduce fuzz command
diff mbox

Message ID 148498614970.16675.13773024118867518517.stgit@birch.djwong.org
State Superseded
Headers show

Commit Message

Darrick J. Wong Jan. 21, 2017, 8:09 a.m. UTC
Introduce a new 'fuzz' command to write creative values into
disk structure fields.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 db/Makefile       |    3 
 db/bit.c          |   17 +-
 db/bit.h          |    5 -
 db/command.c      |    2 
 db/fuzz.c         |  461 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 db/fuzz.h         |   21 ++
 db/io.c           |    9 +
 db/io.h           |    1 
 db/type.c         |   44 ++++-
 db/type.h         |    1 
 man/man8/xfs_db.8 |   55 ++++++
 11 files changed, 598 insertions(+), 21 deletions(-)
 create mode 100644 db/fuzz.c
 create mode 100644 db/fuzz.h



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Patch
diff mbox

diff --git a/db/Makefile b/db/Makefile
index cdc0b99..feeacf6 100644
--- a/db/Makefile
+++ b/db/Makefile
@@ -12,7 +12,8 @@  HFILES = addr.h agf.h agfl.h agi.h attr.h attrshort.h bit.h block.h bmap.h \
 	dir2.h dir2sf.h dquot.h echo.h faddr.h field.h \
 	flist.h fprint.h frag.h freesp.h hash.h help.h init.h inode.h input.h \
 	io.h logformat.h malloc.h metadump.h output.h print.h quit.h sb.h \
-	 sig.h strvec.h text.h type.h write.h attrset.h symlink.h fsmap.h
+	 sig.h strvec.h text.h type.h write.h attrset.h symlink.h fsmap.h \
+	fuzz.h
 CFILES = $(HFILES:.h=.c)
 LSRCFILES = xfs_admin.sh xfs_ncheck.sh xfs_metadump.sh
 
diff --git a/db/bit.c b/db/bit.c
index 24872bf..3fcb085 100644
--- a/db/bit.c
+++ b/db/bit.c
@@ -19,13 +19,8 @@ 
 #include "libxfs.h"
 #include "bit.h"
 
-#undef setbit	/* defined in param.h on Linux */
-
-static int	getbit(char *ptr, int bit);
-static void	setbit(char *ptr, int bit, int val);
-
-static int
-getbit(
+int
+getbit_l(
 	char	*ptr,
 	int	bit)
 {
@@ -39,8 +34,8 @@  getbit(
 	return (*ptr & mask) >> shift;
 }
 
-static void
-setbit(
+void
+setbit_l(
 	char *ptr,
 	int  bit,
 	int  val)
@@ -106,7 +101,7 @@  getbitval(
 
 
 	for (i = 0, rval = 0LL; i < nbits; i++) {
-		if (getbit(p, bit + i)) {
+		if (getbit_l(p, bit + i)) {
 			/* If the last bit is on and we care about sign
 			 * bits and we don't have a full 64 bit
 			 * container, turn all bits on between the
@@ -162,7 +157,7 @@  setbitval(
 
 	if (bitoff % NBBY || nbits % NBBY) {
 		for (bit = 0; bit < nbits; bit++)
-			setbit(out, bit + bitoff, getbit(in, bit));
+			setbit_l(out, bit + bitoff, getbit_l(in, bit));
 	} else
 		memcpy(out + byteize(bitoff), in, byteize(nbits));
 }
diff --git a/db/bit.h b/db/bit.h
index 80ba24c..4506679 100644
--- a/db/bit.h
+++ b/db/bit.h
@@ -21,9 +21,12 @@ 
 #define	bitszof(x,y)	bitize(szof(x,y))
 #define	byteize(s)	((s) / NBBY)
 #define	bitoffs(s)	((s) % NBBY)
+#define	byteize_up(s)	(((s) + NBBY - 1) / NBBY)
 
