[PATCH v2 04/14] 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 returning 0 in such a case and freeing the page array if
allocated here rather than being passed in.

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
Link: https://sashiko.dev/#/patchset/20260616100821.2062304-1-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 | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 243662af1af7..5f60733f3280 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1568,6 +1568,7 @@ static ssize_t iov_iter_extract_xarray_pages(struct iov_iter *i,
 	struct folio *folio;
 	unsigned int nr = 0, offset;
 	loff_t pos = i->xarray_start + i->iov_offset;
+	bool will_alloc = !*pages;
 	XA_STATE(xas, i->xarray, pos >> PAGE_SHIFT);
 
 	offset = pos & ~PAGE_MASK;
@@ -1595,6 +1596,14 @@ static ssize_t iov_iter_extract_xarray_pages(struct iov_iter *i,
 	}
 	rcu_read_unlock();
 
+	if (!nr) {
+		if (will_alloc) {
+			kvfree(*pages);
+			*pages = NULL;
+		}
+		return 0;
+	}
+
 	maxsize = min_t(size_t, nr * PAGE_SIZE - offset, maxsize);
 	iov_iter_advance(i, maxsize);
 	return maxsize;