Merge patch series 'Fix shmem_rename2 directory offset calculation' of https://lore.kernel.org/r/20240415152057.4605-1-cel@kernel.org

Pull shmem_rename2() offset fixes from Chuck Lever:

The existing code in shmem_rename2() allocates a fresh directory
offset value when renaming over an existing destination entry. User
space does not expect this behavior. In particular, applications
that rename while walking a directory can loop indefinitely because
they never reach the end of the directory.

* 'Fix shmem_rename2 directory offset calculation' of https://lore.kernel.org/r/20240415152057.4605-1-cel@kernel.org: (3 commits)
  shmem: Fix shmem_rename2()
  libfs: Add simple_offset_rename() API
  libfs: Fix simple_offset_rename_exchange()

 fs/libfs.c         | 55 +++++++++++++++++++++++++++++++++++++++++-----
 include/linux/fs.h |  2 ++
 mm/shmem.c         |  3 +--
 3 files changed, 52 insertions(+), 8 deletions(-)

Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 49 insertions, 6 deletions

diff --git a/fs/libfs.c b/fs/libfs.c
index 3a6f2cb364f8..b635ee5adbcc 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -295,6 +295,18 @@ int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry)
 	return 0;
 }
 
+static int simple_offset_replace(struct offset_ctx *octx, struct dentry *dentry,
+				 long offset)
+{
+	int ret;
+
+	ret = mtree_store(&octx->mt, offset, dentry, GFP_KERNEL);
+	if (ret)
+		return ret;
+	offset_set(dentry, offset);
+	return 0;
+}
+
 /**
  * simple_offset_remove - Remove an entry to a directory's offset map
  * @octx: directory offset ctx to be updated
@@ -346,12 +358,45 @@ int simple_offset_empty(struct dentry *dentry)
 }
 
 /**
+ * simple_offset_rename - handle directory offsets for rename
+ * @old_dir: parent directory of source entry
+ * @old_dentry: dentry of source entry
+ * @new_dir: parent_directory of destination entry
+ * @new_dentry: dentry of destination
+ *
+ * Caller provides appropriate serialization.
+ *
+ * User space expects the directory offset value of the replaced
+ * (new) directory entry to be unchanged after a rename.
+ *
+ * Returns zero on success, a negative errno value on failure.
+ */
+int simple_offset_rename(struct inode *old_dir, struct dentry *old_dentry,
+			 struct inode *new_dir, struct dentry *new_dentry)
+{
+	struct offset_ctx *old_ctx = old_dir->i_op->get_offset_ctx(old_dir);
+	struct offset_ctx *new_ctx = new_dir->i_op->get_offset_ctx(new_dir);
+	long new_offset = dentry2offset(new_dentry);
+
+	simple_offset_remove(old_ctx, old_dentry);
+
+	if (new_offset) {
+		offset_set(new_dentry, 0);
+		return simple_offset_replace(new_ctx, old_dentry, new_offset);
+	}
+	return simple_offset_add(new_ctx, old_dentry);
+}
+
+/**
  * simple_offset_rename_exchange - exchange rename with directory offsets
  * @old_dir: parent of dentry being moved
  * @old_dentry: dentry being moved
  * @new_dir: destination parent
  * @new_dentry: destination dentry
  *
+ * This API preserves the directory offset values. Caller provides
+ * appropriate serialization.
+ *
  * Returns zero on success. Otherwise a negative errno is returned and the
  * rename is rolled back.
  */
@@ -369,11 +414,11 @@ int simple_offset_rename_exchange(struct inode *old_dir,
 	simple_offset_remove(old_ctx, old_dentry);
 	simple_offset_remove(new_ctx, new_dentry);
 
-	ret = simple_offset_add(new_ctx, old_dentry);
+	ret = simple_offset_replace(new_ctx, old_dentry, new_index);
 	if (ret)
 		goto out_restore;
 
-	ret = simple_offset_add(old_ctx, new_dentry);
+	ret = simple_offset_replace(old_ctx, new_dentry, old_index);
 	if (ret) {
 		simple_offset_remove(new_ctx, old_dentry);
 		goto out_restore;
@@ -388,10 +433,8 @@ int simple_offset_rename_exchange(struct inode *old_dir,
 	return 0;
 
 out_restore:
-	offset_set(old_dentry, old_index);
-	mtree_store(&old_ctx->mt, old_index, old_dentry, GFP_KERNEL);
-	offset_set(new_dentry, new_index);
-	mtree_store(&new_ctx->mt, new_index, new_dentry, GFP_KERNEL);
+	(void)simple_offset_replace(old_ctx, old_dentry, old_index);
+	(void)simple_offset_replace(new_ctx, new_dentry, new_index);
 	return ret;
 }