[RFC PATCH 4/6] shiftfs: translate uids using s_user_ns from lower fs

[Seth Forshee <seth.forshee@canonical.com>]

Do not assume that ids from the lower filesystem are from
init_user_ns. Instead, translate them from that filesystem's
s_user_ns and then to the shiftfs user ns.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
---
 fs/shiftfs.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/shiftfs.c b/fs/shiftfs.c
index 556594988dd2..226c03d8588b 100644
--- a/fs/shiftfs.c
+++ b/fs/shiftfs.c
@@ -613,6 +613,8 @@ static int shiftfs_getattr(const struct path *path, struct kstat *stat,
 	struct inode *reali = real->d_inode;
 	const struct inode_operations *iop = reali->i_op;
 	struct path newpath = { .mnt = path->dentry->d_sb->s_fs_info, .dentry = real };
+	struct user_namespace *from_ns = reali->i_sb->s_user_ns;
+	struct user_namespace *to_ns = inode->i_sb->s_user_ns;
 	int err = 0;
 
 	if (iop->getattr)
@@ -624,8 +626,8 @@ static int shiftfs_getattr(const struct path *path, struct kstat *stat,
 		return err;
 
 	/* transform the underlying id */
-	stat->uid = make_kuid(inode->i_sb->s_user_ns, __kuid_val(stat->uid));
-	stat->gid = make_kgid(inode->i_sb->s_user_ns, __kgid_val(stat->gid));
+	stat->uid = shift_kuid(from_ns, to_ns, stat->uid);
+	stat->gid = shift_kgid(from_ns, to_ns, stat->gid);
 	return 0;
 }
 
-- 
2.19.1