[PATCHv6 10/36] kernel: Add do_timens_ktime_to_host() helper

[Dmitry Safonov <dima@arista.com>]

From: Andrei Vagin <avagin@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 <avagin@gmail.com>
Co-developed-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
---
 include/linux/time_namespace.h | 17 ++++++++++++++
 kernel/time_namespace.c        | 43 ++++++++++++++++++++++++++++++++++
 2 files changed, 60 insertions(+)

diff --git a/include/linux/time_namespace.h b/include/linux/time_namespace.h
index 334c1a1c6607..9ba9664ff0ab 100644
--- a/include/linux/time_namespace.h
+++ b/include/linux/time_namespace.h
@@ -56,6 +56,18 @@ 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;
+
+	if (!offsets)
+		return tim;
+
+	return do_timens_ktime_to_host(clockid, tim, offsets);
+}
+
 #else
 static inline struct time_namespace *get_time_ns(struct time_namespace *ns)
 {
@@ -82,6 +94,11 @@ 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 394a9e168e7c..ff2c5de7e815 100644
--- a/kernel/time_namespace.c
+++ b/kernel/time_namespace.c
@@ -16,6 +16,49 @@
 #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 if (KTIME_MAX - tim < -offset) {
+		/*
+		 * User-supplied @tim may be close or even equal KTIME_MAX
+		 * and time namespace offset can be negative.
+		 * Let's check (tim - offset) for an overflow.
+		 * Return KTIME_MAX in such case, as the time value is
+		 * thousands *years* in future anyway.
+		 */
+		tim = KTIME_MAX;
+	} else {
+		tim = ktime_sub(tim, offset);
+	}
+
+	return tim;
+}
+
 static struct ucounts *inc_time_namespaces(struct user_namespace *ns)
 {
 	return inc_ucount(ns, current_euid(), UCOUNT_TIME_NAMESPACES);
-- 
2.22.0