Further improve 'zpool labelclear' command

That patch follows:
  openzfs#8500
and is a port to ZoL of:
  openzfs/openzfs#424

It adds the following:
  - ability to clear a specific label (options -b, -e and -i)
  - label invalidation using a single-byte modification (wipe with option -w)
  - option -ff to force invalidation even for invalid labels

OpenZFS-issue: https://www.illumos.org/issues/7584

cmd/zpool/zpool_main.c

@@ -347,7 +347,8 @@ get_usage(zpool_help_t idx)
 		    "\t    [[pool ...]|[pool vdev ...]|[vdev ...]]"
 		    " [[-n] interval [count]]\n"));
 	case HELP_LABELCLEAR:
-		return (gettext("\tlabelclear [-f] <vdev>\n"));
+		return (gettext("\tlabelclear [-b | -e | -i index] [-w] "
+		    "[-f] <vdev>\n"));
 	case HELP_LIST:
 		return (gettext("\tlist [-gHLpPv] [-o property[,...]] "
 		    "[-T d|u] [pool] ... \n"
@@ -1038,12 +1039,22 @@ zpool_do_remove(int argc, char **argv)
 }
 
 /*
- * zpool labelclear [-f] <vdev>
+ * zpool labelclear [-b | -e | -i index] [-w] [-f] <vdev>
  *
- *	-f	Force clearing the label for the vdevs which are members of
- *		the exported or foreign pools.
+ *     -b              Only work on labels located at the beginning of the
+ *                     device.
  *
- * Verifies that the vdev is not active and zeros out the label information
+ *     -e              Only work on labels located at the end of the device.
+ *
+ *     -i index        Only work on label located at specified index.
+ *
+ *     -w              Wipe label area entirely and replace it with zeroes.
+ *
+ *     -f              Force clearing the label for the vdevs which are
+ *                     members of the exported or foreign pools. If specified
+ *                     twice, clear even seemingly-invalid labels.
+ *
+ * Verifies that the vdev is not active and invalidates the label information
  * on the device.
  */
 int
@@ -1057,11 +1068,42 @@ zpool_do_labelclear(int argc, char **argv)
 	pool_state_t state;
 	boolean_t inuse = B_FALSE;
 	boolean_t force = B_FALSE;
+	boolean_t force_invalid = B_FALSE;
+	boolean_t wipe = B_FALSE;
+	unsigned int start = 0, n = VDEV_LABELS;
+	char *end = NULL;
+	long long index = 0;
 
 	/* check options */
-	while ((c = getopt(argc, argv, "f")) != -1) {
+	while ((c = getopt(argc, argv, "bei:wf")) != -1) {
 		switch (c) {
+		case 'b':
+			start = 0;
+			n = VDEV_LABELS / 2;
+			break;
+		case 'e':
+			start = VDEV_LABELS / 2;
+			n = VDEV_LABELS / 2;
+			break;
+		case 'i':
+			index = strtoll(optarg, &end, 10);
+			if((end == optarg) || (*end != '\0') ||
+			    (index < 0) || (index >= VDEV_LABELS)) {
+				(void) fprintf(stderr,
+				gettext("Invalid index value provided\n"));
+				return (1);
+			}
+			start = (unsigned int)index;
+			n = 1;
+			break;
+		case 'w':
+			wipe = B_TRUE;
+			break;
 		case 'f':
+			if (force) {
+				/* -f specified twice */
+				force_invalid = B_TRUE;
+			}
 			force = B_TRUE;
 			break;
 		default:
@@ -1123,8 +1165,13 @@ zpool_do_labelclear(int argc, char **argv)
 		    "cache for %s: %s\n"), vdev, strerror(errno));
 
 	if (zpool_read_label(fd, &config, NULL) != 0) {
-		(void) fprintf(stderr,
-		    gettext("failed to read label from %s\n"), vdev);
+		if (force) {
+			goto wipe_label;
+		}
+		(void) fprintf(stderr, gettext(
+		    "use '-f' to override the following error:\n"
+		    "failed to read label from \"%s\"\n"),
+		    vdev);
 		ret = 1;
 		goto errout;
 	}
@@ -1179,7 +1226,7 @@ zpool_do_labelclear(int argc, char **argv)
 	}
 
 wipe_label:
-	ret = zpool_clear_label(fd);
+	ret = zpool_clear_n_labels(fd, start, n, force_invalid, wipe);
 	if (ret != 0) {
 		(void) fprintf(stderr,
 		    gettext("failed to clear label for %s\n"), vdev);

include/libzfs.h

@@ -808,6 +808,8 @@ extern int zpool_in_use(libzfs_handle_t *, int, pool_state_t *, char **,
  * Label manipulation.
  */
 extern int zpool_clear_label(int);
+extern int zpool_clear_n_labels(int, unsigned int, unsigned int, boolean_t,
+    boolean_t);
 
