perf_counter: sysctl for system wide perf counters

Impact: add sysctl for paranoid/relaxed perfcounters policy

Allow the use of system wide perf counters to everybody, but provide
a sysctl to disable it for the paranoid security minded.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
LKML-Reference: <20090409085524.514046352@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/perf_counter.h

@@ -568,6 +568,8 @@ struct perf_callchain_entry {
 
 extern struct perf_callchain_entry *perf_callchain(struct pt_regs *regs);
 
+extern int sysctl_perf_counter_priv;
+
 #else
 static inline void
 perf_counter_task_sched_in(struct task_struct *task, int cpu)		{ }

kernel/perf_counter.c

@@ -42,6 +42,8 @@ static atomic_t nr_mmap_tracking __read_mostly;
 static atomic_t nr_munmap_tracking __read_mostly;
 static atomic_t nr_comm_tracking __read_mostly;
 
+int sysctl_perf_counter_priv __read_mostly; /* do we need to be privileged */
+
 /*
  * Mutex for (sysadmin-configurable) counter reservations:
  */
@@ -1132,7 +1134,7 @@ static struct perf_counter_context *find_get_context(pid_t pid, int cpu)
 	 */
 	if (cpu != -1) {
 		/* Must be root to operate on a CPU counter: */
-		if (!capable(CAP_SYS_ADMIN))
+		if (sysctl_perf_counter_priv && !capable(CAP_SYS_ADMIN))
 			return ERR_PTR(-EACCES);
 
 		if (cpu < 0 || cpu > num_possible_cpus())

kernel/sysctl.c

@@ -49,6 +49,7 @@
 #include <linux/reboot.h>
 #include <linux/ftrace.h>
 #include <linux/slow-work.h>
+#include <linux/perf_counter.h>
 
 #include <asm/uaccess.h>
 #include <asm/processor.h>
@@ -920,6 +921,16 @@ static struct ctl_table kern_table[] = {
 		.child		= slow_work_sysctls,
 	},
 #endif
+#ifdef CONFIG_PERF_COUNTERS
+	{
+		.ctl_name	= CTL_UNNUMBERED,
+		.procname	= "perf_counter_privileged",
+		.data		= &sysctl_perf_counter_priv,
+		.maxlen		= sizeof(sysctl_perf_counter_priv),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec,
+	},
+#endif
 /*
  * NOTE: do not add new entries to this table unless you have read
  * Documentation/sysctl/ctl_unnumbered.txt