[PATCH block/for-5.2-fixes] blkcg: update blkcg_print_stat() to handle larger outputs

[Tejun Heo]

Depending on the number of devices, blkcg stats can go over the
default seqfile buf size.  seqfile normally retries with a larger
buffer but since the ->pd_stat() addition, blkcg_print_stat() doesn't
tell seqfile that overflow has happened and the output gets printed
truncated.  Fix it by calling seq_commit() w/ -1 on possible
overflows.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Fixes: 903d23f0a354 ("blk-cgroup: allow controllers to output their own stats")
Cc: stable@xxxxxxxxxxxxxxx # v4.19+
Cc: Josef Bacik <jbacik@xxxxxx>
---
 block/blk-cgroup.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1005,8 +1005,12 @@ static int blkcg_print_stat(struct seq_f
 		}
 next:
 		if (has_stats) {
-			off += scnprintf(buf+off, size-off, "\n");
-			seq_commit(sf, off);
+			if (off < size - 1) {
+				off += scnprintf(buf+off, size-off, "\n");
+				seq_commit(sf, off);
+			} else {
+				seq_commit(sf, -1);
+			}
 		}
 	}