ring-buffer: Check for valid buffer before changing size

On some machines the number of possible CPUS is not the same as the
number of CPUs that is on the machine. Ftrace uses possible_cpus to
update the tracing structures but the ring buffer only allocates
per cpu buffers for online CPUs when they come up.

When the wakeup tracer was enabled in such a case, the ftrace code
enabled all possible cpu buffers, but the code in ring_buffer_resize()
did not check to see if the buffer in question was allocated. Since
boot up CPUs did not match possible CPUs it caused the following
crash:

BUG: unable to handle kernel NULL pointer dereference at 00000020
IP: [<c1097851>] ring_buffer_resize+0x16a/0x28d
*pde = 00000000
Oops: 0000 [#1] PREEMPT SMP
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in: [last unloaded: scsi_wait_scan]

Pid: 1387, comm: bash Not tainted 3.4.0-test+ #13                  /DG965MQ
EIP: 0060:[<c1097851>] EFLAGS: 00010217 CPU: 0
EIP is at ring_buffer_resize+0x16a/0x28d
EAX: f5a14340 EBX: f6026b80 ECX: 00000ff4 EDX: 00000ff3
ESI: 00000000 EDI: 00000002 EBP: f4275ecc ESP: f4275eb0
 DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
CR0: 80050033 CR2: 00000020 CR3: 34396000 CR4: 000007d0
DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
DR6: ffff0ff0 DR7: 00000400
Process bash (pid: 1387, ti=f4274000 task=f4380cb0 task.ti=f4274000)
Stack:
 c109cf9a f6026b98 00000162 00160f68 00000006 00160f68 00000002 f4275ef0
 c109d013 f4275ee8 c123b72a c1c0bf00 c1cc81dc 00000005 f4275f98 00000007
 f4275f70 c109d0c7 7700000e 75656b61 00000070 f5e90900 f5c4e198 00000301
Call Trace:
 [<c109cf9a>] ? tracing_set_tracer+0x115/0x1e9
 [<c109d013>] tracing_set_tracer+0x18e/0x1e9
 [<c123b72a>] ? _copy_from_user+0x30/0x46
 [<c109d0c7>] tracing_set_trace_write+0x59/0x7f
 [<c10ec01e>] ? fput+0x18/0x1c6
 [<c11f8732>] ? security_file_permission+0x27/0x2b
 [<c10eaacd>] ? rw_verify_area+0xcf/0xf2
 [<c10ec01e>] ? fput+0x18/0x1c6
 [<c109d06e>] ? tracing_set_tracer+0x1e9/0x1e9
 [<c10ead77>] vfs_write+0x8b/0xe3
 [<c10ebead>] ? fget_light+0x30/0x81
 [<c10eaf54>] sys_write+0x42/0x63
 [<c1834fbf>] sysenter_do_call+0x12/0x28

This happens with the latency tracer as the ftrace code updates the
saved max buffer via its cpumask and not with a global setting.

Adding a check in ring_buffer_resize() to make sure the buffer being resized
exists, fixes the problem.

Cc: Vaibhav Nagarnaik <vnagarnaik@google.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/ring_buffer.c

@@ -1486,6 +1486,11 @@ int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size,
 	if (!buffer)
 		return size;
 
+	/* Make sure the requested buffer exists */
+	if (cpu_id != RING_BUFFER_ALL_CPUS &&
+	    !cpumask_test_cpu(cpu_id, buffer->cpumask))
+		return size;
+
 	size = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
 	size *= BUF_PAGE_SIZE;