[RFC PATCH 3/6] shiftfs: copy inode attrs up from underlying fs

[Seth Forshee <seth.forshee@xxxxxxxxxxxxx>]

Not all inode permission checks go through the permission
callback, e.g. some checks related to file capabilities. Always
copy up the inode attrs to ensure these checks work as expected.

Also introduce helpers helpers for shifting kernel ids from one
user ns to another, as this is an operation that is going to be
repeated.

Signed-off-by: Seth Forshee <seth.forshee@xxxxxxxxxxxxx>
---
 fs/shiftfs.c | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/fs/shiftfs.c b/fs/shiftfs.c
index b179a1be7bc1..556594988dd2 100644
--- a/fs/shiftfs.c
+++ b/fs/shiftfs.c
@@ -266,6 +266,33 @@ static int shiftfs_xattr_set(const struct xattr_handler *handler,
 	return shiftfs_setxattr(dentry, inode, name, value, size, flags);
 }
 
+static kuid_t shift_kuid(struct user_namespace *from, struct user_namespace *to,
+			 kuid_t kuid)
+{
+	uid_t uid = from_kuid(from, kuid);
+	return make_kuid(to, uid);
+}
+
+static kgid_t shift_kgid(struct user_namespace *from, struct user_namespace *to,
+			 kgid_t kgid)
+{
+	gid_t gid = from_kgid(from, kgid);
+	return make_kgid(to, gid);
+}
+
+static void shiftfs_copyattr(struct inode *from, struct inode *to)
+{
+	struct user_namespace *from_ns = from->i_sb->s_user_ns;
+	struct user_namespace *to_ns = to->i_sb->s_user_ns;
+
+	to->i_uid = shift_kuid(from_ns, to_ns, from->i_uid);
+	to->i_gid = shift_kgid(from_ns, to_ns, from->i_gid);
+	to->i_mode = from->i_mode;
+	to->i_atime = from->i_atime;
+	to->i_mtime = from->i_mtime;
+	to->i_ctime = from->i_ctime;
+}
+
 static void shiftfs_fill_inode(struct inode *inode, struct dentry *dentry)
 {
 	struct inode *reali;
@@ -278,6 +305,7 @@ static void shiftfs_fill_inode(struct inode *inode, struct dentry *dentry)
 	if (!reali->i_op->get_link)
 		inode->i_opflags |= IOP_NOFOLLOW;
 
+	shiftfs_copyattr(reali, inode);
 	inode->i_mapping = reali->i_mapping;
 	inode->i_private = reali;
 	set_nlink(inode, reali->i_nlink);
@@ -573,7 +601,7 @@ static int shiftfs_setattr(struct dentry *dentry, struct iattr *attr)
 		return err;
 
 	/* all OK, reflect the change on our inode */
-	setattr_copy(d_inode(dentry), attr);
+	shiftfs_copyattr(reali, d_inode(dentry));
 	return 0;
 }
 
-- 
2.19.1