Re: [PATCH] perf/core: fix perf_proc_update_handler() bug

[Jiri Olsa <jolsa@redhat.com>]

On Thu, Jan 10, 2019 at 05:17:16PM -0800, Stephane Eranian 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>

Reviewed-by: Jiri Olsa <jolsa@kernel.org>

jirka

> ---
>  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
>