block: allow for_each_bvec to support zero len bvec

Block layer usually doesn't support or allow zero-length bvec. Since
commit fc6952cecf59 ("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: fc6952cecf59 ("iov_iter: use bvec iterator to implement iterate_bvec()")
Reported-by: syzbot <syzbot+61acc40a49a3e46e25ea@syzkaller.appspotmail.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Tested-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: <stable@vger.kernel.org>
Link: https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg2262077.html
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index ac0c7299d5b8a2aa203037cd9fbc825bf6c29e0c..dd74503f7e5ea997c9075c6fc1097e776af9642b 100644 (file)
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -117,11 +117,18 @@ static inline bool bvec_iter_advance(const struct bio_vec *bv,
        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)                          \