hw breakpoints: Fix pid namespace bug

Hardware breakpoints can't be registered within pid namespaces
because tsk->pid is passed rather than the pid in the current
namespace.

(See https://bugzilla.kernel.org/show_bug.cgi?id=17281 )

This is a quick fix demonstrating the problem but is not the
best method of solving the problem since passing pids internally
is not the best way to avoid pid namespace bugs. Subsequent patches
will show a better solution.

Much thanks to Frederic Weisbecker <fweisbec@gmail.com> for doing
the bulk of the work finding this bug.
Reported-by: NRobin Green <greenrd@greenrd.org>
Signed-off-by: NMatt Helsley <matthltc@us.ibm.com>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Prasad <prasad@linux.vnet.ibm.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Cc: 2.6.33-2.6.35 <stable@kernel.org>
LKML-Reference: <f63454af09fb1915717251570423eb9ddd338340.1284407762.git.matthltc@us.ibm.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>

kernel/hw_breakpoint.c

@@ -433,7 +433,8 @@ register_user_hw_breakpoint(struct perf_event_attr *attr,
 			    perf_overflow_handler_t triggered,
 			    struct task_struct *tsk)
 {
-	return perf_event_create_kernel_counter(attr, -1, tsk->pid, triggered);
+	return perf_event_create_kernel_counter(attr, -1, task_pid_vnr(tsk),
+						triggered);
 }
 EXPORT_SYMBOL_GPL(register_user_hw_breakpoint);