 #define	BVUNSIGNED	0
 #define	BVSIGNED	1
 
 extern __int64_t	getbitval(void *obj, int bitoff, int nbits, int flags);
-extern void             setbitval(void *obuf, int bitoff, int nbits, void *ibuf);
+extern void		setbitval(void *obuf, int bitoff, int nbits, void *ibuf);
+extern int		getbit_l(char *ptr, int bit);
+extern void		setbit_l(char *ptr, int bit, int val);
diff --git a/db/command.c b/db/command.c
index 3d7cfd7..0eb4944 100644
--- a/db/command.c
+++ b/db/command.c
@@ -51,6 +51,7 @@ 
 #include "dquot.h"
 #include "fsmap.h"
 #include "crc.h"
+#include "fuzz.h"
 
 cmdinfo_t	*cmdtab;
 int		ncmds;
@@ -146,4 +147,5 @@  init_commands(void)
 	type_init();
 	write_init();
 	dquot_init();
+	fuzz_init();
 }
diff --git a/db/fuzz.c b/db/fuzz.c
new file mode 100644
index 0000000..061ecd1
--- /dev/null
+++ b/db/fuzz.c
@@ -0,0 +1,461 @@ 
+/*
+ * Copyright (c) 2000-2002,2005 Silicon Graphics, Inc.
+ * All Rights Reserved.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it would be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write the Free Software Foundation,
+ * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+#include "libxfs.h"
+#include <ctype.h>
+#include <time.h>
+#include "bit.h"
+#include "block.h"
+#include "command.h"
+#include "type.h"
+#include "faddr.h"
+#include "fprint.h"
+#include "field.h"
+#include "flist.h"
+#include "io.h"
+#include "init.h"
+#include "output.h"
+#include "print.h"
+#include "write.h"
+#include "malloc.h"
+
+static int	fuzz_f(int argc, char **argv);
+static void     fuzz_help(void);
+
+static const cmdinfo_t	fuzz_cmd =
+	{ "fuzz", NULL, fuzz_f, 0, -1, 0, N_("[-c] [-d] field fuzzcmd..."),
+	  N_("fuzz values on disk"), fuzz_help };
+
+void
+fuzz_init(void)
+{
+	if (!expert_mode)
+		return;
+
+	add_command(&fuzz_cmd);
+	srand48(clock());
+}
+
+static void
+fuzz_help(void)
+{
+	dbprintf(_(
+"\n"
+" The 'fuzz' command fuzzes fields in any on-disk data structure.  For\n"
+" block fuzzing, see the 'blocktrash' or 'write' commands."
+"\n"
+" Examples:\n"
+"  Struct mode: 'fuzz core.uid zeroes'    - set an inode uid field to 0.\n"
+"               'fuzz crc ones'           - set a crc filed to all ones.\n"
+"               'fuzz bno[11] firstbit'   - set the high bit of a block array.\n"
+"               'fuzz keys[5].startblock add'    - increase a btree key value.\n"
+"               'fuzz uuid random'        - randomize the superblock uuid.\n"
+"\n"
+" In data mode type 'fuzz' by itself for a list of specific commands.\n\n"
+" Specifying the -c option will allow writes of invalid (corrupt) data with\n"
+" an invalid CRC. Specifying the -d option will allow writes of invalid data,\n"
+" but still recalculate the CRC so we are forced to check and detect the\n"
+" invalid data appropriately.\n\n"
+));
+
+}
+
+static int
+fuzz_f(
+	int		argc,
+	char		**argv)
+{
+	pfunc_t	pf;
+	extern char *progname;
+	int c;
+	bool corrupt = false;	/* Allow write of bad data w/ invalid CRC */
+	bool invalid_data = false; /* Allow write of bad data w/ valid CRC */
+	struct xfs_buf_ops local_ops;
+	const struct xfs_buf_ops *stashed_ops = NULL;
+
+	if (x.isreadonly & LIBXFS_ISREADONLY) {
+		dbprintf(_("%s started in read only mode, fuzzing disabled\n"),
+			progname);
+		return 0;
+	}
+
+	if (cur_typ == NULL) {
+		dbprintf(_("no current type\n"));
+		return 0;
+	}
+
+	pf = cur_typ->pfunc;
+	if (pf == NULL) {
+		dbprintf(_("no handler function for type %s, fuzz unsupported.\n"),
+			 cur_typ->name);
+		return 0;
+	}
+
+	while ((c = getopt(argc, argv, "cd")) != EOF) {
+		switch (c) {
+		case 'c':
+			corrupt = true;
+			break;
+		case 'd':
+			invalid_data = true;
+			break;
+		default:
+			dbprintf(_("bad option for fuzz command\n"));
+			return 0;
+		}
+	}
+
+	if (corrupt && invalid_data) {
+		dbprintf(_("Cannot specify both -c and -d options\n"));
+		return 0;
+	}
+
+	if (invalid_data && iocur_top->typ->crc_off == TYP_F_NO_CRC_OFF &&
+			!iocur_top->ino_buf) {
+		dbprintf(_("Cannot recalculate CRCs on this type of object\n"));
+		return 0;
+	}
+
+	argc -= optind;
