libceph: validate timespec conversions

A ceph timespec contains 32-bit unsigned values for its seconds and
nanoseconds components.  For a standard timespec, both fields are
signed, and the seconds field is almost surely 64 bits.

Add some explicit casts so the fact that this conversion is taking
place is obvious.  Also trip a bug if we ever try to put out of
range (negative or too big) values into a ceph timespec.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

include/linux/ceph/decode.h

@@ -154,14 +154,19 @@ static inline char *ceph_extract_encoded_string(void **p, void *end,
 static inline void ceph_decode_timespec(struct timespec *ts,
 					const struct ceph_timespec *tv)
 {
-	ts->tv_sec = le32_to_cpu(tv->tv_sec);
-	ts->tv_nsec = le32_to_cpu(tv->tv_nsec);
+	ts->tv_sec = (__kernel_time_t)le32_to_cpu(tv->tv_sec);
+	ts->tv_nsec = (long)le32_to_cpu(tv->tv_nsec);
 }
 static inline void ceph_encode_timespec(struct ceph_timespec *tv,
 					const struct timespec *ts)
 {
-	tv->tv_sec = cpu_to_le32(ts->tv_sec);
-	tv->tv_nsec = cpu_to_le32(ts->tv_nsec);
+	BUG_ON(ts->tv_sec < 0);
+	BUG_ON(ts->tv_sec > (__kernel_time_t)U32_MAX);
+	BUG_ON(ts->tv_nsec < 0);
+	BUG_ON(ts->tv_nsec > (long)U32_MAX);
+
+	tv->tv_sec = cpu_to_le32((u32)ts->tv_sec);
+	tv->tv_nsec = cpu_to_le32((u32)ts->tv_nsec);
 }
 
 /*