linux-kernel - Re: [PATCH v2] sched/rt: fix the case where sched_rt_period

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Date:   Mon, 16 May 2022 16:04:48 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Yajun Deng <yajun.deng@...ux.dev>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com
Cc:     linux-kernel@...r.kernel.org, Yajun Deng <yajun.deng@...ux.dev>
Subject: Re: [PATCH v2] sched/rt: fix the case where sched_rt_period_us is
 negative

On 12/05/22 08:39, Yajun Deng wrote:
> The proc_dointvec() is for integer, but sysctl_sched_rt_period is a
> unsigned integer, proc_dointvec() would convert negative number into
> positive number. So both proc_dointvec() and sched_rt_global_validate()
> aren't return error even if we set a negative number.
>
> Use proc_dointvec_minmax() instead of proc_dointvec() and use extra1
> limit the minimum value for sched_rt_period_us/sched_rt_runtime_us.
>
> Fixes: 391e43da797a ("sched: Move all scheduler bits into kernel/sched/")

That's just the last apparent change of the incriminated variable. AFAICT
the issue stems from:

- sysctl_sched_rt_period being unsigned int
- the ctl entry using proc_dointvec()
- the bounds check on sysctl_sched_rt_period being just <= 0 which doesn't
  actually respect the [1, INT_MAX] stated in
  Documentation/scheduler/sched-rt-group.rst

The <= thing was added by:

  ec5d498991e8 ("sched: fix deadlock in setting scheduler parameter to zero")

but AFAICT the unsigned int vs proc_dointvec() thing dates back to:

  d0b27fa77854 ("sched: rt-group: synchonised bandwidth period")

> Signed-off-by: Yajun Deng <yajun.deng@...ux.dev>
> ---

In the absence of a cover letter (e.g. single-patch submission), this is
where you should write patch version changelogs (see
Documentation/process).

>  kernel/sched/rt.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index b491a0f8c25d..3add32679885 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -37,6 +37,7 @@ static struct ctl_table sched_rt_sysctls[] = {
>               .maxlen         = sizeof(unsigned int),
>               .mode           = 0644,
>               .proc_handler   = sched_rt_handler,
> +		.extra1		= SYSCTL_ONE,

Per earlier comment, the Documentation says that this needs to be within
[1, INT_MAX], which you do get by excluding negative values when casting to
int...

How about we make sysctl_sched_rt_period int on top of this, then all variables
modified by the sched_rt_handler() proc_dointvec() are *actually* int, and
the upper bound requires less mental gymnastics to be figured out?

>       },
>       {
>               .procname       = "sched_rt_runtime_us",
> @@ -44,6 +45,8 @@ static struct ctl_table sched_rt_sysctls[] = {
>               .maxlen         = sizeof(int),
>               .mode           = 0644,
>               .proc_handler   = sched_rt_handler,
> +		.extra1		= SYSCTL_NEG_ONE,
> +		.extra2		= (void *)&sysctl_sched_rt_period,

Per this, you could also remove the

                ((sysctl_sched_rt_runtime > sysctl_sched_rt_period) ||

from sched_rt_global_validate(), no?

>       },
>       {
>               .procname       = "sched_rr_timeslice_ms",
> @@ -2959,9 +2962,6 @@ static int sched_rt_global_constraints(void)
>  #ifdef CONFIG_SYSCTL
>  static int sched_rt_global_validate(void)
>  {
> -	if (sysctl_sched_rt_period <= 0)
> -		return -EINVAL;
> -
>       if ((sysctl_sched_rt_runtime != RUNTIME_INF) &&
>               ((sysctl_sched_rt_runtime > sysctl_sched_rt_period) ||
>                ((u64)sysctl_sched_rt_runtime *
> @@ -2992,7 +2992,7 @@ static int sched_rt_handler(struct ctl_table *table, int write, void *buffer,
>       old_period = sysctl_sched_rt_period;
>       old_runtime = sysctl_sched_rt_runtime;
>
> -	ret = proc_dointvec(table, write, buffer, lenp, ppos);
> +	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
>
>       if (!ret && write) {
>               ret = sched_rt_global_validate();
> --
> 2.25.1