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From:   Stephane Eranian <eranian@google.com>
Date:   Thu, 17 Jan 2019 13:03:20 -0800
Message-ID: <CABPqkBS30_Afu3a1hycMAnB5KaqdFe=JYPoENfZG4ZtUYAE7mQ@mail.gmail.com>
Subject: Re: [PATCH] perf/core: fix perf_proc_update_handler() bug
To:     LKML <linux-kernel@vger.kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>, mingo@elte.hu,
        Andi Kleen <ak@linux.intel.com>,
        "Liang, Kan" <kan.liang@intel.com>, Jiri Olsa <jolsa@redhat.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>
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On Thu, Jan 10, 2019 at 5:17 PM Stephane Eranian <eranian@google.com> wrote:
>
> The perf_proc_update_handler() handles /proc/sys/kernel/perf_event_max_sample_rate
> syctl variable.  When the PMU IRQ handler timing monitoring is disabled, i.e,
> when /proc/sys/kernel/perf_cpu_time_max_percent is equal to 0 or 100,
> then no modification to sysctl_perf_event_sample_rate is allowed to prevent
> possible hang from wrong values.
>
> The problem is that the test to prevent modification is made after the
> sysctl variable is modified in perf_proc_update_handler().
>
> You get an error:
>
> $ echo 10001 >/proc/sys/kernel/perf_event_max_sample_rate
> echo: write error: invalid argument
>
> But the value is still modified causing all sorts of inconsistencies:
>
> $ cat /proc/sys/kernel/perf_event_max_sample_rate
> 10001
>
> This patch fixes the problem by moving the parsing of the value after
> the test.
>
> Signed-off-by: Stephane Eranian <eranian@google.com>

Ping.

> ---
>  kernel/events/core.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 3cd13a30f732..e5ede6918050 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -436,18 +436,18 @@ int perf_proc_update_handler(struct ctl_table *table, int write,
>                 void __user *buffer, size_t *lenp,
>                 loff_t *ppos)
>  {
> -       int ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> -
> -       if (ret || !write)
> -               return ret;
> -
> +       int ret;
> +       int perf_cpu = sysctl_perf_cpu_time_max_percent;
>         /*
>          * If throttling is disabled don't allow the write:
>          */
> -       if (sysctl_perf_cpu_time_max_percent == 100 ||
> -           sysctl_perf_cpu_time_max_percent == 0)
> +       if (write && (perf_cpu == 100 || perf_cpu == 0))
>                 return -EINVAL;
>
> +       ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> +       if (ret || !write)
> +               return ret;
> +
>         max_samples_per_tick = DIV_ROUND_UP(sysctl_perf_event_sample_rate, HZ);
>         perf_sample_period_ns = NSEC_PER_SEC / sysctl_perf_event_sample_rate;
>         update_perf_cpu_limits();
> --
> 2.7.4
>