bcachefs: check for inode.bi_sectors underflow

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: Kent Overstreet

fs/bcachefs/io_write.c

@@ -255,6 +255,27 @@ static inline int bch2_extent_update_i_size_sectors(struct btree_trans *trans,
 	}
 
 	if (i_sectors_delta) {
+		s64 bi_sectors = le64_to_cpu(inode->v.bi_sectors);
+		if (unlikely(bi_sectors + i_sectors_delta < 0)) {
+			struct bch_fs *c = trans->c;
+			struct printbuf buf = PRINTBUF;
+			bch2_log_msg_start(c, &buf);
+			prt_printf(&buf, "inode %llu i_sectors underflow: %lli + %lli < 0",
+				   extent_iter->pos.inode, bi_sectors, i_sectors_delta);
+
+			bool repeat = false, print = false, suppress = false;
+			bch2_count_fsck_err(c, inode_i_sectors_underflow, buf.buf,
+					    &repeat, &print, &suppress);
+			if (print)
+				bch2_print_str(c, buf.buf);
+			printbuf_exit(&buf);
+
+			if (i_sectors_delta < 0)
+				i_sectors_delta = -bi_sectors;
+			else
+				i_sectors_delta = 0;
+		}
+
 		le64_add_cpu(&inode->v.bi_sectors, i_sectors_delta);
 		inode_update_flags = 0;
 	}

fs/bcachefs/sb-errors_format.h

@@ -236,6 +236,7 @@ enum bch_fsck_flags {
 	x(inode_has_child_snapshots_wrong,			287,	0)		\
 	x(inode_unreachable,					210,	FSCK_AUTOFIX)	\
 	x(inode_journal_seq_in_future,				299,	FSCK_AUTOFIX)	\
+	x(inode_i_sectors_underflow,				312,	FSCK_AUTOFIX)	\
 	x(vfs_inode_i_blocks_underflow,				311,	FSCK_AUTOFIX)	\
 	x(deleted_inode_but_clean,				211,	FSCK_AUTOFIX)	\
 	x(deleted_inode_missing,				212,	FSCK_AUTOFIX)	\
@@ -320,7 +321,7 @@ enum bch_fsck_flags {
 	x(dirent_stray_data_after_cf_name,			305,	0)		\
 	x(rebalance_work_incorrectly_set,			309,	FSCK_AUTOFIX)	\
 	x(rebalance_work_incorrectly_unset,			310,	FSCK_AUTOFIX)	\
-	x(MAX,							312,	0)
+	x(MAX,							313,	0)
 
 enum bch_sb_error_id {
 #define x(t, n, ...) BCH_FSCK_ERR_##t = n,