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From:   Stephane Eranian <eranian@google.com>
To:     linux-kernel@vger.kernel.org
Cc:     peterz@infradead.org, mingo@elte.hu, ak@linux.intel.com,
        kan.liang@intel.com, jolsa@redhat.com, acme@redhat.com
Subject: [PATCH] perf/core: fix perf_proc_update_handler() bug
Date:   Thu, 10 Jan 2019 17:17:16 -0800
Message-Id: <1547169436-6266-1-git-send-email-eranian@google.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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>
---
 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