+	argv += optind;
+
+	/*
+	 * If the buffer has no verifier or we are using standard verifier
+	 * paths, then just fuzz it and return
+	 */
+	if (!iocur_top->bp->b_ops ||
+	    !(corrupt || invalid_data)) {
+		(*pf)(DB_FUZZ, cur_typ->fields, argc, argv);
+		return 0;
+	}
+
+
+	/* Temporarily remove write verifier to write bad data */
+	stashed_ops = iocur_top->bp->b_ops;
+	local_ops.verify_read = stashed_ops->verify_read;
+	iocur_top->bp->b_ops = &local_ops;
+
+	if (corrupt) {
+		local_ops.verify_write = xfs_dummy_verify;
+		dbprintf(_("Allowing fuzz of corrupted data and bad CRC\n"));
+	} else if (iocur_top->ino_buf) {
+		local_ops.verify_write = xfs_verify_recalc_inode_crc;
+		dbprintf(_("Allowing fuzz of corrupted inode with good CRC\n"));
+	} else { /* invalid data */
+		local_ops.verify_write = xfs_verify_recalc_crc;
+		dbprintf(_("Allowing fuzz of corrupted data with good CRC\n"));
+	}
+
+	(*pf)(DB_FUZZ, cur_typ->fields, argc, argv);
+
+	iocur_top->bp->b_ops = stashed_ops;
+
+	return 0;
+}
+
+/* Write zeroes to the field */
+static bool
+fuzz_zeroes(
+	void		*buf,
+	int		bitoff,
+	int		nbits)
+{
+	char		*out = buf;
+	int		bit;
+
+	if (bitoff % NBBY || nbits % NBBY) {
+		for (bit = 0; bit < nbits; bit++)
+			setbit_l(out, bit + bitoff, 0);
+	} else
+		memset(out + byteize(bitoff), 0, byteize(nbits));
+	return true;
+}
+
+/* Write ones to the field */
+static bool
+fuzz_ones(
+	void		*buf,
+	int		bitoff,
+	int		nbits)
+{
+	char		*out = buf;
+	int		bit;
+
+	if (bitoff % NBBY || nbits % NBBY) {
+		for (bit = 0; bit < nbits; bit++)
+			setbit_l(out, bit + bitoff, 1);
+	} else
+		memset(out + byteize(bitoff), 0xFF, byteize(nbits));
+	return true;
+}
+
+/* Flip the high bit in the (presumably big-endian) field */
+static bool
+fuzz_firstbit(
+	void		*buf,
+	int		bitoff,
+	int		nbits)
+{
+	setbit_l((char *)buf, bitoff, !getbit_l((char *)buf, bitoff));
+	return true;
+}
+
+/* Flip the low bit in the (presumably big-endian) field */
+static bool
+fuzz_lastbit(
+	void		*buf,
+	int		bitoff,
+	int		nbits)
+{
+	setbit_l((char *)buf, bitoff + nbits - 1,
+			!getbit_l((char *)buf, bitoff));
+	return true;
+}
+
+/* Flip the middle bit in the (presumably big-endian) field */
+static bool
+fuzz_middlebit(
+	void		*buf,
+	int		bitoff,
+	int		nbits)
+{
+	setbit_l((char *)buf, bitoff + nbits / 2,
+			!getbit_l((char *)buf, bitoff));
+	return true;
+}
+
+/* Format and shift a number into a buffer for setbitval. */
+static char *
+format_number(
+	uint64_t	val,
+	__be64		*out,
+	int		bit_length)
+{
+	int		offset;
+	char		*rbuf = (char *)out;
+
+	/*
+	 * If the length of the field is not a multiple of a byte, push
+	 * the bits up in the field, so the most signicant field bit is
+	 * the most significant bit in the byte:
+	 *
+	 * before:
+	 * val  |----|----|----|----|----|--MM|mmmm|llll|
+	 * after
+	 * val  |----|----|----|----|----|MMmm|mmll|ll00|
+	 */
+	offset = bit_length % NBBY;
+	if (offset)
+		val <<= (NBBY - offset);
+
+	/*
+	 * convert to big endian and copy into the array
+	 * rbuf |----|----|----|----|----|MMmm|mmll|ll00|
+	 */
+	*out = cpu_to_be64(val);
+
+	/*
+	 * Align the array to point to the field in the array.
+	 *  rbuf  = |MMmm|mmll|ll00|
+	 */
+	offset = sizeof(__be64) - 1 - ((bit_length - 1) / sizeof(__be64));
+	return rbuf + offset;
+}
+
+/* Increase the value by some small prime number. */
+static bool
+fuzz_add(
+	void		*buf,
+	int		bitoff,
+	int		nbits)
+{
+	uint64_t	val;
+	__be64		out;
+	char		*b;
+
+	if (nbits > 64)
+		return false;
+
+	val = getbitval(buf, bitoff, nbits, BVUNSIGNED);
+	val += (nbits > 8 ? 2017 : 137);
+	b = format_number(val, &out, nbits);
+	setbitval(buf, bitoff, nbits, b);
+
+	return true;
+}
+
+/* Decrease the value by some small prime number. */
+static bool
+fuzz_sub(
+	void		*buf,
+	int		bitoff,
+	int		nbits)
+{
+	uint64_t	val;
+	__be64		out;
+	char		*b;
+
+	if (nbits > 64)
+		return false;
+
+	val = getbitval(buf, bitoff, nbits, BVUNSIGNED);
+	val -= (nbits > 8 ? 2017 : 137);
+	b = format_number(val, &out, nbits);
+	setbitval(buf, bitoff, nbits, b);
