[PATCH 4.19 268/271] blkcg: update blkcg_print_stat() to handle larger outputs

[Greg Kroah-Hartman]

From: Tejun Heo <t...@kernel.org>

commit f539da82f2158916e154d206054e0efd5df7ab61 upstream.

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 <t...@kernel.org>
Fixes: 903d23f0a354 ("blk-cgroup: allow controllers to output their own stats")
Cc: sta...@vger.kernel.org # v4.19+
Cc: Josef Bacik <jba...@fb.com>
Signed-off-by: Jens Axboe <ax...@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 block/blk-cgroup.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1016,8 +1016,12 @@ 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);
+                       }
                }
        }