iov_iter: Fix iov_iter_extract_pages() with zero-sized entries

iov_iter_extract_pages() doesn't correctly handle skipping over initial
zero-length entries in ITER_KVEC and ITER_BVEC-type iterators.

The problem is that it accidentally reduces maxsize to 0 when it
skipping and thus runs to the end of the array and returns 0.

Fix this by sticking the calculated size-to-copy in a new variable
rather than back in maxsize.

Fixes: 7d58fe7 ("iov_iter: Add a function to extract a page list from an iterator")
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: David Hildenbrand <david@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/iov_iter.c

@@ -1654,14 +1654,14 @@ static ssize_t iov_iter_extract_bvec_pages(struct iov_iter *i,
 					   size_t *offset0)
 {
 	struct page **p, *page;
-	size_t skip = i->iov_offset, offset;
+	size_t skip = i->iov_offset, offset, size;
 	int k;
 
 	for (;;) {
 		if (i->nr_segs == 0)
 			return 0;
-		maxsize = min(maxsize, i->bvec->bv_len - skip);
-		if (maxsize)
+		size = min(maxsize, i->bvec->bv_len - skip);
+		if (size)
 			break;
 		i->iov_offset = 0;
 		i->nr_segs--;
@@ -1674,16 +1674,16 @@ static ssize_t iov_iter_extract_bvec_pages(struct iov_iter *i,
 	offset = skip % PAGE_SIZE;
 	*offset0 = offset;
 
-	maxpages = want_pages_array(pages, maxsize, offset, maxpages);
+	maxpages = want_pages_array(pages, size, offset, maxpages);
 	if (!maxpages)
 		return -ENOMEM;
 	p = *pages;
 	for (k = 0; k < maxpages; k++)
 		p[k] = page + k;
 
-	maxsize = min_t(size_t, maxsize, maxpages * PAGE_SIZE - offset);
-	iov_iter_advance(i, maxsize);
-	return maxsize;
+	size = min_t(size_t, size, maxpages * PAGE_SIZE - offset);
+	iov_iter_advance(i, size);
+	return size;
 }
 
 /*
@@ -1698,14 +1698,14 @@ static ssize_t iov_iter_extract_kvec_pages(struct iov_iter *i,
 {
 	struct page **p, *page;
 	const void *kaddr;
-	size_t skip = i->iov_offset, offset, len;
+	size_t skip = i->iov_offset, offset, len, size;
 	int k;
 
 	for (;;) {
 		if (i->nr_segs == 0)
 			return 0;
-		maxsize = min(maxsize, i->kvec->iov_len - skip);
-		if (maxsize)
+		size = min(maxsize, i->kvec->iov_len - skip);
+		if (size)
 			break;
 		i->iov_offset = 0;
 		i->nr_segs--;
@@ -1717,13 +1717,13 @@ static ssize_t iov_iter_extract_kvec_pages(struct iov_iter *i,
 	offset = (unsigned long)kaddr & ~PAGE_MASK;
 	*offset0 = offset;
 
-	maxpages = want_pages_array(pages, maxsize, offset, maxpages);
+	maxpages = want_pages_array(pages, size, offset, maxpages);
 	if (!maxpages)
 		return -ENOMEM;
 	p = *pages;
 
 	kaddr -= offset;
-	len = offset + maxsize;
+	len = offset + size;
 	for (k = 0; k < maxpages; k++) {
 		size_t seg = min_t(size_t, len, PAGE_SIZE);
 
@@ -1737,9 +1737,9 @@ static ssize_t iov_iter_extract_kvec_pages(struct iov_iter *i,
 		kaddr += PAGE_SIZE;
 	}
 
-	maxsize = min_t(size_t, maxsize, maxpages * PAGE_SIZE - offset);
-	iov_iter_advance(i, maxsize);
-	return maxsize;
+	size = min_t(size_t, size, maxpages * PAGE_SIZE - offset);
+	iov_iter_advance(i, size);
+	return size;
 }
 
 /*