linux-kernel - Re: [PATCH] ktime: Fix ktime

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Date:	Fri, 1 May 2015 19:54:00 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	John Stultz <john.stultz@...aro.org>
cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Josh Boyer <jwboyer@...hat.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] ktime: Fix ktime_divns to do signed division

On Fri, 1 May 2015, John Stultz wrote:

> It was noted that the 32bit implementation of ktime_divns
> was doing unsgined division adn didn't properly handle
> negative values.
> 
> This patch fixes the problem by checking and preserving
> the sign bit, and then reapplying it if appropriate
> after the division.
> 
> Unfortunately there is some duplication since we have
> the optimized version for constant 32bit divider. I
> was considering reworkign the __ktime_divns helper
> to simplify the sign-handling logic, but then it
> would likely just be a s64/s64 divide, and probably
> should be more generic.
> 
> Thoughts?

Wouldn't it be better to simply forbid negative time altogether?  Given 
it's been broken for quite a while, there must not be that many 
instances of such usage and fixing them would avoid the useless sign 
handling overhead to 99.9% of the cases.

> Nicolas also notes that the ktime_divns() function
> breaks if someone passes in a negative divisor as
> well. This patch doesn't yet address that issue.

GRanted, a negative divisor here would be even weirder and should 
definitely be rejected.  Maybe the infinite loop is a good thing in that 
case, probably better than producing wrong numbers.

> Cc: Nicolas Pitre <nicolas.pitre@...aro.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Josh Boyer <jwboyer@...hat.com>
> Cc: One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
> Reported-by: Trevor Cordes <trevor@...nopolis.ca>
> Signed-off-by: John Stultz <john.stultz@...aro.org>
> ---
>  include/linux/ktime.h | 12 ++++++++++--
>  kernel/time/hrtimer.c | 11 +++++++++--
>  2 files changed, 19 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/ktime.h b/include/linux/ktime.h
> index 5fc3d10..d947263 100644
> --- a/include/linux/ktime.h
> +++ b/include/linux/ktime.h
> @@ -166,12 +166,20 @@ 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);
> +extern s64 __ktime_divns(const ktime_t kt, s64 div);
>  static inline u64 ktime_divns(const ktime_t kt, s64 div)
>  {
>  	if (__builtin_constant_p(div) && !(div >> 32)) {
> -		u64 ns = kt.tv64;
> +		s64 ns = kt.tv64;
> +		int neg = 0;
> +
> +		if (ns < 0) {
> +			neg = 1;
> +			ns = -ns;
> +		}
>  		do_div(ns, div);
> +		if (neg)
> +			ns = -ns;
>  		return ns;
>  	} else {
>  		return __ktime_divns(kt, div);
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 76d4bd9..4c1b294 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -266,12 +266,17 @@ 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;
> +	s64 dclc;
>  	int sft = 0;
> +	int neg = 0;
>  
>  	dclc = ktime_to_ns(kt);
> +	if (dclc < 0) {
> +		neg = 1;
> +		dclc = -dclc;
> +	}
>  	/* Make sure the divisor is less than 2^32: */
>  	while (div >> 32) {
>  		sft++;
> @@ -279,6 +284,8 @@ 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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