[PATCHv7 10/33] kernel: Add do_timens_ktime_to_host() helper

[Dmitry Safonov]

From: Andrei Vagin <ava...@gmail.com>

The helper subtracts namespace's clock offset from the given time
and checks that the result is in [0, KTIME_MAX].

Signed-off-by: Andrei Vagin <ava...@gmail.com>
Co-developed-by: Dmitry Safonov <d...@arista.com>
Signed-off-by: Dmitry Safonov <d...@arista.com>
---
 include/linux/time_namespace.h | 14 +++++++++++++
 kernel/time/namespace.c        | 36 ++++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+)

diff --git a/include/linux/time_namespace.h b/include/linux/time_namespace.h
index 3d429c7ecca5..9a77d3854830 100644
--- a/include/linux/time_namespace.h
+++ b/include/linux/time_namespace.h
@@ -57,6 +57,15 @@ static inline void timens_add_boottime(struct timespec64 *ts)
        *ts = timespec64_add(*ts, ns_offsets->boottime);
 }
 
+ktime_t do_timens_ktime_to_host(clockid_t clockid, ktime_t tim,
+                               struct timens_offsets *offsets);
+static inline ktime_t timens_ktime_to_host(clockid_t clockid, ktime_t tim)
+{
+       struct timens_offsets *offsets = &current->nsproxy->time_ns->offsets;
+
+       return do_timens_ktime_to_host(clockid, tim, offsets);
+}
+
 #else
 static inline struct time_namespace *get_time_ns(struct time_namespace *ns)
 {
@@ -83,6 +92,11 @@ static inline int timens_on_fork(struct nsproxy *nsproxy, 
struct task_struct *ts
 
 static inline void timens_add_monotonic(struct timespec64 *ts) {}
 static inline void timens_add_boottime(struct timespec64 *ts) {}
+
+static inline ktime_t timens_ktime_to_host(clockid_t clockid, ktime_t tim)
+{
+       return tim;
+}
 #endif
 
 #endif /* _LINUX_TIMENS_H */
diff --git a/kernel/time/namespace.c b/kernel/time/namespace.c
index c2a58e45fc4b..1a0fbaa5d2d4 100644
--- a/kernel/time/namespace.c
+++ b/kernel/time/namespace.c
@@ -16,6 +16,42 @@
 #include <linux/err.h>
 #include <linux/mm.h>
 
+ktime_t do_timens_ktime_to_host(clockid_t clockid, ktime_t tim,
+                               struct timens_offsets *ns_offsets)
+{
+       ktime_t offset;
+
+       switch (clockid) {
+       case CLOCK_MONOTONIC:
+               offset = timespec64_to_ktime(ns_offsets->monotonic);
+               break;
+       case CLOCK_BOOTTIME:
+       case CLOCK_BOOTTIME_ALARM:
+               offset = timespec64_to_ktime(ns_offsets->boottime);
+               break;
+       default:
+               return tim;
+       }
+
+       /*
+        * Check that @tim value is in [offset, KTIME_MAX + offset]
+        * and subtract offset.
+        */
+       if (tim < offset) {
+               /*
+                * User can specify @tim *absolute* value - if it's lesser than
+                * the time namespace's offset - it's already expired.
+                */
+               tim = 0;
+       } else {
+               tim = ktime_sub(tim, offset);
+               if (unlikely(tim > KTIME_MAX))
+                       tim = KTIME_MAX;
+       }
+
+       return tim;
+}
+
 static struct ucounts *inc_time_namespaces(struct user_namespace *ns)
 {
        return inc_ucount(ns, current_euid(), UCOUNT_TIME_NAMESPACES);
-- 
2.23.0