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.pi...@linaro.org>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Cc: Josh Boyer <jwbo...@redhat.com>
> Cc: One Thousand Gnomes <gno...@lxorguk.ukuu.org.uk>
> Reported-by: Trevor Cordes <tre...@tecnopolis.ca>
> Signed-off-by: John Stultz <john.stu...@linaro.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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