Re: [PATCH] sched: Make uclamp changes depend on CAP_SYS_NICE

[Xuewen Yan]

On Thu, Jun 10, 2021 at 2:16 AM Quentin Perret <qperret@xxxxxxxxxx> wrote:
>
> There is currently nothing preventing tasks from changing their per-task
> clamp values in anyway that they like. The rationale is probably that
> systems administrator are still able to limit those clamps thanks to the
> cgroup interface. However, this causes pain in a system where both
> per-task and per-cgroup clamps values are expected to be under the
> control of core system components (as is the case for Android).
>
> To fix this, let's require CAP_SYS_NICE to increase per-task clamp
> values. This allows unprivileged tasks to lower their requests, but not
> increase them, which is consistent with the existing behaviour for nice
> values.
>
> Signed-off-by: Quentin Perret <qperret@xxxxxxxxxx>
> ---
>  kernel/sched/core.c | 48 ++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 41 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 1d4aedbbcf96..1e5f9ae441a0 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1430,6 +1430,11 @@ static int uclamp_validate(struct task_struct *p,
>         if (util_min != -1 && util_max != -1 && util_min > util_max)
>                 return -EINVAL;
>
> +       return 0;
> +}
> +
> +static void uclamp_enable(void)
> +{
>         /*
>          * We have valid uclamp attributes; make sure uclamp is enabled.
>          *
> @@ -1438,8 +1443,25 @@ static int uclamp_validate(struct task_struct *p,
>          * scheduler locks.
>          */
>         static_branch_enable(&sched_uclamp_used);
> +}
>
> -       return 0;
> +static bool uclamp_reduce(struct task_struct *p, const struct sched_attr *attr)
> +{
> +       int util_min, util_max;
> +
> +       if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
> +               util_min = p->uclamp_req[UCLAMP_MIN].value;
> +               if (attr->sched_util_min > util_min)
> +                       return false;
> +       }
> +
> +       if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX) {
> +               util_max = p->uclamp_req[UCLAMP_MAX].value;
> +               if (attr->sched_util_max > util_max)
> +                       return false;

when the attr->sched_util_max = -1, and the util_max < 1024, here may
should return false, but it would return ture.

Thanks
xuewen
> +       }
> +
> +       return true;
>  }
>
>  static bool uclamp_reset(const struct sched_attr *attr,
> @@ -1580,6 +1602,11 @@ static inline int uclamp_validate(struct task_struct *p,
>  {
>         return -EOPNOTSUPP;
>  }
> +static inline void uclamp_enable(void) { }
> +static bool uclamp_reduce(struct task_struct *p, const struct sched_attr *attr)
> +{
> +       return true;
> +}
>  static void __setscheduler_uclamp(struct task_struct *p,
>                                   const struct sched_attr *attr) { }
>  static inline void uclamp_fork(struct task_struct *p) { }
> @@ -6116,6 +6143,13 @@ static int __sched_setscheduler(struct task_struct *p,
>             (rt_policy(policy) != (attr->sched_priority != 0)))
>                 return -EINVAL;
>
> +       /* Update task specific "requested" clamps */
> +       if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP) {
> +               retval = uclamp_validate(p, attr);
> +               if (retval)
> +                       return retval;
> +       }
> +
>         /*
>          * Allow unprivileged RT tasks to decrease priority:
>          */
> @@ -6165,6 +6199,10 @@ static int __sched_setscheduler(struct task_struct *p,
>                 /* Normal users shall not reset the sched_reset_on_fork flag: */
>                 if (p->sched_reset_on_fork && !reset_on_fork)
>                         return -EPERM;
> +
> +               /* Can't increase util-clamps */
> +               if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP && !uclamp_reduce(p, attr))
> +                       return -EPERM;
>         }
>
>         if (user) {
> @@ -6176,12 +6214,8 @@ static int __sched_setscheduler(struct task_struct *p,
>                         return retval;
>         }
>
> -       /* Update task specific "requested" clamps */
> -       if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP) {
> -               retval = uclamp_validate(p, attr);
> -               if (retval)
> -                       return retval;
> -       }
> +       if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)
> +               uclamp_enable();
>
>         if (pi)
>                 cpuset_read_lock();
> --
> 2.32.0.272.g935e593368-goog
>