tracing/syscalls: Fix perf syscall tracing when syscall_nr == -1

syscall_get_nr can return -1 in the case that the task is not executing
a system call.

This patch fixes perf_syscall_{enter,exit} to check that the syscall
number is valid before using it as an index into a bitmap.

Link: http://lkml.kernel.org/r/[email protected]

Change-Id: Iedc719957e184c6572b3ad94e241ae2a97a0b533
Cc: Jason Baron <[email protected]>
Cc: Wade Farnsworth <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Signed-off-by: Will Deacon <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>

kernel/trace/trace_syscalls.c

@@ -506,6 +506,8 @@ static void perf_syscall_enter(void *ignore, struct pt_regs *regs, long id)
 	int size;
 
 	syscall_nr = syscall_get_nr(current, regs);
+	if (syscall_nr < 0)
+		return;
 	if (!test_bit(syscall_nr, enabled_perf_enter_syscalls))
 		return;
 
@@ -580,6 +582,8 @@ static void perf_syscall_exit(void *ignore, struct pt_regs *regs, long ret)
 	int size;
 
 	syscall_nr = syscall_get_nr(current, regs);
+	if (syscall_nr < 0)
+		return;
 	if (!test_bit(syscall_nr, enabled_perf_exit_syscalls))
 		return;