 /*
  * Management interfaces for SMB ACL files

include/sys/fs/zfs.h

@@ -748,6 +748,8 @@ typedef struct zpool_load_policy {
 #define	ZPOOL_CONFIG_LOAD_DATA_ERRORS	"verify_data_errors"
 #define	ZPOOL_CONFIG_REWIND_TIME	"seconds_of_rewind"
 
+#define	VDEV_LABELS				4
+
 #define	VDEV_TYPE_ROOT			"root"
 #define	VDEV_TYPE_MIRROR		"mirror"
 #define	VDEV_TYPE_REPLACING		"replacing"

include/sys/nvpair.h

@@ -94,6 +94,7 @@ typedef struct nvlist {
 #define	NV_VERSION	0
 
 /* nvlist pack encoding */
+#define	NV_ENCODE_INVALID	(-1)
 #define	NV_ENCODE_NATIVE	0
 #define	NV_ENCODE_XDR		1
 
@@ -150,6 +151,7 @@ void nv_alloc_fini(nv_alloc_t *);
 /* list management */
 int nvlist_alloc(nvlist_t **, uint_t, int);
 void nvlist_free(nvlist_t *);
+int nvlist_invalidate(char *, size_t);
 int nvlist_size(nvlist_t *, size_t *, int);
 int nvlist_pack(nvlist_t *, char **, size_t *, int, int);
 int nvlist_unpack(char *, size_t, nvlist_t **, int);

include/sys/vdev_impl.h

@@ -441,8 +441,6 @@ typedef struct vdev_label {
  */
 #define	VDEV_LABEL_START_SIZE	(2 * sizeof (vdev_label_t) + VDEV_BOOT_SIZE)
 #define	VDEV_LABEL_END_SIZE	(2 * sizeof (vdev_label_t))
-#define	VDEV_LABELS		4
-#define	VDEV_BEST_LABEL		VDEV_LABELS
 
 #define	VDEV_ALLOC_LOAD		0
 #define	VDEV_ALLOC_ADD		1

lib/libzfs/libzfs_import.c

@@ -137,16 +137,17 @@ label_offset(uint64_t size, int l)
 }
 
 /*
- * Given a file descriptor, clear (zero) the label information.  This function
- * is used in the appliance stack as part of the ZFS sysevent module and
- * to implement the "zpool labelclear" command.
+ * Given a file descriptor, a starting label and a number of labels to clear,
+ * invalidate or clear (zero) the label information. This function is used in
+ * the appliance stack as part of the ZFS sysevent module and to implement the
+ * "zpool labelclear" command.
  */
 int
-zpool_clear_label(int fd)
+zpool_clear_n_labels(int fd, unsigned int start, unsigned int n,
+    boolean_t force, boolean_t wipe)
 {
 	struct stat64 statbuf;
-	int l;
-	vdev_label_t *label;
+	unsigned int l, end;
 	uint64_t size;
 	int labels_cleared = 0;
 
@@ -155,62 +156,86 @@ zpool_clear_label(int fd)
 
 	size = P2ALIGN_TYPED(statbuf.st_size, sizeof (vdev_label_t), uint64_t);
 
-	if ((label = calloc(1, sizeof (vdev_label_t))) == NULL)
-		return (-1);
+	end = start + n;
+	if (end > VDEV_LABELS)
+	    return (-1);
+
+	for (l = start; l < end; l++) {
+		vdev_label_t label;
+		char *nvbuf = label.vl_vdev_phys.vp_nvlist;
+		size_t nvbuflen = sizeof (label.vl_vdev_phys.vp_nvlist);
 
-	for (l = 0; l < VDEV_LABELS; l++) {
 		uint64_t state, guid;
 		nvlist_t *config;
 
-		if (pread64(fd, label, sizeof (vdev_label_t),
-		    label_offset(size, l)) != sizeof (vdev_label_t)) {
-			continue;
-		}
-
-		if (nvlist_unpack(label->vl_vdev_phys.vp_nvlist,
-		    sizeof (label->vl_vdev_phys.vp_nvlist), &config, 0) != 0) {
-			continue;
-		}
+		if (!(force && wipe)) {
+			if (pread64(fd, &label, sizeof (vdev_label_t),
+			    label_offset(size, l)) != sizeof (vdev_label_t)) {
+				continue;
+			}
 
-		/* Skip labels which do not have a valid guid. */
-		if (nvlist_lookup_uint64(config, ZPOOL_CONFIG_GUID,
-		    &guid) != 0 || guid == 0) {
-			nvlist_free(config);
-			continue;
+			if (!force) {
+				if (nvlist_unpack(nvbuf, nvbuflen, &config, 0) != 0) {
+					continue;
+				}
+
+				/* Skip labels which do not have a valid guid. */
+				if (nvlist_lookup_uint64(config, ZPOOL_CONFIG_GUID,
+				    &guid) != 0 || guid == 0) {
+					nvlist_free(config);
+					continue;
+				}
+
+				/* Skip labels which are not in a known valid state. */
+				if (nvlist_lookup_uint64(config, ZPOOL_CONFIG_POOL_STATE,
+				    &state) != 0 || state > POOL_STATE_L2CACHE) {
+					nvlist_free(config);
+					continue;
+				}
+
+				nvlist_free(config);
+			}
 		}
 
