linux-kernel - Re: [PATCH v2] timekeeping: always make sure wall_to

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Message-ID: <20150602151238.GA2081@udknight>
Date:	Tue, 2 Jun 2015 23:12:38 +0800
From:	Wang YanQing <udknight@...il.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] timekeeping: always make sure wall_to_monotonic isn't
 positive

On Mon, Jun 01, 2015 at 04:55:48PM -0700, John Stultz wrote:
> On Sat, May 30, 2015 at 10:10 PM, Wang YanQing <udknight@...il.com> wrote:
> > 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;
> >         }
> > "
> > Reason:
> > The reason is positive wall_to_monotonic push boot time back to the time
> > before Epoch, getboottime will return negative value.
> >
> > In issue 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 issue 2:
> >           show_stat use "unsigned long" to print
> >           negative value return by getboottime.
> >
> > This patch fix these two issues.
> 
> If there is two issues, we probably should have two patches, each
> clearly fixing one issue.  If there is one problem with multiple
> symptoms, then a single patch is fine but we want to be clear there.
> 
> 
> > Note: this patch will cause we can't use settimeofday with time
> >       earlier than current time on system which timekeeping_init
> >       initialize the xtime, boot and monotonic to 0 before set
> >       current time to a more reasonable time point.
> 
> If everything is initialized to 0 (aka 1970), then setting the time to
> prior to (relatively) shortly after boot is a pretty reasonable
> constraint. So you might want to reword this a little bit.
I get it and will reword this in v3.

> This basically seems to come down to the fact that you can't set the
> CLOCK_REALTIME time prior to (1970 + system uptime), right?
Yes, that's strict and precise representation!

> 
> 
> >
> > Signed-off-by: Wang YanQing <udknight@...il.com>
> > ---
> >  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.
> >
> >  It really take me some times to realize how stupid and
> >  buggy the version 1 patch is, but I am ready to be told
> >  this version is even stupider:)
> >
> >  Thanks.
> >
> >  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 0d784b3..b501aa6 100644
> > --- a/kernel/time/timekeeping.c
> > +++ b/kernel/time/timekeeping.c
> > @@ -895,6 +895,7 @@ 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;
> > @@ -908,10 +909,15 @@ 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);
> 
> If we didn't set the time, should we be calling timekeeping_update here?

Because we have called timekeeping_forward_now(tk), I found a same situation 
in timekeeping_inject_offset:

"error: /* even if we error out, we forwarded the time, so call update */"

Thanks.
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