[PATCH] perf: Prevent divide by zero exception in kernel/events/core.c

[PATCH] perf: Prevent divide by zero exception in kernel/events/core.c

[Knut Petersen]

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Any value written to /proc/sys/kernel/perf_event_max_sample_rate
is accepted and used as a divisor in perf_proc_update_handler().

Obviously that is wrong. 0 will cause a divide by 0 exception,
and negative values are unreasonable.

The attached patch enforces a lower limit of 1.

cu,
  Knut

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>From 7f468bd7bad2f2fec2e93c89a48008d1cba99001 Mon Sep 17 00:00:00 2001
From: Knut Petersen <Knut_Petersen@t-online.de>
Date: Wed, 25 Sep 2013 13:34:57 +0200
Subject: [PATCH] Enforce 1 as lower limit for perf_event_max_sample_rate

/proc/sys/kernel/perf_event_max_sample_rate will accept
negative values as well as 0.

Negative values are unreasonable, and 0 causes a
divide by zero exception in perf_proc_update_handler.

This patch enforces a lower limit of 1.

Signed-off-by: Knut Petersen <Knut_Petersen@t-online.de>
---
 kernel/events/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index dd236b6..350bb53 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -198,6 +198,9 @@ int perf_proc_update_handler(struct ctl_table *table, int write,
 	if (ret || !write)
 		return ret;
 
+	if (sysctl_perf_event_sample_rate <= 0)
+		sysctl_perf_event_sample_rate = 1;
+
 	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();
-- 
1.8.1.4

Re: [PATCH] perf: Prevent divide by zero exception in kernel/events/core.c

[Peter Zijlstra]

On Wed, Sep 25, 2013 at 02:07:35PM +0200, Knut Petersen wrote:
> From: Knut Petersen <Knut_Petersen@t-online.de>
> Date: Wed, 25 Sep 2013 13:34:57 +0200
> Subject: [PATCH] Enforce 1 as lower limit for perf_event_max_sample_rate
> 
> /proc/sys/kernel/perf_event_max_sample_rate will accept
> negative values as well as 0.
> 
> Negative values are unreasonable, and 0 causes a
> divide by zero exception in perf_proc_update_handler.
> 
> This patch enforces a lower limit of 1.
> 
> Signed-off-by: Knut Petersen <Knut_Petersen@t-online.de>
> ---
>  kernel/events/core.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index dd236b6..350bb53 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -198,6 +198,9 @@ int perf_proc_update_handler(struct ctl_table *table, int write,
>  	if (ret || !write)
>  		return ret;
>  
> +	if (sysctl_perf_event_sample_rate <= 0)
> +		sysctl_perf_event_sample_rate = 1;
> +
>  	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();

The 'normal' way of doing that is changing the sysctl table entry to use
proc_dointvec_minmax and set .extra = &one. See for example
&sysctl_sched_cfs_bandwidth_slice in kernel/sysctl.c

Re: [PATCH] perf: Prevent divide by zero exception in kernel/events/core.c

[Knut Petersen]

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On 25.09.2013 14:12, Peter Zijlstra wrote:
> The 'normal' way of doing that is changing the sysctl table entry to use
> proc_dointvec_minmax and set .extra = &one. See for example
> &sysctl_sched_cfs_bandwidth_slice in kernel/sysctl.c

Thanks. V2 attached.

cu,
  Knut

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>From 4ef9dabdd27a02e2ebd45c2d98afda7e43e82ab3 Mon Sep 17 00:00:00 2001
From: Knut Petersen <Knut_Petersen@t-online.de>
Date: Wed, 25 Sep 2013 14:29:37 +0200
Subject: [PATCH] Enforce 1 as lower limit for perf_event_max_sample_rate

/proc/sys/kernel/perf_event_max_sample_rate will accept
negative values as well as 0.

Negative values are unreasonable, and 0 causes a
divide by zero exception in perf_proc_update_handler.

This patch enforces a lower limit of 1.

v2: Use proc_dointvec_minmax() as suggested by
Peter Zijlstra.

Signed-off-by: Knut Petersen <Knut_Petersen@t-online.de>
---
 kernel/events/core.c | 2 +-
 kernel/sysctl.c      | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index dd236b6..8721a17 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -193,7 +193,7 @@ int perf_proc_update_handler(struct ctl_table *table, int write,
 		void __user *buffer, size_t *lenp,
 		loff_t *ppos)
 {
-	int ret = proc_dointvec(table, write, buffer, lenp, ppos);
+	int ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
 
 	if (ret || !write)
 		return ret;
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index b2f06f3..2a9db91 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1049,6 +1049,7 @@ static struct ctl_table kern_table[] = {
 		.maxlen		= sizeof(sysctl_perf_event_sample_rate),
 		.mode		= 0644,
 		.proc_handler	= perf_proc_update_handler,
+		.extra1		= &one,
 	},
 	{
 		.procname	= "perf_cpu_time_max_percent",
-- 
1.8.1.4

[tip:perf/core] perf: Enforce 1 as lower limit for perf_event_max_sample_rate

[tip-bot for Knut Petersen]

Commit-ID:  723478c8a471403c53cf144999701f6e0c4bbd11
Gitweb:     http://git.kernel.org/tip/723478c8a471403c53cf144999701f6e0c4bbd11
Author:     Knut Petersen <Knut_Petersen@t-online.de>
AuthorDate: Wed, 25 Sep 2013 14:29:37 +0200
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 4 Oct 2013 10:06:07 +0200

perf: Enforce 1 as lower limit for perf_event_max_sample_rate

/proc/sys/kernel/perf_event_max_sample_rate will accept
negative values as well as 0.

Negative values are unreasonable, and 0 causes a
divide by zero exception in perf_proc_update_handler.

This patch enforces a lower limit of 1.

Signed-off-by: Knut Petersen <Knut_Petersen@t-online.de>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/5242DB0C.4070005@t-online.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/events/core.c | 2 +-
 kernel/sysctl.c      | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index d49a9d2..b25d65c 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -193,7 +193,7 @@ int perf_proc_update_handler(struct ctl_table *table, int write,
 		void __user *buffer, size_t *lenp,
 		loff_t *ppos)
 {
-	int ret = proc_dointvec(table, write, buffer, lenp, ppos);
+	int ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
 
 	if (ret || !write)
 		return ret;
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index b2f06f3..2a9db91 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1049,6 +1049,7 @@ static struct ctl_table kern_table[] = {
 		.maxlen		= sizeof(sysctl_perf_event_sample_rate),
 		.mode		= 0644,
 		.proc_handler	= perf_proc_update_handler,
+		.extra1		= &one,
 	},
 	{
 		.procname	= "perf_cpu_time_max_percent",