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

@@ -1826,22 +1826,30 @@ static int check_dirent_inode_dirent(struct btree_trans *trans,
 	if (inode_points_to_dirent(target, d))
 		return 0;
 
-	if (bch2_inode_should_have_bp(target) &&
-	    !fsck_err(trans, inode_wrong_backpointer,
-		      "dirent points to inode that does not point back:\n  %s",
+	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)))
-		goto err;
+			     buf.buf));
 
-	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);
 	}
 
+	if (bch2_inode_should_have_bp(target) &&
+	    !fsck_err(trans, inode_wrong_backpointer,
+		      "dirent points to inode that does not point back:\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)))
+		goto err;
+
 	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,