diff --git a/include/linux/posix-timers.h b/include/linux/posix-timers.h
index 79a1cea7f6ed..88b9256169f8 100644
--- a/include/linux/posix-timers.h
+++ b/include/linux/posix-timers.h
@@ -18,6 +18,17 @@ struct cpu_timer_list {
 	int firing;
 };
 
+/*
+ * Bit fields within a clockid:
+ *
+ * The most significant 29 bits hold either a pid or a file descriptor.
+ *
+ * Bit 2 indicates whether a cpu clock refers to a thread or a process.
+ *
+ * Bits 1 and 0 give the type: PROF=0, VIRT=1, SCHED=2, or FD=3.
+ *
+ * A clockid is invalid if bits 2, 1, and 0 are all set.
+ */
 #define CPUCLOCK_PID(clock)		((pid_t) ~((clock) >> 3))
 #define CPUCLOCK_PERTHREAD(clock) \
 	(((clock) & (clockid_t) CPUCLOCK_PERTHREAD_MASK) != 0)
@@ -29,6 +40,8 @@ struct cpu_timer_list {
 #define CPUCLOCK_VIRT		1
 #define CPUCLOCK_SCHED		2
 #define CPUCLOCK_MAX		3
+#define CLOCKFD			CPUCLOCK_MAX
+#define CLOCKFD_MASK		(CPUCLOCK_PERTHREAD_MASK|CPUCLOCK_CLOCK_MASK)
 
 #define MAKE_PROCESS_CPUCLOCK(pid, clock) \
 	((~(clockid_t) (pid) << 3) | (clockid_t) (clock))
diff --git a/kernel/posix-timers.c b/kernel/posix-timers.c
index 5a5a4f1c0971..df629d853a81 100644
--- a/kernel/posix-timers.c
+++ b/kernel/posix-timers.c
@@ -488,7 +488,7 @@ static void release_posix_timer(struct k_itimer *tmr, int it_id_set)
 static struct k_clock *clockid_to_kclock(const clockid_t id)
 {
 	if (id < 0)
-		return &clock_posix_cpu;
+		return (id & CLOCKFD_MASK) == CLOCKFD ? NULL : &clock_posix_cpu;
 
 	if (id >= MAX_CLOCKS || !posix_clocks[id].clock_getres)
 		return NULL;