[PATCH 5.4 107/129] block: allow for_each_bvec to support zero len bvec

[Greg Kroah-Hartman]

From: Ming Lei <ming.lei@xxxxxxxxxx>

commit 7e24969022cbd61ddc586f14824fc205661bb124 upstream.

Block layer usually doesn't support or allow zero-length bvec. Since
commit 1bdc76aea115 ("iov_iter: use bvec iterator to implement
iterate_bvec()"), iterate_bvec() switches to bvec iterator. However,
Al mentioned that 'Zero-length segments are not disallowed' in iov_iter.

Fixes for_each_bvec() so that it can move on after seeing one zero
length bvec.

Fixes: 1bdc76aea115 ("iov_iter: use bvec iterator to implement iterate_bvec()")
Reported-by: syzbot <syzbot+61acc40a49a3e46e25ea@xxxxxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
Tested-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Link: https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg2262077.html
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 include/linux/bvec.h |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -110,11 +110,18 @@ static inline bool bvec_iter_advance(con
 	return true;
 }
 
+static inline void bvec_iter_skip_zero_bvec(struct bvec_iter *iter)
+{
+	iter->bi_bvec_done = 0;
+	iter->bi_idx++;
+}
+
 #define for_each_bvec(bvl, bio_vec, iter, start)			\
 	for (iter = (start);						\
 	     (iter).bi_size &&						\
 		((bvl = bvec_iter_bvec((bio_vec), (iter))), 1);	\
-	     bvec_iter_advance((bio_vec), &(iter), (bvl).bv_len))
+	     (bvl).bv_len ? (void)bvec_iter_advance((bio_vec), &(iter),	\
+		     (bvl).bv_len) : bvec_iter_skip_zero_bvec(&(iter)))
 
 /* for iterating one bio from start to end */
 #define BVEC_ITER_ALL_INIT (struct bvec_iter)				\