tracing/eprobe: Update cond flag before enabling trigger

The eprobe's trigger, for the event it is attached to, needs access to the
trace record in order to perform its function. This need is indicated by
the TRIGGER_COND flag and properly set by the update_cond_flag() function.
Currently, the cond flag is set after calling
trace_event_trigger_enable_disable(), which leaves a small time gap in
which the eprobe's trigger could be invoked without a trace record, causing
a NULL pointer dereference issue.

This issue can be easily reproduced by enabling an event probe for
kmem/mm_page_alloc using any of its fields as such:

root@localhost:/sys/kernel/tracing# echo 'e kmem/mm_page_alloc $gfp_flags' > dynamic_events
root@localhost:/sys/kernel/tracing# echo 1 > events/eprobes/enable
[   30.847000] general protection fault, probably for non-canonical address 0xdffffc0000000003: 0000 [#1] PREEMPT SMP KASAN PTI
[   30.848618] KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
[   30.849498] CPU: 0 PID: 223 Comm: bash Not tainted 6.1.0-rc5+ torvalds#98
[...] stripped
[   30.850623] RIP: 0010:get_event_field.isra.0+0x1b8/0x3f0
[...] stripped
[   30.856173] Call Trace:
[   30.856340]  <TASK>
[   30.856489]  process_fetch_insn+0x1289/0x19b0
[   30.856789]  ? write_comp_data+0x2f/0x90
[   30.857061]  ? __sanitizer_cov_trace_pc+0x25/0x60
[   30.857379]  ? __trace_eprobe_create+0x1d30/0x1d30
[   30.857701]  ? __sanitizer_cov_trace_pc+0x25/0x60
[   30.858019]  ? write_comp_data+0x2f/0x90
[   30.858289]  ? write_comp_data+0x2f/0x90
[   30.858562]  eprobe_trigger_func+0x1253/0x2180
[   30.858867]  ? kasan_poison.part.0+0x3c/0x50
[   30.859165]  ? process_fetch_insn+0x19b0/0x19b0
[   30.859473]  ? get_page_from_freelist+0x10c1/0x29c0
[   30.859806]  ? rcu_read_lock_bh_held+0xd0/0xd0
[   30.860114]  ? write_comp_data+0x2f/0x90
[   30.860387]  event_triggers_call+0x293/0x410
[   30.860683]  __trace_trigger_soft_disabled+0xbb/0xe0
[   30.861019]  trace_event_raw_event_mm_page_alloc+0xb5/0xc0
[   30.861389]  __alloc_pages+0x4cd/0x760
[   30.861651]  ? __alloc_pages_slowpath.constprop.0+0x2480/0x2480
[   30.862047]  ? tracepoint_probe_register_prio_may_exist+0x100/0x100
[   30.862464]  ? __sanitizer_cov_trace_pc+0x25/0x60
[   30.862780]  ? write_comp_data+0x2f/0x90
[   30.863055]  trace_buffered_event_enable+0xc6/0x440
[   30.863381]  __ftrace_event_enable_disable+0x16a/0x880
[   30.863728]  trace_event_enable_disable+0x2a/0x40
[   30.864043]  trace_event_trigger_enable_disable.part.0+0x70/0x90
[   30.864446]  trace_event_trigger_enable_disable+0x31/0xc0
[   30.864806]  eprobe_register+0x49a/0xdf0
[   30.865080]  ? disable_eprobe+0x370/0x370
[   30.865360]  ? mutex_unlock+0x16/0x20
[   30.865618]  ? write_comp_data+0x2f/0x90
[   30.865897]  __ftrace_event_enable_disable+0x4c1/0x880
[   30.866247]  __ftrace_set_clr_event_nolock+0x297/0x390
[   30.866598]  __ftrace_set_clr_event+0x43/0x70
[   30.866901]  system_enable_write+0x1f7/0x2a0
[   30.867214]  ? event_enable_write+0x290/0x290
[   30.867521]  ? rcu_read_lock_held+0xc0/0xc0
[   30.867842]  ? write_comp_data+0x2f/0x90
[   30.868176]  ? write_comp_data+0x2f/0x90
[   30.868467]  vfs_write+0x311/0xe50
[   30.868752]  ? event_enable_write+0x290/0x290
[   30.869067]  ? kernel_write+0x5b0/0x5b0
[   30.869329]  ? lockdep_hardirqs_on_prepare+0x410/0x410
[   30.869675]  ? lock_is_held_type+0xaf/0x120
[   30.869954]  ? close_fd+0x6e/0xb0
[   30.870182]  ? write_comp_data+0x2f/0x90
[   30.870453]  ? __sanitizer_cov_trace_pc+0x25/0x60
[   30.870772]  ? write_comp_data+0x2f/0x90
[   30.871043]  ? write_comp_data+0x2f/0x90
[   30.871315]  ksys_write+0x158/0x2a0
[   30.871558]  ? __ia32_sys_read+0xc0/0xc0
[   30.871827]  ? syscall_enter_from_user_mode+0x25/0x70
[   30.872169]  ? do_syscall_64+0x3b/0x90
[   30.872432]  __x64_sys_write+0x7c/0xc0
[   30.872691]  ? syscall_enter_from_user_mode+0x25/0x70
[   30.873028]  do_syscall_64+0x60/0x90
[   30.873279]  ? syscall_exit_to_user_mode+0x4a/0x60
[   30.873597]  ? do_syscall_64+0x6d/0x90
[   30.873857]  ? fpregs_assert_state_consistent+0x90/0xf0
[   30.874214]  ? syscall_exit_to_user_mode+0x4a/0x60
[   30.874535]  ? do_syscall_64+0x6d/0x90
[   30.874794]  ? do_syscall_64+0x6d/0x90
[   30.875118]  ? do_syscall_64+0x6d/0x90
[   30.875379]  entry_SYSCALL_64_after_hwframe+0x63/0xcd
[   30.875712] RIP: 0033:0x7f7cc14da8f3
[...] stripped
[   30.879875]  </TASK>
[...] stripped
[   30.881930] ---[ end trace 0000000000000000 ]---
[   30.882237] RIP: 0010:get_event_field.isra.0+0x1b8/0x3f0
[...] stripped
[   30.887734] note: bash[223] exited with preempt_count 2

