[PATCH v4 03/30] iov_iter: Fix potential underflow in iov_iter_extract_xarray_pages()

[David Howells <dhowells@redhat.com>]

In iov_iter_extract_xarray_pages(), if no pages are extracted because
there's a hole (or something otherwise unextractable) in the xarray, then
the calculation of maxsize at the end can go wrong if the starting offset
is not zero.

Fix this by setting maxsize to 0 if nr is 0.

Note that in the near future, ITER_XARRAY should be removed.

Fixes: 7d58fe731028 ("iov_iter: Add a function to extract a page list from an iterator")
Link: https://sashiko.dev/#/patchset/20260608145432.681865-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Paulo Alcantara <pc@manguebit.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: Christoph Hellwig <hch@infradead.org>
cc: Jens Axboe <axboe@kernel.dk>
cc: Mike Marshall <hubcap@omnibond.com>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
---
 lib/iov_iter.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 243662af1af7..dc9c6eb21bdb 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1595,7 +1595,10 @@ static ssize_t iov_iter_extract_xarray_pages(struct iov_iter *i,
 	}
 	rcu_read_unlock();
 
-	maxsize = min_t(size_t, nr * PAGE_SIZE - offset, maxsize);
+	if (nr > 0)
+		maxsize = min_t(size_t, nr * PAGE_SIZE - offset, maxsize);
+	else
+		maxsize = 0;
 	iov_iter_advance(i, maxsize);
 	return maxsize;
 }