ext4: teach ext4_ext_find_extent() to free path on error

Right now, there are a places where it is all to easy to leak memory
on an error path, via a usage like this:

	struct ext4_ext_path *path = NULL

	while (...) {
		...
		path = ext4_ext_find_extent(inode, block, path, 0);
		if (IS_ERR(path)) {
			/* oops, if path was non-NULL before the call to
			   ext4_ext_find_extent, we've leaked it!  :-(  */
			...
			return PTR_ERR(path);
		}
		...
	}

Unfortunately, there some code paths where we are doing the following
instead:

	path = ext4_ext_find_extent(inode, block, orig_path, 0);

and where it's important that we _not_ free orig_path in the case
where ext4_ext_find_extent() returns an error.

So change the function signature of ext4_ext_find_extent() so that it
takes a struct ext4_ext_path ** for its third argument, and by
default, on an error, it will free the struct ext4_ext_path, and then
zero out the struct ext4_ext_path * pointer.  In order to avoid
causing problems, we add a flag EXT4_EX_NOFREE_ON_ERR which causes
ext4_ext_find_extent() to use the original behavior of forcing the
caller to deal with freeing the original path pointer on the error
case.

The goal is to get rid of EXT4_EX_NOFREE_ON_ERR entirely, but this
allows for a gentle transition and makes the patches easier to verify.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ext4.h

@@ -582,6 +582,7 @@ enum {
  */
 #define EXT4_EX_NOCACHE				0x0800
 #define EXT4_EX_FORCE_CACHE			0x1000
+#define EXT4_EX_NOFREE_ON_ERR			0x2000
 
 /*
  * Flags used by ext4_free_blocks
@@ -2733,7 +2734,7 @@ extern int ext4_ext_insert_extent(handle_t *, struct inode *,
 				  struct ext4_ext_path *,
 				  struct ext4_extent *, int);
 extern struct ext4_ext_path *ext4_ext_find_extent(struct inode *, ext4_lblk_t,
-						  struct ext4_ext_path *,
+						  struct ext4_ext_path **,
 						  int flags);
 extern void ext4_ext_drop_refs(struct ext4_ext_path *);
 extern int ext4_ext_check_inode(struct inode *inode);

fs/ext4/extents.c

@@ -855,11 +855,13 @@ int ext4_ext_tree_init(handle_t *handle, struct inode *inode)
 
 struct ext4_ext_path *
 ext4_ext_find_extent(struct inode *inode, ext4_lblk_t block,
-		     struct ext4_ext_path *path, int flags)
+		     struct ext4_ext_path **orig_path, int flags)
 {
 	struct ext4_extent_header *eh;
 	struct buffer_head *bh;
-	short int depth, i, ppos = 0, alloc = 0;
+	struct ext4_ext_path *path = orig_path ? *orig_path : NULL;
+	short int depth, i, ppos = 0;
+	short free_on_err = (flags & EXT4_EX_NOFREE_ON_ERR) == 0;
 	int ret;
 
 	eh = ext_inode_hdr(inode);
@@ -871,7 +873,7 @@ ext4_ext_find_extent(struct inode *inode, ext4_lblk_t block,
 				GFP_NOFS);
 		if (unlikely(!path))
 			return ERR_PTR(-ENOMEM);
-		alloc = 1;
+		free_on_err = 1;
 	}
 	path[0].p_hdr = eh;
 	path[0].p_bh = NULL;
@@ -923,8 +925,11 @@ ext4_ext_find_extent(struct inode *inode, ext4_lblk_t block,
 
 err:
 	ext4_ext_drop_refs(path);
-	if (alloc)
+	if (free_on_err) {
 		kfree(path);
+		if (orig_path)
+			*orig_path = NULL;
+	}
 	return ERR_PTR(ret);
 }
 
@@ -1356,7 +1361,7 @@ repeat:
 		ext4_ext_drop_refs(path);
 		path = ext4_ext_find_extent(inode,
 				    (ext4_lblk_t)le32_to_cpu(newext->ee_block),
-				    path, gb_flags);
+				    &path, gb_flags | EXT4_EX_NOFREE_ON_ERR);
 		if (IS_ERR(path))
 			err = PTR_ERR(path);
 	} else {
@@ -1369,7 +1374,7 @@ repeat:
 		ext4_ext_drop_refs(path);
 		path = ext4_ext_find_extent(inode,
 				   (ext4_lblk_t)le32_to_cpu(newext->ee_block),
-				    path, gb_flags);
+				    &path, gb_flags | EXT4_EX_NOFREE_ON_ERR);
 		if (IS_ERR(path)) {
 			err = PTR_ERR(path);
 			goto out;
@@ -2152,7 +2157,7 @@ static int ext4_fill_fiemap_extents(struct inode *inode,
 			path = NULL;
 		}
 
-		path = ext4_ext_find_extent(inode, block, path, 0);
+		path = ext4_ext_find_extent(inode, block, &path, 0);
 		if (IS_ERR(path)) {
 			up_read(&EXT4_I(inode)->i_data_sem);
 			err = PTR_ERR(path);
@@ -3313,7 +3318,8 @@ static int ext4_split_extent(handle_t *handle,
 	 * result in split of original leaf or extent zeroout.
 	 */
 	ext4_ext_drop_refs(path);
-	path = ext4_ext_find_extent(inode, map->m_lblk, path, 0);
+	path = ext4_ext_find_extent(inode, map->m_lblk, &path,
+				    EXT4_EX_NOFREE_ON_ERR);
 	if (IS_ERR(path))
 		return PTR_ERR(path);
 	depth = ext_depth(inode);
@@ -3697,7 +3703,8 @@ static int ext4_convert_initialized_extents(handle_t *handle,
 		if (err < 0)
 			goto out;
 		ext4_ext_drop_refs(path);
-		path = ext4_ext_find_extent(inode, map->m_lblk, path, 0);
+		path = ext4_ext_find_extent(inode, map->m_lblk, &path,
+					    EXT4_EX_NOFREE_ON_ERR);
 		if (IS_ERR(path)) {
 			err = PTR_ERR(path);
 			goto out;
@@ -3769,7 +3776,8 @@ static int ext4_convert_unwritten_extents_endio(handle_t *handle,
 		if (err < 0)
 			goto out;
 		ext4_ext_drop_refs(path);
-		path = ext4_ext_find_extent(inode, map->m_lblk, path, 0);
+		path = ext4_ext_find_extent(inode, map->m_lblk, &path,
+					    EXT4_EX_NOFREE_ON_ERR);
 		if (IS_ERR(path)) {
 			err = PTR_ERR(path);
 			goto out;

fs/ext4/move_extent.c

@@ -37,7 +37,7 @@ get_ext_path(struct inode *inode, ext4_lblk_t lblock,
 	int ret = 0;
 	struct ext4_ext_path *path;
 
-	path = ext4_ext_find_extent(inode, lblock, *orig_path, EXT4_EX_NOCACHE);
+	path = ext4_ext_find_extent(inode, lblock, orig_path, EXT4_EX_NOCACHE);
 	if (IS_ERR(path))
 		ret = PTR_ERR(path);
 	else if (path[ext_depth(inode)].p_ext == NULL)