OpenZFS 6842 - Fix empty xattr dir causing lockup

Reviewed by: Brian Behlendorf <[email protected]>
Reviewed by: Dan McDonald <[email protected]>
Reviewed by: Matthew Ahrens <[email protected]>
Approved by: Robert Mustacchi <[email protected]>
Ported-by: Denys Rtveliashvili <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>

An initial version of this patch was applied in commit 29572cc and
subsequently refined upstream.  Since the implementations do not
conflict with each other both are left applied for now.

OpenZFS-issue: https://www.illumos.org/issues/6842
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/02525cd
Closes #4615

1 files changed, 28 insertions, 10 deletions

diff --git a/module/zfs/zap_micro.c b/module/zfs/zap_micro.c
index 85b465b05..3faf27ce3 100644
--- a/module/zfs/zap_micro.c
+++ b/module/zfs/zap_micro.c
@@ -366,6 +366,9 @@ mzap_open(objset_t *os, uint64_t obj, dmu_buf_t *db)
 	zap_t *winner;
 	zap_t *zap;
 	int i;
+	uint64_t *zap_hdr = (uint64_t *)db->db_data;
+	uint64_t zap_block_type = zap_hdr[0];
+	uint64_t zap_magic = zap_hdr[1];
 
 	ASSERT3U(MZAP_ENT_LEN, ==, sizeof (mzap_ent_phys_t));
 
@@ -376,9 +379,13 @@ mzap_open(objset_t *os, uint64_t obj, dmu_buf_t *db)
 	zap->zap_object = obj;
 	zap->zap_dbuf = db;
 
-	if (*(uint64_t *)db->db_data != ZBT_MICRO) {
+	if (zap_block_type != ZBT_MICRO) {
 		mutex_init(&zap->zap_f.zap_num_entries_mtx, 0, 0, 0);
 		zap->zap_f.zap_block_shift = highbit64(db->db_size) - 1;
+		if (zap_block_type != ZBT_HEADER || zap_magic != ZAP_MAGIC) {
+			winner = NULL;	/* No actual winner here... */
+			goto handle_winner;
+		}
 	} else {
 		zap->zap_ismicro = TRUE;
 	}
@@ -391,14 +398,8 @@ mzap_open(objset_t *os, uint64_t obj, dmu_buf_t *db)
 	dmu_buf_init_user(&zap->zap_dbu, zap_evict, &zap->zap_dbuf);
 	winner = dmu_buf_set_user(db, &zap->zap_dbu);
 
-	if (winner != NULL) {
-		rw_exit(&zap->zap_rwlock);
-		rw_destroy(&zap->zap_rwlock);
-		if (!zap->zap_ismicro)
-			mutex_destroy(&zap->zap_f.zap_num_entries_mtx);
-		kmem_free(zap, sizeof (zap_t));
-		return (winner);
-	}
+	if (winner != NULL)
+		goto handle_winner;
 
 	if (zap->zap_ismicro) {
 		zap->zap_salt = zap_m_phys(zap)->mz_salt;
@@ -445,6 +446,14 @@ mzap_open(objset_t *os, uint64_t obj, dmu_buf_t *db)
 	}
 	rw_exit(&zap->zap_rwlock);
 	return (zap);
+
+handle_winner:
+	rw_exit(&zap->zap_rwlock);
+	rw_destroy(&zap->zap_rwlock);
+	if (!zap->zap_ismicro)
+		mutex_destroy(&zap->zap_f.zap_num_entries_mtx);
+	kmem_free(zap, sizeof (zap_t));
+	return (winner);
 }
 
 int
@@ -468,8 +477,17 @@ zap_lockdir(objset_t *os, uint64_t obj, dmu_tx_t *tx,
 		return (SET_ERROR(EINVAL));
 
 	zap = dmu_buf_get_user(db);
-	if (zap == NULL)
+	if (zap == NULL) {
 		zap = mzap_open(os, obj, db);
+		if (zap == NULL) {
+			/*
+			 * mzap_open() didn't like what it saw on-disk.
+			 * Check for corruption!
+			 */
+			dmu_buf_rele(db, NULL);
+			return (SET_ERROR(EIO));
+		}
+	}
 
 	/*
 	 * We're checking zap_ismicro without the lock held, in order to