Re: [PATCH v3] ktime: Fix ktime_divns to do signed division

[Nicolas Pitre]

On Fri, 8 May 2015, John Stultz wrote:

> It was noted that the 32bit implementation of ktime_divns()
> was doing unsigned division and didn't properly handle
> negative values.
> 
> And when a ktime helper was changed to utilize
> ktime_divns, it caused a regression on some IR blasters.
> See the following bugzilla for details:
>   https://bugzilla.redhat.com/show_bug.cgi?id=1200353
> 
> This patch fixes the problem in ktime_divns by checking
> and preserving the sign bit, and then reapplying it if
> appropriate after the division, it also changes the return
> type to a s64 to make it more obvious this is expected.
> 
> Nicolas also pointed out that negative dividers would
> cause infinite loops on 32bit systems, negative dividers
> is unlikely for users of this function, but out of caution
> this patch adds checks for negative dividers for both
> 32-bit (BUG_ON) and 64-bit(WARN_ON) versions to make sure
> no such use cases creep in.
> 
> Cc: Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Josh Boyer <jwboyer@xxxxxxxxxx>
> Cc: One Thousand Gnomes <gnomes@xxxxxxxxxxxxxxxxxxx>
> Cc: Trevor Cordes <trevor@xxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # 3.17+ for regression
> Tested-by: Trevor Cordes <trevor@xxxxxxxxxxxxx>
> Reported-by: Trevor Cordes <trevor@xxxxxxxxxxxxx>
> Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>

Acked-by: Nicolas Pitre <nico@xxxxxxxxxx>

> ---
> 
> New in v3:
> * Fix casting issue Nicolas pointed out
> * Use WARN_ON for 64bit case
> 
>  include/linux/ktime.h | 27 +++++++++++++++++++++++----
>  kernel/time/hrtimer.c | 11 ++++++++---
>  2 files changed, 31 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/ktime.h b/include/linux/ktime.h
> index 5fc3d10..ab2de1c7 100644
> --- a/include/linux/ktime.h
> +++ b/include/linux/ktime.h
> @@ -166,19 +166,38 @@ static inline bool ktime_before(const ktime_t cmp1, const ktime_t cmp2)
>  }
>  
>  #if BITS_PER_LONG < 64
> -extern u64 __ktime_divns(const ktime_t kt, s64 div);
> -static inline u64 ktime_divns(const ktime_t kt, s64 div)
> +extern s64 __ktime_divns(const ktime_t kt, s64 div);
> +static inline s64 ktime_divns(const ktime_t kt, s64 div)
>  {
> +	/*
> +	 * Negative divisors could cause an inf loop,
> +	 * so bug out here.
> +	 */
> +	BUG_ON(div < 0);
>  	if (__builtin_constant_p(div) && !(div >> 32)) {
> -		u64 ns = kt.tv64;
> +		s64 ns = kt.tv64;
> +		int neg = (ns < 0);
> +
> +		if (neg)
> +			ns = -ns;
>  		do_div(ns, div);
> +		if (neg)
> +			ns = -ns;
>  		return ns;
>  	} else {
>  		return __ktime_divns(kt, div);
>  	}
>  }
>  #else /* BITS_PER_LONG < 64 */
> -# define ktime_divns(kt, div)		(u64)((kt).tv64 / (div))
> +static inline s64 ktime_divns(const ktime_t kt, s64 div)
> +{
> +	/*
> +	 * 32-bit implementation cannot handle negative divisors,
> +	 * so catch them on 64bit as well.
> +	 */
> +	WARN_ON(div < 0);
> +	return kt.tv64 / div;
> +}
>  #endif
>  
>  static inline s64 ktime_to_us(const ktime_t kt)
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 76d4bd9..c98ce4d 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -266,12 +266,15 @@ lock_hrtimer_base(const struct hrtimer *timer, unsigned long *flags)
>  /*
>   * Divide a ktime value by a nanosecond value
>   */
> -u64 __ktime_divns(const ktime_t kt, s64 div)
> +s64 __ktime_divns(const ktime_t kt, s64 div)
>  {
> -	u64 dclc;
> -	int sft = 0;
> +	s64 dclc;
> +	int neg, sft = 0;
>  
>  	dclc = ktime_to_ns(kt);
> +	neg = (dclc < 0);
> +	if (neg)
> +		dclc = -dclc;
>  	/* Make sure the divisor is less than 2^32: */
>  	while (div >> 32) {
>  		sft++;
> @@ -279,6 +282,8 @@ u64 __ktime_divns(const ktime_t kt, s64 div)
>  	}
>  	dclc >>= sft;
>  	do_div(dclc, (unsigned long) div);
> +	if (neg)
> +		dclc = -dclc;
>  
>  	return dclc;
>  }
> -- 
> 1.9.1
> 
> 
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