On Sun, Nov 15, 2020 at 11:36:46AM +0100, Christian Brauner wrote:

+static inline struct user_namespace *mnt_user_ns(const struct vfsmount *mnt)
+{
+	return mnt->mnt_user_ns;
+}

I think you might want a READ_ONCE() here. Right now it seems ok, since the
mnt_user_ns can't change, but if we ever allow it to change (and I see you have
a idmapped_mounts_wip_v2_allow_to_change_idmapping branch on your public tree
:D), the pattern of,

        user_ns = mnt_user_ns(path->mnt);
        if (mnt_idmapped(path->mnt)) {
                uid = kuid_from_mnt(user_ns, uid);
                gid = kgid_from_mnt(user_ns, gid);
        }

could race.

Tycho