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SubjectRe: [PATCH v3] ktime: Fix ktime_divns to do signed division
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@linaro.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Josh Boyer <jwboyer@redhat.com>
> Cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
> Cc: Trevor Cordes <trevor@tecnopolis.ca>
> Cc: <stable@vger.kernel.org> # 3.17+ for regression
> Tested-by: Trevor Cordes <trevor@tecnopolis.ca>
> Reported-by: Trevor Cordes <trevor@tecnopolis.ca>
> Signed-off-by: John Stultz <john.stultz@linaro.org>

Acked-by: Nicolas Pitre <nico@linaro.org>

> ---
>
> 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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