That happens because enable_eprobe() will eventually trigger the
kmem/mm_page_alloc trace event:

- enable_eprobe [trace_eprobe.c]
  - trace_event_trigger_enable_disable [trace_events_trigger.c]
    - trace_event_enable_disable [trace_events.c]
      - __ftrace_event_enable_disable [trace_events.c]
        - trace_buffered_event_enable [trace.c]
          - alloc_pages_node [gfp.h]
	   ...
            - __alloc_pages [page_alloc.c]
              - trace_mm_page_alloc // eprobe event file without TRIGGER_COND bit set

By the time kmem/mm_page_alloc trace event is hit, the eprobe event file
does not have the TRIGGER_COND flag set yet, which causes the eprobe's
trigger to be invoked (through the trace_trigger_soft_disabled() path)
without a trace record, causing a NULL pointer dereference when fetching
the event fields.

Fix this by setting the cond flag beforehand when enabling the eprobe's
trigger.

Fixes: 7491e2c ("tracing: Add a probe that attaches to trace events")
Signed-off-by: Rafael Mendonca <rafaelmendsr@gmail.com>

kernel/trace/trace_eprobe.c

@@ -727,8 +727,8 @@ static int enable_eprobe(struct trace_eprobe *ep,
 
  list_add_tail_rcu(&trigger->list, &file->triggers);
 
- trace_event_trigger_enable_disable(file, 1);
  update_cond_flag(file);
+ trace_event_trigger_enable_disable(file, 1);
 
  return 0;
 }