Re: splice: infinite busy loop lockup bug

[Ming Lei]

On Fri, Aug 07, 2020 at 01:38:54PM +0100, Al Viro wrote:
> On Fri, Aug 07, 2020 at 01:27:27PM +0100, Al Viro wrote:
> > On Fri, Aug 07, 2020 at 07:35:08PM +0900, Tetsuo Handa wrote:
> > > syzbot is reporting hung task at pipe_release() [1], for for_each_bvec() from
> > > iterate_bvec() from iterate_all_kinds() from iov_iter_alignment() from
> > > ext4_unaligned_io() from ext4_dio_write_iter() from ext4_file_write_iter() from
> > > call_write_iter() from do_iter_readv_writev() from do_iter_write() from
> > > vfs_iter_write() from iter_file_splice_write() falls into infinite busy loop
> > > with pipe->mutex held.
> > > 
> > > The reason of falling into infinite busy loop is that iter_file_splice_write()
> > > for some reason generates "struct bio_vec" entry with .bv_len=0 and .bv_offset=0
> > > while for_each_bvec() cannot handle .bv_len == 0.
> > 
> > broken in 1bdc76aea115 "iov_iter: use bvec iterator to implement iterate_bvec()",
> > unless I'm misreading it...
> > 
> > Zero-length segments are not disallowed; it's not all that hard to filter them
> > out in iter_file_splice_write(), but the intent had always been to have
> > iterate_all_kinds() et.al. able to cope with those.
> > 
> > How are these pipe_buffers with ->len == 0 generated in that reproducer, BTW?
> > There might be something else fishy going on...
> 
> FWIW, my preference would be to have for_each_bvec() advance past zero-length
> segments; I'll need to go through its uses elsewhere in the tree first, though
> (after I grab some sleep),

Usually block layer doesn't allow/support zero bvec, however we can make
for_each_bvec() to support it only.

Tetsuo, can you try the following patch?

diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index ac0c7299d5b8..b03c793dd28d 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -117,11 +117,19 @@ static inline bool bvec_iter_advance(const struct bio_vec *bv,
 	return true;
 }
 
+static inline void bvec_iter_skip_zero_vec(const struct bio_vec *bv,
+		struct bvec_iter *iter)
+{
+	iter->bi_idx++;
+	iter->bi_bvec_done = 0;
+}
+
 #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 = bvec_iter_bvec((bio_vec), (iter))), 1);		\
+	  (bvl).bv_len ? bvec_iter_advance((bio_vec), &(iter), (bvl).bv_len) : \
+			bvec_iter_skip_zero_vec((bio_vec), &(iter)))
 
 /* for iterating one bio from start to end */
 #define BVEC_ITER_ALL_INIT (struct bvec_iter)				\

Thanks,
Ming