fix iov_iter_advance() for ITER_PIPE

iov_iter_advance() needs to decrement iter->count by the number of
bytes we'd moved beyond.  Normal flavours do that, but ITER_PIPE
doesn't and ITER_PIPE generic_file_read_iter() for O_DIRECT files
ends up with a bogus fallback to page cache read, resulting in incorrect
values for file offset and bytes read.

Signed-off-by: Abhi Das <adas@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 3 insertions, 1 deletions

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index f0c7f1481bae..f2bd21b93dfc 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -683,10 +683,11 @@ static void pipe_advance(struct iov_iter *i, size_t size)
         struct pipe_inode_info *pipe = i->pipe;
         struct pipe_buffer *buf;
         int idx = i->idx;
-        size_t off = i->iov_offset;
+        size_t off = i->iov_offset, orig_sz;
         
         if (unlikely(i->count < size))
                 size = i->count;
+        orig_sz = size;
 
         if (size) {
                 if (off) /* make it relative to the beginning of buffer */
@@ -713,6 +714,7 @@ static void pipe_advance(struct iov_iter *i, size_t size)
                         pipe->nrbufs--;
                 }
         }
+        i->count -= orig_sz;
 }
 
 void iov_iter_advance(struct iov_iter *i, size_t size)