time: Add timens_offsets to be used for tasks in time namespace

Introduce offsets for time namespace. They will contain an adjustment
needed to convert clocks to/from host's.

A new namespace is created with the same offsets as the time namespace
of the current process.
Co-developed-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrei Vagin <avagin@openvz.org>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20191112012724.250792-5-dima@arista.com

include/linux/time_namespace.h

@@ -12,11 +12,17 @@
 struct user_namespace;
 extern struct user_namespace init_user_ns;
 
+struct timens_offsets {
+	struct timespec64 monotonic;
+	struct timespec64 boottime;
+};
+
 struct time_namespace {
 	struct kref		kref;
 	struct user_namespace	*user_ns;
 	struct ucounts		*ucounts;
 	struct ns_common	ns;
+	struct timens_offsets	offsets;
 } __randomize_layout;
 
 extern struct time_namespace init_time_ns;
@@ -39,6 +45,20 @@ static inline void put_time_ns(struct time_namespace *ns)
 	kref_put(&ns->kref, free_time_ns);
 }
 
+static inline void timens_add_monotonic(struct timespec64 *ts)
+{
+	struct timens_offsets *ns_offsets = &current->nsproxy->time_ns->offsets;
+
+	*ts = timespec64_add(*ts, ns_offsets->monotonic);
+}
+
+static inline void timens_add_boottime(struct timespec64 *ts)
+{
+	struct timens_offsets *ns_offsets = &current->nsproxy->time_ns->offsets;
+
+	*ts = timespec64_add(*ts, ns_offsets->boottime);
+}
+
 #else
 static inline struct time_namespace *get_time_ns(struct time_namespace *ns)
 {
@@ -66,6 +86,8 @@ static inline int timens_on_fork(struct nsproxy *nsproxy,
 	return 0;
 }
 
+static inline void timens_add_monotonic(struct timespec64 *ts) { }
+static inline void timens_add_boottime(struct timespec64 *ts) { }
 #endif
 
 #endif /* _LINUX_TIMENS_H */

kernel/time/namespace.c

@@ -14,6 +14,7 @@
 #include <linux/slab.h>
 #include <linux/cred.h>
 #include <linux/err.h>
+#include <linux/mm.h>
 
 static struct ucounts *inc_time_namespaces(struct user_namespace *ns)
 {
@@ -60,6 +61,7 @@ static struct time_namespace *clone_time_ns(struct user_namespace *user_ns,
 	ns->ucounts = ucounts;
 	ns->ns.ops = &timens_operations;
 	ns->user_ns = get_user_ns(user_ns);
+	ns->offsets = old_ns->offsets;
 	return ns;
 
 fail_free: