btrfs: handle non-fatal errors in btrfs_qgroup_inherit()

create_pending_snapshot() will go readonly on _any_ error return from
btrfs_qgroup_inherit(). If qgroups are enabled, a user can crash their fs by
just making a snapshot and asking it to inherit from an invalid qgroup. For
example:

$ btrfs sub snap -i 1/10 /btrfs/ /btrfs/foo

Will cause a transaction abort.

Fix this by only throwing errors in btrfs_qgroup_inherit() when we know
going readonly is acceptable.

The following xfstests test case reproduces this bug:

  seq=`basename $0`
  seqres=$RESULT_DIR/$seq
  echo "QA output created by $seq"

  here=`pwd`
  tmp=/tmp/$$
  status=1	# failure is the default!
  trap "_cleanup; exit \$status" 0 1 2 3 15

  _cleanup()
  {
  	cd /
  	rm -f $tmp.*
  }

  # get standard environment, filters and checks
  . ./common/rc
  . ./common/filter

  # remove previous $seqres.full before test
  rm -f $seqres.full

  # real QA test starts here
  _supported_fs btrfs
  _supported_os Linux
  _require_scratch

  rm -f $seqres.full

  _scratch_mkfs
  _scratch_mount
  _run_btrfs_util_prog quota enable $SCRATCH_MNT
  # The qgroup '1/10' does not exist and should be silently ignored
  _run_btrfs_util_prog subvolume snapshot -i 1/10 $SCRATCH_MNT $SCRATCH_MNT/snap1

  _scratch_unmount

  echo "Silence is golden"

  status=0
  exit
Signed-off-by: Mark Fasheh <mfasheh@suse.de>
Reviewed-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/qgroup.c

@@ -1842,8 +1842,10 @@ int btrfs_run_qgroups(struct btrfs_trans_handle *trans,
 }
 
 /*
- * copy the acounting information between qgroups. This is necessary when a
- * snapshot or a subvolume is created
+ * Copy the acounting information between qgroups. This is necessary
+ * when a snapshot or a subvolume is created. Throwing an error will
+ * cause a transaction abort so we take extra care here to only error
+ * when a readonly fs is a reasonable outcome.
  */
 int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans,
 			 struct btrfs_fs_info *fs_info, u64 srcid, u64 objectid,
@@ -1873,15 +1875,15 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans,
 		       2 * inherit->num_excl_copies;
 		for (i = 0; i < nums; ++i) {
 			srcgroup = find_qgroup_rb(fs_info, *i_qgroups);
-			if (!srcgroup) {
-				ret = -EINVAL;
-				goto out;
-			}
 
-			if ((srcgroup->qgroupid >> 48) <= (objectid >> 48)) {
-				ret = -EINVAL;
-				goto out;
-			}
+			/*
+			 * Zero out invalid groups so we can ignore
+			 * them later.
+			 */
+			if (!srcgroup ||
+			    ((srcgroup->qgroupid >> 48) <= (objectid >> 48)))
+				*i_qgroups = 0ULL;
+
 			++i_qgroups;
 		}
 	}
@@ -1916,17 +1918,19 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans,
 	 */
 	if (inherit) {
 		i_qgroups = (u64 *)(inherit + 1);
-		for (i = 0; i < inherit->num_qgroups; ++i) {
+		for (i = 0; i < inherit->num_qgroups; ++i, ++i_qgroups) {
+			if (*i_qgroups == 0)
+				continue;
 			ret = add_qgroup_relation_item(trans, quota_root,
 						       objectid, *i_qgroups);
-			if (ret)
+			if (ret && ret != -EEXIST)
 				goto out;
 			ret = add_qgroup_relation_item(trans, quota_root,
 						       *i_qgroups, objectid);
-			if (ret)
+			if (ret && ret != -EEXIST)
 				goto out;
-			++i_qgroups;
 		}
+		ret = 0;
 	}
 
 
@@ -1987,17 +1991,22 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans,
 
 	i_qgroups = (u64 *)(inherit + 1);
 	for (i = 0; i < inherit->num_qgroups; ++i) {
+		if (*i_qgroups) {
 			ret = add_relation_rb(quota_root->fs_info, objectid,
 					      *i_qgroups);
 			if (ret)
 				goto unlock;
+		}
 		++i_qgroups;
 	}
 
-	for (i = 0; i <  inherit->num_ref_copies; ++i) {
+	for (i = 0; i <  inherit->num_ref_copies; ++i, i_qgroups += 2) {
 		struct btrfs_qgroup *src;
 		struct btrfs_qgroup *dst;
 
+		if (!i_qgroups[0] || !i_qgroups[1])
+			continue;
+
 		src = find_qgroup_rb(fs_info, i_qgroups[0]);
 		dst = find_qgroup_rb(fs_info, i_qgroups[1]);
 
@@ -2008,12 +2017,14 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans,
 
 		dst->rfer = src->rfer - level_size;
 		dst->rfer_cmpr = src->rfer_cmpr - level_size;
-		i_qgroups += 2;
 	}
-	for (i = 0; i <  inherit->num_excl_copies; ++i) {
+	for (i = 0; i <  inherit->num_excl_copies; ++i, i_qgroups += 2) {
 		struct btrfs_qgroup *src;
 		struct btrfs_qgroup *dst;
 
+		if (!i_qgroups[0] || !i_qgroups[1])
+			continue;
+
 		src = find_qgroup_rb(fs_info, i_qgroups[0]);
 		dst = find_qgroup_rb(fs_info, i_qgroups[1]);
 
@@ -2024,7 +2035,6 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans,
 
 		dst->excl = src->excl + level_size;
 		dst->excl_cmpr = src->excl_cmpr + level_size;
-		i_qgroups += 2;
 	}
 
 unlock: