Both iov_iter_get_pages and iov_iter_get_pages_alloc return the number of bytes
of the iovec they could get the pages for.  When they cannot get any pages,
they're supposed to return 0, but when the start of the iovec isn't page
aligned, the calculation goes wrong and they return a negative value.  Fix both
functions.

In addition, change iov_iter_get_pages_alloc to return NULL in that case to
prevent resource leaks.

It seems that the cifs and nfs filesystems don't handle the zero case very
well.  Could the maintainers please have a look?

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
 lib/iov_iter.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index e23123ae3a13..25dfc48536d7 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c

@@ -1484,7 +1484,7 @@ ssize_t iov_iter_get_pages(struct iov_iter *i,
 		res = get_user_pages_fast(addr, n,
 				iov_iter_rw(i) != WRITE ?  FOLL_WRITE : 0,
 				pages);
-		if (unlikely(res < 0))
+		if (unlikely(res <= 0))
 			return res;
 		return (res == n ? len : res * PAGE_SIZE) - *start;
 	}

@@ -1608,8 +1608,9 @@ ssize_t iov_iter_get_pages_alloc(struct iov_iter *i,
 			return -ENOMEM;
 		res = get_user_pages_fast(addr, n,
 				iov_iter_rw(i) != WRITE ?  FOLL_WRITE : 0, p);
-		if (unlikely(res < 0)) {
+		if (unlikely(res <= 0)) {
 			kvfree(p);
+			*pages = NULL;
 			return res;
 		}
 		*pages = p;

-- 
2.26.3