-		/* Skip labels which are not in a known valid state. */
-		if (nvlist_lookup_uint64(config, ZPOOL_CONFIG_POOL_STATE,
-		    &state) != 0 || state > POOL_STATE_L2CACHE) {
-			nvlist_free(config);
-			continue;
+		if (wipe) {
+			/*
+			 * A valid label was found, overwrite this label's nvlist
+			 * and uberblocks with zeros on disk.  This is done to prevent
+			 * system utilities, like blkid, from incorrectly detecting a
+			 * partial label.  The leading pad space is left untouched.
+			 */
+			memset(&label, 0, sizeof (vdev_label_t));
+		} else {
+			/*
+			 * Perform minimal on-disk change
+			 */
+			if (nvlist_invalidate(nvbuf, nvbuflen) != 0)
+			    continue;
 		}
 
-		nvlist_free(config);
-
-		/*
-		 * A valid label was found, overwrite this label's nvlist
-		 * and uberblocks with zeros on disk.  This is done to prevent
-		 * system utilities, like blkid, from incorrectly detecting a
-		 * partial label.  The leading pad space is left untouched.
-		 */
-		memset(label, 0, sizeof (vdev_label_t));
 		size_t label_size = sizeof (vdev_label_t) - (2 * VDEV_PAD_SIZE);
 
-		if (pwrite64(fd, label, label_size, label_offset(size, l) +
-		    (2 * VDEV_PAD_SIZE)) == label_size) {
+		if (pwrite64(fd, (void*)&label + (2 * VDEV_PAD_SIZE), label_size,
+		    label_offset(size, l) + (2 * VDEV_PAD_SIZE)) == label_size) {
 			labels_cleared++;
 		}
 	}
 
-	free(label);
-
 	if (labels_cleared == 0)
 		return (-1);
 
 	return (0);
 }
 
+/*
+ * Given a file descriptor, clear (zero) the label information.
+ */
+int
+zpool_clear_label(int fd)
+{
+	return (zpool_clear_n_labels(fd, 0, VDEV_LABELS, B_TRUE, B_TRUE));
+}
+
 static boolean_t
 find_guid(nvlist_t *nv, uint64_t guid)
 {

man/man8/zpool.8

@@ -128,7 +128,8 @@
 .Op Ar interval Op Ar count
 .Nm
 .Cm labelclear
-.Op Fl f
+.Op Fl fw
+.Op Fl Sy b Ns | Ns Sy e Ns | Ns Sy i Ar index
 .Ar device
 .Nm
 .Cm list
@@ -1854,17 +1855,38 @@ will be sampled from the end of the interval.
 .It Xo
 .Nm
 .Cm labelclear
-.Op Fl f
+.Op Fl fw
+.Op Fl Sy b Ns | Ns Sy e Ns | Ns Sy i Ar index
 .Ar device
 .Xc
 Removes ZFS label information from the specified
 .Ar device .
 The
 .Ar device
 must not be part of an active pool configuration.
+Options
+.Fl b ,
+.Fl e
+and
+.Fl i
+can be used for clearing specific labels.
+They are mutually exclusive and the last one will supersede others, if any.
 .Bl -tag -width Ds
 .It Fl f
-Treat exported or foreign devices as inactive.
+Force mode: treat exported or foreign devices as inactive.
+Specify twice to clear even seemingly-invalid labels.
+.It Fl w
+Instead of invalidating the label information, wipe label area entirely and
+replace it with zeroes.
+.It Fl b
+Only remove ZFS labels located at the beginning of
+.Ar device .
+.It Fl e
+Only remove ZFS labels located at the end of
+.Ar device .
+.It Fl i Ar index
+Remove a single label entry located at position
+.Ar index .
 .El
 .It Xo
 .Nm

module/nvpair/nvpair.c

@@ -2629,6 +2629,18 @@ nvlist_common(nvlist_t *nvl, char *buf, size_t *buflen, int encoding,
 	return (err);
 }
 
+int
+nvlist_invalidate(char *buf, size_t buflen)
+{
+	if (buf == NULL || buflen < sizeof (nvs_header_t))
+	    return (EINVAL);
+
+	nvs_header_t *nvh = (void *)buf;
+	nvh->nvh_encoding = NV_ENCODE_INVALID;
+
+	return (0);
+}
+
 int
 nvlist_size(nvlist_t *nvl, size_t *size, int encoding)
 {
@@ -3656,6 +3668,7 @@ EXPORT_SYMBOL(nvlist_prev_nvpair);
 EXPORT_SYMBOL(nvlist_empty);
 EXPORT_SYMBOL(nvlist_add_hrtime);
 
+EXPORT_SYMBOL(nvlist_invalidate);
 EXPORT_SYMBOL(nvlist_remove);
 EXPORT_SYMBOL(nvlist_remove_nvpair);
 EXPORT_SYMBOL(nvlist_remove_all);