ceph: fix crash after fscrypt_encrypt_pagecache_blocks() error

The function move_dirty_folio_in_page_array() was created by commit
ce80b76 ("ceph: introduce ceph_process_folio_batch() method") by
moving code from ceph_writepages_start() to this function.

This new function is supposed to return an error code which is checked
by the caller (now ceph_process_folio_batch()), and on error, the
caller invokes redirty_page_for_writepage() and then breaks from the
loop.

However, the refactoring commit has gone wrong, and it by accident, it
always returns 0 (= success) because it first NULLs the pointer and
then returns PTR_ERR(NULL) which is always 0.  This means errors are
silently ignored, leaving NULL entries in the page array, which may
later crash the kernel.

The simple solution is to call PTR_ERR() before clearing the pointer.

Cc: stable@vger.kernel.org
Fixes: ce80b76 ("ceph: introduce ceph_process_folio_batch() method")
Link: https://lore.kernel.org/ceph-devel/aK4v548CId5GIKG1@swift.blarg.de/
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

Author: MaxKellermann

fs/ceph/addr.c

@@ -1264,7 +1264,9 @@ static inline int move_dirty_folio_in_page_array(struct address_space *mapping,
 								0,
 								gfp_flags);
 		if (IS_ERR(pages[index])) {
-			if (PTR_ERR(pages[index]) == -EINVAL) {
+			int err = PTR_ERR(pages[index]);
+
+			if (err == -EINVAL) {
 				pr_err_client(cl, "inode->i_blkbits=%hhu\n",
 						inode->i_blkbits);
 			}
@@ -1273,7 +1275,7 @@ static inline int move_dirty_folio_in_page_array(struct address_space *mapping,
 			BUG_ON(ceph_wbc->locked_pages == 0);
 
 			pages[index] = NULL;
-			return PTR_ERR(pages[index]);
+			return err;
 		}
 	} else {
 		pages[index] = &folio->page;