[PATCH v1 1/2] shmem: Fix shmem_rename2()

[cel@xxxxxxxxxx]

From: Chuck Lever <chuck.lever@xxxxxxxxxx>

When renaming onto an existing directory entry, user space expects
the replacement entry to have the same directory offset as the
original one.

The details of handling directory offsets during a rename are moved
to fs/libfs.c so that they can be reused by other API consumers.

For backporting to stable kernels: use xa_store() rather than
mtree_store(), and octx->xa rather than octx->mt. See commit
0e4a862174f2 ("libfs: Convert simple directory offsets to use a
Maple Tree") for details.

Link: https://gitlab.alpinelinux.org/alpine/aports/-/issues/15966
Fixes: a2e459555c5f ("shmem: stable directory offsets")
Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
---
 fs/libfs.c         | 53 ++++++++++++++++++++++++++++++++++++++++------
 include/linux/fs.h |  2 ++
 mm/shmem.c         |  3 +--
 3 files changed, 50 insertions(+), 8 deletions(-)

diff --git a/fs/libfs.c b/fs/libfs.c
index 3a6f2cb364f8..ff767288e5dd 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -295,6 +295,17 @@ int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry)
 	return 0;
 }
 
+/*
+ * Internal helper for use when it is known that the tree entry at
+ * @index is already NULL.
+ */
+static int simple_offset_store(struct offset_ctx *octx, struct dentry *dentry,
+			       long index)
+{
+	offset_set(dentry, index);
+	return mtree_store(&octx->mt, index, dentry, GFP_KERNEL);
+}
+
 /**
  * simple_offset_remove - Remove an entry to a directory's offset map
  * @octx: directory offset ctx to be updated
@@ -345,6 +356,35 @@ int simple_offset_empty(struct dentry *dentry)
 	return ret;
 }
 
+/**
+ * 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.
+ *
+ * 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_index = dentry2offset(new_dentry);
+
+	simple_offset_remove(old_ctx, old_dentry);
+
+	/*
+	 * When the destination entry already exists, user space expects
+	 * its directory offset value to be unchanged after the rename.
+	 */
+	if (new_index)
+		return simple_offset_store(new_ctx, old_dentry, new_index);
+	return simple_offset_add(new_ctx, old_dentry);
+}
+
 /**
  * simple_offset_rename_exchange - exchange rename with directory offsets
  * @old_dir: parent of dentry being moved
@@ -352,6 +392,9 @@ int simple_offset_empty(struct dentry *dentry)
  * @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 +412,11 @@ 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_store(new_ctx, old_dentry, new_index);
 	if (ret)
 		goto out_restore;
 
-	ret = simple_offset_add(old_ctx, new_dentry);
+	ret = simple_offset_store(old_ctx, new_dentry, old_index);
 	if (ret) {
 		simple_offset_remove(new_ctx, old_dentry);
 		goto out_restore;
@@ -388,10 +431,8 @@ 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_store(old_ctx, old_dentry, old_index);
+	(void)simple_offset_store(new_ctx, new_dentry, new_index);
 	return ret;
 }
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 8dfd53b52744..b09f14132110 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3340,6 +3340,8 @@ void simple_offset_init(struct offset_ctx *octx);
 int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry);
 void simple_offset_remove(struct offset_ctx *octx, struct dentry *dentry);
 int simple_offset_empty(struct dentry *dentry);
+int simple_offset_rename(struct inode *old_dir, struct dentry *old_dentry,
+			 struct inode *new_dir, struct dentry *new_dentry);
 int simple_offset_rename_exchange(struct inode *old_dir,
 				  struct dentry *old_dentry,
 				  struct inode *new_dir,
diff --git a/mm/shmem.c b/mm/shmem.c
index 0aad0d9a621b..c0fb65223963 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3473,8 +3473,7 @@ static int shmem_rename2(struct mnt_idmap *idmap,
 			return error;
 	}
 
-	simple_offset_remove(shmem_get_offset_ctx(old_dir), old_dentry);
-	error = simple_offset_add(shmem_get_offset_ctx(new_dir), old_dentry);
+	error = simple_offset_rename(old_dir, old_dentry, new_dir, new_dentry);
 	if (error)
 		return error;
 
-- 
2.44.0