+
+	return true;
+}
+
+/* Randomize the field. */
+static bool
+fuzz_random(
+	void		*buf,
+	int		bitoff,
+	int		nbits)
+{
+	int		i, bytes;
+	char		*b, *rbuf;
+
+	bytes = byteize_up(nbits);
+	rbuf = b = malloc(bytes);
+	if (!b) {
+		perror("fuzz_random");
+		return false;
+	}
+
+	for (i = 0; i < bytes; i++)
+		*b++ = (char)lrand48();
+
+	setbitval(buf, bitoff, nbits, rbuf);
+	free(rbuf);
+
+	return true;
+}
+
+struct fuzzcmd {
+	const char	*verb;
+	bool		(*fn)(void *buf, int bitoff, int nbits);
+};
+
+/* Keep these verbs in sync with enum fuzzcmds. */
+static struct fuzzcmd fuzzverbs[] = {
+	{"zeroes",		fuzz_zeroes},
+	{"ones",		fuzz_ones},
+	{"firstbit",		fuzz_firstbit},
+	{"middlebit",		fuzz_middlebit},
+	{"lastbit",		fuzz_lastbit},
+	{"add",			fuzz_add},
+	{"sub",			fuzz_sub},
+	{"random",		fuzz_random},
+	{NULL,			NULL},
+};
+
+/* ARGSUSED */
+void
+fuzz_struct(
+	const field_t	*fields,
+	int		argc,
+	char		**argv)
+{
+	const ftattr_t	*fa;
+	flist_t		*fl;
+	flist_t		*sfl;
+	int		bit_length;
+	struct fuzzcmd	*fc;
+	bool		success;
+	int		parentoffset;
+
+	if (argc != 2) {
+		dbprintf(_("Usage: fuzz fieldname verb\n"));
+		dbprintf("Verbs: %s", fuzzverbs->verb);
+		for (fc = fuzzverbs + 1; fc->verb != NULL; fc++)
+			dbprintf(", %s", fc->verb);
+		dbprintf(".\n");
+		return;
+	}
+
+	fl = flist_scan(argv[0]);
+	if (!fl) {
+		dbprintf(_("unable to parse '%s'.\n"), argv[0]);
+		return;
+	}
+
+	/* Find our fuzz verb */
+	for (fc = fuzzverbs; fc->verb != NULL; fc++)
+		if (!strcmp(fc->verb, argv[1]))
+			break;
+	if (fc->fn == NULL) {
+		dbprintf(_("Unknown fuzz command '%s'.\n"), argv[1]);
+		return;
+	}
+
+	/* if we're a root field type, go down 1 layer to get field list */
+	if (fields->name[0] == '\0') {
+		fa = &ftattrtab[fields->ftyp];
+		ASSERT(fa->ftyp == fields->ftyp);
+		fields = fa->subfld;
+	}
+
+	/* run down the field list and set offsets into the data */
+	if (!flist_parse(fields, fl, iocur_top->data, 0)) {
+		flist_free(fl);
+		dbprintf(_("parsing error\n"));
+		return;
+	}
+
+	sfl = fl;
+	parentoffset = 0;
+	while (sfl->child) {
+		parentoffset = sfl->offset;
+		sfl = sfl->child;
+	}
+
+	/*
+	 * For structures, fsize * fcount tells us the size of the region we are
+	 * modifying, which is usually a single structure member and is pointed
+	 * to by the last child in the list.
+	 *
+	 * However, if the base structure is an array and we have a direct index
+	 * into the array (e.g. write bno[5]) then we are returned a single
+	 * flist object with the offset pointing directly at the location we
+	 * need to modify. The length of the object we are modifying is then
+	 * determined by the size of the individual array entry (fsize) and the
+	 * indexes defined in the object, not the overall size of the array
+	 * (which is what fcount returns).
+	 */
+	bit_length = fsize(sfl->fld, iocur_top->data, parentoffset, 0);
+	if (sfl->fld->flags & FLD_ARRAY)
+		bit_length *= sfl->high - sfl->low + 1;
+	else
+		bit_length *= fcount(sfl->fld, iocur_top->data, parentoffset);
+
+	/* Fuzz the value */
+	success = fc->fn(iocur_top->data, sfl->offset, bit_length);
+	if (!success) {
+		dbprintf(_("unable to fuzz field '%s'\n"), argv[0]);
+		flist_free(fl);
+		return;
+	}
+
+	/* Write the fuzzed value back */
+	write_cur();
+
+	flist_print(fl);
+	print_flist(fl);
+	flist_free(fl);
+}
diff --git a/db/fuzz.h b/db/fuzz.h
new file mode 100644
index 0000000..c203eb5
--- /dev/null
+++ b/db/fuzz.h
@@ -0,0 +1,21 @@ 
+/*
+ * Copyright (C) 2017 Oracle.  All Rights Reserved.
+ *
+ * Author: Darrick J. Wong <darrick.wong@oracle.com>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it would be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write the Free Software Foundation,
+ * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301, USA.
+ */
+extern void	fuzz_init(void);
+extern void	fuzz_struct(const field_t *fields, int argc, char **argv);
diff --git a/db/io.c b/db/io.c
index f398195..1f316d8 100644
--- a/db/io.c
+++ b/db/io.c
@@ -465,6 +465,15 @@  xfs_dummy_verify(
 }
 
