OpenZFS 7163 - ztest failures due to excess error injection

Authored by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>

OpenZFS-issue: https://www.illumos.org/issues/7163
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/f34284d
Closes #4484 
Closes #5661

cmd/ztest/ztest.c

@@ -5214,7 +5214,7 @@ ztest_fault_inject(ztest_ds_t *zd, uint64_t id)
 	char *path0;
 	char *pathrand;
 	size_t fsize;
-	int bshift = SPA_MAXBLOCKSHIFT + 2;	/* don't scrog all labels */
+	int bshift = SPA_MAXBLOCKSHIFT + 2;
 	int iters = 1000;
 	int maxfaults;
 	int mirror_save;
@@ -5407,7 +5407,29 @@ ztest_fault_inject(ztest_ds_t *zd, uint64_t id)
 		    (leaves << bshift) + (leaf << bshift) +
 		    (ztest_random(1ULL << (bshift - 1)) & -8ULL);
 
-		if (offset >= fsize)
+		/*
+		 * Only allow damage to the labels at one end of the vdev.
+		 *
+		 * If all labels are damaged, the device will be totally
+		 * inaccessible, which will result in loss of data,
+		 * because we also damage (parts of) the other side of
+		 * the mirror/raidz.
+		 *
+		 * Additionally, we will always have both an even and an
+		 * odd label, so that we can handle crashes in the
+		 * middle of vdev_config_sync().
+		 */
+		if ((leaf & 1) == 0 && offset < VDEV_LABEL_START_SIZE)
+			continue;
+
+		/*
+		 * The two end labels are stored at the "end" of the disk, but
+		 * the end of the disk (vdev_psize) is aligned to
+		 * sizeof (vdev_label_t).
+		 */
+		uint64_t psize = P2ALIGN(fsize, sizeof (vdev_label_t));
+		if ((leaf & 1) == 1 &&
+		    offset + sizeof (bad) > psize - VDEV_LABEL_END_SIZE)
 			continue;
 
 		mutex_enter(&ztest_vdev_lock);