kprobes: Do not disarm disabled ftrace kprobe

Only disable a ftrace probe if it is enabled, otherwise:

The following triggers a warning:

  # modprobe trace_printk
  # echo "p:kprobes1/event1 trace_printk:trace_printk_irq_work" > /sys/kernel/tracing/kprobe_events
  # rmmod trace_printk

 ------------[ cut here ]------------
 Failed to disarm kprobe-ftrace at trace_printk_irq_work+0x0/0x76 [trace_printk] (-2)
 WARNING: CPU: 5 PID: 4852 at kernel/kprobes.c:1100 __disarm_kprobe_ftrace.isra.0+0x78/0xa0
 Modules linked in: trace_printk(-) [..] [last unloaded: trace_printk]
 CPU: 5 PID: 4852 Comm: rmmod Tainted: G        W         5.9.0-rc4-test+ torvalds#506
 Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01 v03.03 07/14/2016
 RIP: 0010:__disarm_kprobe_ftrace.isra.0+0x78/0xa0
 Code: 8b 03 eb cb 80 3d 5d 95 58 01 00 75 de 48 8b 75 00 89 c2 89 44 24 04 48 c7 c7 38 e3 33 8b c6 05 43 95 58 01 01 e8 c8 1d ef ff <0f> 0b 8b 44 24 04 eb b9 89 c6 48 c7 c7 08 e3 33 8b 89 44 24 04 e8
 RSP: 0018:ffff971ce04b7e38 EFLAGS: 00010282
 RAX: 0000000000000000 RBX: ffffffff8c900b30 RCX: 0000000000000000
 RDX: 0000000000000001 RSI: ffffffff8a16c5af RDI: ffffffff8a16c5af
 RBP: ffff971cf2722440 R08: 0000000000000001 R09: 0000000000000001
 R10: 0000000000000000 R11: 0000000000000046 R12: ffffffff8b7b33a0
 R13: ffffffff8c901eb0 R14: 0000000000000000 R15: 0000000000000000
 FS:  00007f4fe349b740(0000) GS:ffff971d5ab40000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 000055d07b0148b8 CR3: 00000000b76cc006 CR4: 00000000001706e0
 Call Trace:
  kprobes_module_callback+0x1b3/0x3c0
  notifier_call_chain+0x47/0x70
  blocking_notifier_call_chain+0x43/0x60
  __x64_sys_delete_module+0x161/0x2a0
  do_syscall_64+0x33/0x40
  entry_SYSCALL_64_after_hwframe+0x44/0xa9
 RIP: 0033:0x7f4fe35cb00b
 Code: 73 01 c3 48 8b 0d 7d fe 0b 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa b8 b0 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 4d fe 0b 00 f7 d8 64 89 01 48
 RSP: 002b:00007ffe820f9888 EFLAGS: 00000206 ORIG_RAX: 00000000000000b0
 RAX: ffffffffffffffda RBX: 000055d07b00a800 RCX: 00007f4fe35cb00b
 RDX: 000000000000000a RSI: 0000000000000800 RDI: 000055d07b00a868
 RBP: 00007ffe820f98e8 R08: 0000000000000000 R09: 0000000000000000
 R10: 00007f4fe363eac0 R11: 0000000000000206 R12: 00007ffe820f9ab0
 R13: 00007ffe820fb223 R14: 000055d07b00a2a0 R15: 000055d07b00a800
 irq event stamp: 7463
 hardirqs last  enabled at (7489): [<ffffffff8a1ab4cd>] __tick_nohz_task_switch+0xad/0xc0
 hardirqs last disabled at (7510): [<ffffffff8a1ab4d4>] __tick_nohz_task_switch+0xb4/0xc0
 softirqs last  enabled at (7530): [<ffffffff8ae003b4>] __do_softirq+0x3b4/0x501
 softirqs last disabled at (7545): [<ffffffff8ac01072>] asm_call_on_stack+0x12/0x20
 ---[ end trace 71f3303cdebb63e3 ]---

As well as the following two ftrace selftests fail:

 test.d/kprobe/kprobe_module.tc
 test.d/kprobe/kretprobe_args.tc

This is because we are trying to remove a probe that is not enabled or
registered with ftrace, but exists in the kprobe tables.

Cc: [email protected]
Fixes: 0cb2f13 ("kprobes: Fix NULL pointer dereference at kprobe_ftrace_handler")
Signed-off-by: Steven Rostedt (VMware) <[email protected]>

kernel/kprobes.c

@@ -1088,6 +1088,9 @@ static int __disarm_kprobe_ftrace(struct kprobe *p, struct ftrace_ops *ops,
 {
 	int ret = 0;
 
+	if (kprobe_disabled(p))
+		return ret;
+
 	if (*cnt == 1) {
 		ret = unregister_ftrace_function(ops);
 		if (WARN(ret < 0, "Failed to unregister kprobe-ftrace (%d)\n", ret))