 void
+xfs_verify_recalc_inode_crc(
+	struct xfs_buf *bp)
+{
+	ASSERT(iocur_top->ino_buf);
+	libxfs_dinode_calc_crc(mp, iocur_top->data);
+	iocur_top->ino_crc_ok = 1;
+}
+
+void
 xfs_verify_recalc_crc(
 	struct xfs_buf *bp)
 {
diff --git a/db/io.h b/db/io.h
index c69e9ce..12d96c2 100644
--- a/db/io.h
+++ b/db/io.h
@@ -64,6 +64,7 @@  extern void	set_cur(const struct typ *t, __int64_t d, int c, int ring_add,
 extern void     ring_add(void);
 extern void	set_iocur_type(const struct typ *t);
 extern void	xfs_dummy_verify(struct xfs_buf *bp);
+extern void	xfs_verify_recalc_inode_crc(struct xfs_buf *bp);
 extern void	xfs_verify_recalc_crc(struct xfs_buf *bp);
 
 /*
diff --git a/db/type.c b/db/type.c
index 10fa54e..adab10a 100644
--- a/db/type.c
+++ b/db/type.c
@@ -39,6 +39,7 @@ 
 #include "dir2.h"
 #include "text.h"
 #include "symlink.h"
+#include "fuzz.h"
 
 static const typ_t	*findtyp(char *name);
 static int		type_f(int argc, char **argv);
@@ -254,10 +255,17 @@  handle_struct(
 	int           argc,
 	char          **argv)
 {
-	if (action == DB_WRITE)
+	switch (action) {
+	case DB_FUZZ:
+		fuzz_struct(fields, argc, argv);
+		break;
+	case DB_WRITE:
 		write_struct(fields, argc, argv);
-	else
+		break;
+	case DB_READ:
 		print_struct(fields, argc, argv);
+		break;
+	}
 }
 
