Re: [PATCH] time: Avoid signed overflow in timekeeping_get_ns()

[Laurent Vivier <lvivier@redhat.com>]

On 30/11/2015 02:30, David Gibson wrote:
> 1e75fa8 "time: Condense timekeeper.xtime into xtime_sec" replaced a call to
> clocksource_cyc2ns() from timekeeping_get_ns() with an open-coded version
> of the same logic to avoid keeping a semi-redundant struct timespec
> in struct timekeeper.
> 
> However, the commit also introduced a subtle semantic change - where
> clocksource_cyc2ns() uses purely unsigned math, the new version introduces
> a signed temporary, meaning that if (delta * tk->mult) has a 63-bit
> overflow the following shift will still give a negative result.  The
> choice of 'maxsec' in __clocksource_updatefreq_scale() means this will
> generally happen if there's a ~10 minute pause in examining the
> clocksource.
> 
> This can be triggered on a powerpc KVM guest by stopping it from qemu for
> a bit over 10 minutes.  After resuming time has jumped backwards several
> minutes causing numerous problems (jiffies does not advance, msleep()s can
> be extended by minutes..).  It doesn't happen on x86 KVM guests, because
> the guest TSC is effectively frozen while the guest is stopped, which is
> not the case for the powerpc timebase.
> 
> Obviously an unsigned (64 bit) overflow will only take twice as long as a
> signed, 63-bit overflow.  I don't know the time code well enough to know
> if that will still cause incorrect calculations, or if a 64-bit overflow
> is avoided elsewhere.
> 
> Still, an incorrect forwards clock adjustment will cause less trouble than
> time going backwards.  So, this patch removes the potential for
> intermediate signed overflow.
> 
> Suggested-by: Laurent Vivier <lvivier@redhat.com>
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  kernel/time/timekeeping.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index d563c19..99188ee 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -305,8 +305,7 @@ static inline s64 timekeeping_get_ns(struct tk_read_base *tkr)
>  
>  	delta = timekeeping_get_delta(tkr);
>  
> -	nsec = delta * tkr->mult + tkr->xtime_nsec;
> -	nsec >>= tkr->shift;
> +	nsec = (delta * tkr->mult + tkr->xtime_nsec) >> tkr->shift;
>  
>  	/* If arch requires, add in get_arch_timeoffset() */
>  	return nsec + arch_gettimeoffset();
> 
Tested-by: Laurent Vivier <lvivier@redhat.com>