[PATCH v2 17/21] backing-inode: add backing_inode_copyattr()

[Joanne Koong <joannelkoong@gmail.com>]

Move logic in ovl_copyattr() to a generic backing_inode_copyattr()
function in a new fs/backing-inode.c, which other filesystems that use
backing inodes (eg fuse passthrough) will use.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 fs/Makefile                   |  2 +-
 fs/backing-inode.c            | 43 +++++++++++++++++++++++++++++++++++
 fs/overlayfs/util.c           | 33 +++------------------------
 include/linux/backing-inode.h | 14 ++++++++++++
 4 files changed, 61 insertions(+), 31 deletions(-)
 create mode 100644 fs/backing-inode.c
 create mode 100644 include/linux/backing-inode.h

diff --git a/fs/Makefile b/fs/Makefile
index cf4a745e9679..3f8a227d4938 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -40,7 +40,7 @@ obj-$(CONFIG_COMPAT_BINFMT_ELF)	+= compat_binfmt_elf.o
 obj-$(CONFIG_BINFMT_ELF_FDPIC)	+= binfmt_elf_fdpic.o
 obj-$(CONFIG_BINFMT_FLAT)	+= binfmt_flat.o
 
-obj-$(CONFIG_FS_STACK)		+= backing-file.o
+obj-$(CONFIG_FS_STACK)		+= backing-file.o backing-inode.o
 obj-$(CONFIG_FS_MBCACHE)	+= mbcache.o
 obj-$(CONFIG_FS_POSIX_ACL)	+= posix_acl.o
 obj-$(CONFIG_NFS_COMMON)	+= nfs_common/
diff --git a/fs/backing-inode.c b/fs/backing-inode.c
new file mode 100644
index 000000000000..474770a1fa9d
--- /dev/null
+++ b/fs/backing-inode.c
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Common helpers for stackable filesystems and backing inodes.
+ */
+
+#include <linux/backing-inode.h>
+
+/*
+ * backing_inode_copyattr - copy inode attributes from a backing inode
+ *
+ * When a filesystem copies inode information from a backing layer to its own
+ * inode, it applies the idmapping of the backing mount, ensuring that
+ * the inode ownership will correctly reflect the ownership of the idmapped
+ * backing mount. For example, an idmapped backing mount mapping id 1001 to id
+ * 1000 will take care to map any backing inode owned by id 1001 to id 1000.
+ * These mapping helpers are nops when the backing mount isn't idmapped.
+ */
+void backing_inode_copyattr(struct inode *inode,
+			    const struct path *backing_path)
+{
+	struct inode *realinode;
+	struct mnt_idmap *real_idmap;
+	vfsuid_t vfsuid;
+	vfsgid_t vfsgid;
+
+	realinode = d_inode_rcu(backing_path->dentry);
+	real_idmap = mnt_idmap(backing_path->mnt);
+
+	spin_lock(&inode->i_lock);
+	vfsuid = i_uid_into_vfsuid(real_idmap, realinode);
+	vfsgid = i_gid_into_vfsgid(real_idmap, realinode);
+
+	inode->i_uid = vfsuid_into_kuid(vfsuid);
+	inode->i_gid = vfsgid_into_kgid(vfsgid);
+	inode->i_mode = realinode->i_mode;
+	inode_set_atime_to_ts(inode, inode_get_atime(realinode));
+	inode_set_mtime_to_ts(inode, inode_get_mtime(realinode));
+	inode_set_ctime_to_ts(inode, inode_get_ctime(realinode));
+	i_size_write(inode, i_size_read(realinode));
+	spin_unlock(&inode->i_lock);
+}
+EXPORT_SYMBOL_GPL(backing_inode_copyattr);
+
diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index 3f1b763a8bb4..3e7d66c26c10 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -16,6 +16,7 @@
 #include <linux/namei.h>
 #include <linux/ratelimit.h>
 #include <linux/overflow.h>
+#include <linux/backing-inode.h>
 #include "overlayfs.h"
 
 /* Get write access to upper mnt - may fail if upper sb was remounted ro */
@@ -1503,38 +1504,10 @@ int ovl_sync_status(struct ovl_fs *ofs)
 	return errseq_check(&mnt->mnt_sb->s_wb_err, ofs->errseq);
 }
 
-/*
- * ovl_copyattr() - copy inode attributes from layer to ovl inode
- *
- * When overlay copies inode information from an upper or lower layer to the
- * relevant overlay inode it will apply the idmapping of the upper or lower
- * layer when doing so ensuring that the ovl inode ownership will correctly
- * reflect the ownership of the idmapped upper or lower layer. For example, an
- * idmapped upper or lower layer mapping id 1001 to id 1000 will take care to
- * map any lower or upper inode owned by id 1001 to id 1000. These mapping
- * helpers are nops when the relevant layer isn't idmapped.
- */
 void ovl_copyattr(struct inode *inode)
 {
 	struct path realpath;
-	struct inode *realinode;
-	struct mnt_idmap *real_idmap;
-	vfsuid_t vfsuid;
-	vfsgid_t vfsgid;
 
-	realinode = ovl_i_path_real(inode, &realpath);
-	real_idmap = mnt_idmap(realpath.mnt);
-
-	spin_lock(&inode->i_lock);
-	vfsuid = i_uid_into_vfsuid(real_idmap, realinode);
-	vfsgid = i_gid_into_vfsgid(real_idmap, realinode);
-
-	inode->i_uid = vfsuid_into_kuid(vfsuid);
-	inode->i_gid = vfsgid_into_kgid(vfsgid);
-	inode->i_mode = realinode->i_mode;
-	inode_set_atime_to_ts(inode, inode_get_atime(realinode));
-	inode_set_mtime_to_ts(inode, inode_get_mtime(realinode));
-	inode_set_ctime_to_ts(inode, inode_get_ctime(realinode));
-	i_size_write(inode, i_size_read(realinode));
-	spin_unlock(&inode->i_lock);
+	ovl_i_path_real(inode, &realpath);
+	backing_inode_copyattr(inode, &realpath);
 }
diff --git a/include/linux/backing-inode.h b/include/linux/backing-inode.h
new file mode 100644
index 000000000000..6b43cba9fabd
--- /dev/null
+++ b/include/linux/backing-inode.h
@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Common helpers for stackable filesystems and backing inodes.
+ */
+
+#ifndef _LINUX_BACKING_INODE_H
+#define _LINUX_BACKING_INODE_H
+
+#include <linux/fs.h>
+
+void backing_inode_copyattr(struct inode *inode,
+			    const struct path *backing_path);
+
+#endif /* _LINUX_BACKING_INODE_H */
-- 
2.52.0