ext4: convert from atomic_t to refcount_t on ext4_io_end->count

refcount_t type and corresponding API can protect refcounters from
accidental underflow and overflow and further use-after-free situations.
Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/1626674355-55795-1-git-send-email-xiyuyang19@fudan.edu.cnSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ext4.h

@@ -17,6 +17,7 @@
 #ifndef _EXT4_H
 #define _EXT4_H
 
+#include <linux/refcount.h>
 #include <linux/types.h>
 #include <linux/blkdev.h>
 #include <linux/magic.h>
@@ -241,7 +242,7 @@ typedef struct ext4_io_end {
 	struct bio		*bio;		/* Linked list of completed
 						 * bios covering the extent */
 	unsigned int		flag;		/* unwritten or not */
-	atomic_t		count;		/* reference counter */
+	refcount_t		count;		/* reference counter */
 	struct list_head	list_vec;	/* list of ext4_io_end_vec */
 } ext4_io_end_t;
 

fs/ext4/page-io.c

@@ -279,14 +279,14 @@ ext4_io_end_t *ext4_init_io_end(struct inode *inode, gfp_t flags)
 		io_end->inode = inode;
 		INIT_LIST_HEAD(&io_end->list);
 		INIT_LIST_HEAD(&io_end->list_vec);
-		atomic_set(&io_end->count, 1);
+		refcount_set(&io_end->count, 1);
 	}
 	return io_end;
 }
 
 void ext4_put_io_end_defer(ext4_io_end_t *io_end)
 {
-	if (atomic_dec_and_test(&io_end->count)) {
+	if (refcount_dec_and_test(&io_end->count)) {
 		if (!(io_end->flag & EXT4_IO_END_UNWRITTEN) ||
 				list_empty(&io_end->list_vec)) {
 			ext4_release_io_end(io_end);
@@ -300,7 +300,7 @@ int ext4_put_io_end(ext4_io_end_t *io_end)
 {
 	int err = 0;
 
-	if (atomic_dec_and_test(&io_end->count)) {
+	if (refcount_dec_and_test(&io_end->count)) {
 		if (io_end->flag & EXT4_IO_END_UNWRITTEN) {
 			err = ext4_convert_unwritten_io_end_vec(io_end->handle,
 								io_end);
@@ -314,7 +314,7 @@ int ext4_put_io_end(ext4_io_end_t *io_end)
 
 ext4_io_end_t *ext4_get_io_end(ext4_io_end_t *io_end)
 {
-	atomic_inc(&io_end->count);
+	refcount_inc(&io_end->count);
 	return io_end;
 }