Re: [RFC][PATCH] Use proper casting with signed timespec.tv_nsec values

[George Anzinger <george@mvista.com>]

john stultz wrote:
> All,
> 	I recently ran into a bug with an older kernel where xtime's tv_nsec
> field had accumulated more then 2 seconds worth of time. The timespec's
> tv_nsec is a signed long, however gettimeofday() treats it as an
> unsigned long. Thus when the failure occured, very strange and difficult
> to debug time problems occurred.
> 
> The main cause of the problem I was seeing is already fixed in mainline,
> however just to be safe, I figured the following patch would be wise.
> 
> I only audited i386 and x86_64, however other arches probably could have
> similar signed problems as well.
> 
> Please let me know if you have any further comments or feedback.

John,

There is a problem in the way this code handles the conversion to usec. 
  There is a conversion here and also in the get_offset code.  If the 
nanoseconds are carrier until after the addition of the two about 25% of 
the time you will end up with an additional usec in time.  I strongly 
suggest changing to convert to usec after the addition of xtime and 
get_offset time to avoid this.  If the "correct" thing is done in 
clock_gettime() (i.e. get_offset is in nanoseconds) this actually turns 
up as a back step in time WRT gettimeofday and clock_gettime().

George
-- 
> 
> thanks
> -john
> 
> linux-2.6.13_signed-tv_nsec_A0.patch
> ====================================
> diff --git a/arch/i386/kernel/time.c b/arch/i386/kernel/time.c
> --- a/arch/i386/kernel/time.c
> +++ b/arch/i386/kernel/time.c
> @@ -156,7 +156,7 @@ void do_gettimeofday(struct timeval *tv)
>  			usec += lost * (USEC_PER_SEC / HZ);
>  
>  		sec = xtime.tv_sec;
> -		usec += (xtime.tv_nsec / 1000);
> +		usec += (unsigned long)xtime.tv_nsec / 1000;
>  	} while (read_seqretry(&xtime_lock, seq));
>  
>  	while (usec >= 1000000) {
> diff --git a/arch/x86_64/kernel/time.c b/arch/x86_64/kernel/time.c
> --- a/arch/x86_64/kernel/time.c
> +++ b/arch/x86_64/kernel/time.c
> @@ -128,7 +128,7 @@ void do_gettimeofday(struct timeval *tv)
>  		seq = read_seqbegin(&xtime_lock);
>  
>  		sec = xtime.tv_sec;
> -		usec = xtime.tv_nsec / 1000;
> +		usec = (unsigned long)xtime.tv_nsec / 1000;
>  
>  		/* i386 does some correction here to keep the clock 
>  		   monotonous even when ntpd is fixing drift.
> diff --git a/kernel/timer.c b/kernel/timer.c
> --- a/kernel/timer.c
> +++ b/kernel/timer.c
> @@ -824,7 +824,7 @@ static void update_wall_time(unsigned lo
>  	do {
>  		ticks--;
>  		update_wall_time_one_tick();
> -		if (xtime.tv_nsec >= 1000000000) {
> +		if ((unsigned long)xtime.tv_nsec >= 1000000000) {
>  			xtime.tv_nsec -= 1000000000;
>  			xtime.tv_sec++;
>  			second_overflow();
> 
> 
> -
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-- 
George Anzinger   george@mvista.com
HRT (High-res-timers):  http://sourceforge.net/projects/high-res-timers/