In the loop of ftrace_save_ops_tramp_hash(), it adds all the recs
to the ops hash if the rec has only one callback attached and the
ops is connected to the rec. It gives a nasty warning and shuts down
ftrace if the rec doesn't have a trampoline set for it. But this
can happen with the following scenario:

  # cd /sys/kernel/debug/tracing
  # echo schedule do_IRQ > set_ftrace_filter
  # mkdir instances/foo
  # echo schedule > instances/foo/set_ftrace_filter
  # echo function_graph > current_function
  # echo function > instances/foo/current_function
  # echo nop > instances/foo/current_function

The above would then trigger the following warning and disable
ftrace:

 ------------[ cut here ]------------
 WARNING: CPU: 0 PID: 3145 at kernel/trace/ftrace.c:2212 ftrace_run_update_code+0xe4/0x15b()
 Modules linked in: ipt_MASQUERADE sunrpc ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ip [...]
 CPU: 1 PID: 3145 Comm: bash Not tainted 3.16.0-rc3-test+ #136
 Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./To be filled by O.E.M., BIOS SDBLI944.86P 05/08/2007
  0000000000000000 ffffffff81808a88 ffffffff81502130 0000000000000000
  ffffffff81040ca1 ffff880077c08000 ffffffff810bd286 0000000000000001
  ffffffff81a56830 ffff88007a041be0 ffff88007a872d60 00000000000001be
 Call Trace:
  [<ffffffff81502130>] ? dump_stack+0x4a/0x75
  [<ffffffff81040ca1>] ? warn_slowpath_common+0x7e/0x97
  [<ffffffff810bd286>] ? ftrace_run_update_code+0xe4/0x15b
  [<ffffffff810bd286>] ? ftrace_run_update_code+0xe4/0x15b
  [<ffffffff810bda1a>] ? ftrace_shutdown+0x11c/0x16b
  [<ffffffff810bda87>] ? unregister_ftrace_function+0x1e/0x38
  [<ffffffff810cc7e1>] ? function_trace_reset+0x1a/0x28
  [<ffffffff810c924f>] ? tracing_set_tracer+0xc1/0x276
  [<ffffffff810c9477>] ? tracing_set_trace_write+0x73/0x91
  [<ffffffff81132383>] ? __sb_start_write+0x9a/0xcc
  [<ffffffff8120478f>] ? security_file_permission+0x1b/0x31
  [<ffffffff81130e49>] ? vfs_write+0xac/0x11c
  [<ffffffff8113115d>] ? SyS_write+0x60/0x8e
  [<ffffffff81508112>] ? system_call_fastpath+0x16/0x1b
 ---[ end trace 938c4415cbc7dc96 ]---
 ------------[ cut here ]------------

Link: http://lkml.kernel.org/r/20140723120805.GB21376@redhat.com



Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>