fix braino in generic_file_read_iter()

Wrong sign of iov_iter_revert() argument.  Unfortunately, slipped through
the testing, since most of the time we don't do anything to the iterator
afterwards and potential oops on walking the iter->iov too far backwards
is too infrequent to be easily triggered.

Add a sanity check in iov_iter_revert() to catch bugs like this one;
fortunately, the same braino hadn't happened in other callers, but we'd
better have a warning if such thing crops up.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 1 insertions, 1 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index cc480c07c71b..d6e67be1802e 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2048,7 +2048,7 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
                         iocb->ki_pos += retval;
                         count -= retval;
                 }
-                iov_iter_revert(iter, iov_iter_count(iter) - count);
+                iov_iter_revert(iter, count - iov_iter_count(iter));
 
                 /*
                  * Btrfs can have a short DIO read if we encounter