[PATCH v3] timekeeping: always make sure wall_to_monotonic isn't positive

[Wang YanQing]

I meet two issues on an IMX6 development board without enable
RTC device(so timekeeping_init will initialize the boot time
and monotonic to 0).

Issue 1:exportfs -a generate:
       "exportfs: /opt/nfs/arm does not support NFS export"
Issue 2:cat /proc/stat:
       "btime 4294967236"

Exact reproduction of the same issues on x86 after run below
code:
"	int main(void)
	{
	    struct timeval val;
	    int ret;

	    val.tv_sec = 0;
	    val.tv_usec = 0;
	    ret = settimeofday(&val, NULL);
	    return 0;
	}
"
Two issues are different symptoms of same problem:
The reason is positive wall_to_monotonic push boot time back to the time
before Epoch, getboottime will return negative value.

In symptom 1:
          negative boot time cause get_expiry overflow time_t when input expire
          time is 2147483647, then cache_flush always clear entries just added
          in ip_map_parse.
In symptom 2:
          show_stat use "unsigned long" to print
          negative value return by getboottime.

This patch fix the problem.

Impact:
You can't set the CLOCK_REALTIME time prior to (1970 + system uptime).

Signed-off-by: Wang YanQing <udknight@xxxxxxxxx>
---
 Changes 
 v1-v2:
 1: fix subject, use "isn't positive" instead of "is negative".
 2: rewrite changelog.
 3: simplify code as suggested by John Stultz.

 v2-v3:
 1: reword changelog, no change of source code.
 
 kernel/time/timekeeping.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index bca3667..4cdb771 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -911,6 +911,7 @@ int do_settimeofday64(const struct timespec64 *ts)
 	struct timekeeper *tk = &tk_core.timekeeper;
 	struct timespec64 ts_delta, xt;
 	unsigned long flags;
+	int ret = 0;
 
 	if (!timespec64_valid_strict(ts))
 		return -EINVAL;
@@ -924,10 +925,15 @@ int do_settimeofday64(const struct timespec64 *ts)
 	ts_delta.tv_sec = ts->tv_sec - xt.tv_sec;
 	ts_delta.tv_nsec = ts->tv_nsec - xt.tv_nsec;
 
+	if (timespec64_compare(&tk->wall_to_monotonic, &ts_delta) > 0) {
+		ret = -EINVAL;
+		goto out;
+	}
+
 	tk_set_wall_to_mono(tk, timespec64_sub(tk->wall_to_monotonic, ts_delta));
 
 	tk_set_xtime(tk, ts);
-
+out:
 	timekeeping_update(tk, TK_CLEAR_NTP | TK_MIRROR | TK_CLOCK_WAS_SET);
 
 	write_seqcount_end(&tk_core.seq);
@@ -936,7 +942,7 @@ int do_settimeofday64(const struct timespec64 *ts)
 	/* signal hrtimers about time change */
 	clock_was_set();
 
-	return 0;
+	return ret;
 }
 EXPORT_SYMBOL(do_settimeofday64);
 
@@ -965,7 +971,8 @@ int timekeeping_inject_offset(struct timespec *ts)
 
 	/* Make sure the proposed value is valid */
 	tmp = timespec64_add(tk_xtime(tk),  ts64);
-	if (!timespec64_valid_strict(&tmp)) {
+	if (timespec64_compare(&tk->wall_to_monotonic, &ts64) > 0 ||
+	    !timespec64_valid_strict(&tmp)) {
 		ret = -EINVAL;
 		goto error;
 	}
-- 
1.8.5.6.2.g3d8a54e.dirty
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