time: Always make sure wall_to_monotonic isn't positive
authorWang YanQing <udknight@gmail.com>
Tue, 23 Jun 2015 10:38:54 +0000 (18:38 +0800)
committerJohn Stultz <john.stultz@linaro.org>
Mon, 17 Aug 2015 18:24:54 +0000 (11:24 -0700)
commite1d7ba8735551ed79c7a0463a042353574b96da3
treef15a9ef6516d689f6edaf062079000e32e8a7c77
parentde4a95faf173be1e798c37ca486dfcb234a0941b
time: Always make sure wall_to_monotonic isn't positive

Two issues were found on an IMX6 development board without an
enabled RTC device(resulting in the boot time and monotonic
time being initialized to 0).

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

The same issues can be reproduced on x86 after running the
following 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 a positive wall_to_monotonic pushes boot time back
to the time before Epoch, and getboottime will return negative
value.

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

This patch fix the problem by prohibiting time from being set to a value which
would cause a negative boot time. As a result one can't set the CLOCK_REALTIME
time prior to (1970 + system uptime).

Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Wang YanQing <udknight@gmail.com>
[jstultz: reworded commit message]
Signed-off-by: John Stultz <john.stultz@linaro.org>
kernel/time/timekeeping.c