 void
@@ -267,10 +275,17 @@  handle_string(
 	int           argc,
 	char          **argv)
 {
-	if (action == DB_WRITE)
+	switch (action) {
+	case DB_WRITE:
 		write_string(fields, argc, argv);
-	else
+		break;
+	case DB_READ:
 		print_string(fields, argc, argv);
+		break;
+	case DB_FUZZ:
+		dbprintf(_("string fuzzing not supported.\n"));
+		break;
+	}
 }
 
 void
@@ -280,10 +295,17 @@  handle_block(
 	int           argc,
 	char          **argv)
 {
-	if (action == DB_WRITE)
+	switch (action) {
+	case DB_WRITE:
 		write_block(fields, argc, argv);
-	else
+		break;
+	case DB_READ:
 		print_block(fields, argc, argv);
+		break;
+	case DB_FUZZ:
+		dbprintf(_("use 'blocktrash' or 'write' to fuzz a block.\n"));
+		break;
+	}
 }
 
 void
@@ -293,6 +315,14 @@  handle_text(
 	int           argc,
 	char          **argv)
 {
-	if (action != DB_WRITE)
+	switch (action) {
+	case DB_FUZZ:
+		/* fall through */
+	case DB_WRITE:
+		dbprintf(_("text writing/fuzzing not supported.\n"));
+		break;
+	case DB_READ:
 		print_text(fields, argc, argv);
+		break;
+	}
 }
diff --git a/db/type.h b/db/type.h
index 87ff107..a50d705 100644
--- a/db/type.h
+++ b/db/type.h
@@ -30,6 +30,7 @@  typedef enum typnm
 	TYP_TEXT, TYP_FINOBT, TYP_NONE
 } typnm_t;
 
+#define DB_FUZZ  2
 #define DB_WRITE 1
 #define DB_READ  0
 
diff --git a/man/man8/xfs_db.8 b/man/man8/xfs_db.8
index 460d89d..55e0629 100644
--- a/man/man8/xfs_db.8
+++ b/man/man8/xfs_db.8
@@ -594,6 +594,55 @@  in units of 512-byte blocks, no matter what the filesystem's block size is.
 .BI "The optional " start " and " end " arguments can be used to constrain
 the output to a particular range of disk blocks.
 .TP
+.BI "fuzz [\-c] [\-d] " "field action"
+Write garbage into a specific structure field on disk.
+Expert mode must be enabled to use this command.
+The operation happens immediately; there is no buffering.
+.IP
+The fuzz command can take the following
+.IR action "s"
+against a field:
+.RS 1.0i
+.TP 0.4i
+.B zeroes
+Clears all bits in the field.
+.TP 0.4i
+.B ones
+Sets all bits in the field.
+.TP 0.4i
+.B firstbit
+Flips the first bit in the field.
+For a scalar value, this is the highest bit.
+.TP 0.4i
+.B middlebit
+Flips the middle bit in the field.
+.TP 0.4i
+.B lastbit
+Flips the last bit in the field.
+For a scalar value, this is the lowest bit.
+.TP 0.4i
+.B add
+Adds a small value to a scalar field.
+.TP 0.4i
+.B sub
+Subtracts a small value from a scalar field.
+.TP 0.4i
+.B random
+Randomizes the contents of the field.
+.RE
+.IP
+The following switches affect the write behavior:
+.RS 1.0i
+.TP 0.4i
+.B \-c
+Skip write verifiers and CRC recalculation; allows invalid data to be written
+to disk.
+.TP 0.4i
+.B \-d
+Skip write verifiers but perform CRC recalculation; allows invalid data to be
+written to disk to test detection of invalid data.
+.RE
+.TP
 .BI hash " string
 Prints the hash value of
 .I string
@@ -755,7 +804,7 @@  and
 bits respectively, and their string equivalent reported
 (but no modifications are made).
 .TP
-.BI "write [\-c] [" "field value" "] ..."
+.BI "write [\-c] [\-d] [" "field value" "] ..."
 Write a value to disk.
 Specific fields can be set in structures (struct mode),
 or a block can be set to data values (data mode),
@@ -778,6 +827,10 @@  with no arguments gives more information on the allowed commands.
 .B \-c
 Skip write verifiers and CRC recalculation; allows invalid data to be written
 to disk.
+.TP 0.4i
+.B \-d
+Skip write verifiers but perform CRC recalculation; allows invalid data to be
+written to disk to test detection of invalid data.
 .RE
 .SH TYPES
 This section gives the fields in each structure type and their meanings.