bcachefs: Check for directories with no backpointers

It's legal for regular files to have missing backpointers (due to
hardlinks), and fsck should automatically add them, but for directories
this is an error that should be flagged.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fsck.c

@@ -1826,6 +1826,21 @@ static int check_dirent_inode_dirent(struct btree_trans *trans,
 	if (inode_points_to_dirent(target, d))
 		return 0;
 
+	if (!target->bi_dir &&
+	    !target->bi_dir_offset) {
+		fsck_err_on(S_ISDIR(target->bi_mode),
+			    trans, inode_dir_missing_backpointer,
+			    "directory with missing backpointer\n%s",
+			    (bch2_bkey_val_to_text(&buf, c, d.s_c),
+			     prt_printf(&buf, "\n  "),
+			     bch2_inode_unpacked_to_text(&buf, target),
+			     buf.buf));
+
+		target->bi_dir		= d.k->p.inode;
+		target->bi_dir_offset	= d.k->p.offset;
+		return __bch2_fsck_write_inode(trans, target, target_snapshot);
+	}
+
 	if (bch2_inode_should_have_bp(target) &&
 	    !fsck_err(trans, inode_wrong_backpointer,
 		      "dirent points to inode that does not point back:\n  %s",
@@ -1835,13 +1850,6 @@ static int check_dirent_inode_dirent(struct btree_trans *trans,
 		       buf.buf)))
 		goto err;
 
-	if (!target->bi_dir &&
-	    !target->bi_dir_offset) {
-		target->bi_dir		= d.k->p.inode;
-		target->bi_dir_offset	= d.k->p.offset;
-		return __bch2_fsck_write_inode(trans, target, target_snapshot);
-	}
-
 	struct bkey_s_c_dirent bp_dirent = dirent_get_by_pos(trans, &bp_iter,
 			      SPOS(target->bi_dir, target->bi_dir_offset, target_snapshot));
 	ret = bkey_err(bp_dirent);

fs/bcachefs/sb-errors_format.h

@@ -219,6 +219,7 @@ enum bch_fsck_flags {
 	x(inode_i_sectors_wrong,				204,	FSCK_AUTOFIX)	\
 	x(inode_dir_wrong_nlink,				205,	FSCK_AUTOFIX)	\
 	x(inode_dir_multiple_links,				206,	FSCK_AUTOFIX)	\
+	x(inode_dir_missing_backpointer,			284,	FSCK_AUTOFIX)	\
 	x(inode_multiple_links_but_nlink_0,			207,	FSCK_AUTOFIX)	\
 	x(inode_wrong_backpointer,				208,	FSCK_AUTOFIX)	\
 	x(inode_wrong_nlink,					209,	FSCK_AUTOFIX)	\
@@ -295,7 +296,7 @@ enum bch_fsck_flags {
 	x(accounting_key_replicas_devs_unsorted,		280,	FSCK_AUTOFIX)	\
 	x(accounting_key_version_0,				282,	FSCK_AUTOFIX)	\
 	x(logged_op_but_clean,					283,	FSCK_AUTOFIX)	\
-	x(MAX,							284,	0)
+	x(MAX,							285,	0)
 
 enum bch_sb_error_id {
 #define x(t, n, ...) BCH_FSCK_ERR_##t = n,