linux-kernel - Re: suspicious RCU usage (perf)

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Date:	Mon, 26 Aug 2013 14:18:14 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Dave Jones <davej@...hat.com>
Cc:	paulmck@...ux.vnet.ibm.com,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: suspicious RCU usage (perf)

On Mon, 26 Aug 2013 13:50:12 -0400
Dave Jones <davej@...hat.com> wrote:
> 
> This was triggered as a regular user fwiw.
> I had not been running perf, or any other tracing. It was just left
> fuzzing over the weekend with no interaction at all.

So you are telling me that ftrace was enabled by a regular user? If so,
that's a huge issue.

> [260431.875883]  [<ffffffff816f9c6f>] dump_stack+0x54/0x74
> [260431.876869]  [<ffffffff810be6a7>] lockdep_rcu_suspicious+0xe7/0x120
> [260431.877852]  [<ffffffff81146ef4>] __perf_event_overflow+0x2a4/0x320
> [260431.878834]  [<ffffffff81146d50>] ? __perf_event_overflow+0x100/0x320
> [260431.879811]  [<ffffffff81146e0c>] ? __perf_event_overflow+0x1bc/0x320
> [260431.880784]  [<ffffffff8170ce00>] ? ftrace_call+0x5/0x2f
> [260431.881758]  [<ffffffff81147101>] perf_swevent_overflow+0x51/0xe0
> [260431.882728]  [<ffffffff811471ef>] perf_swevent_event+0x5f/0x90
> [260431.883685]  [<ffffffff81147329>] perf_tp_event+0x109/0x4f0
> [260431.884637]  [<ffffffff81147542>] ? perf_tp_event+0x322/0x4f0
> [260431.885586]  [<ffffffff811079fb>] ? rcu_eqs_enter_common+0x5b/0x420
> [260431.886539]  [<ffffffff8107a320>] ? task_work_run+0xe0/0xe0
> [260431.887486]  [<ffffffff81135910>] perf_ftrace_function_call+0xc0/0xd0

This is the perf function tracing call.

> [260431.888417]  [<ffffffff81114d37>] ? ftrace_ops_control_func+0xe7/0x110
> [260431.889353]  [<ffffffff8107a320>] ? task_work_run+0xe0/0xe0
> [260431.890281]  [<ffffffff811079fb>] ? rcu_eqs_enter_common+0x5b/0x420
> [260431.891191]  [<ffffffff81114d37>] ftrace_ops_control_func+0xe7/0x110
> [260431.892088]  [<ffffffff8170ce00>] ftrace_call+0x5/0x2f

ftrace_call is the mcount trampoline. The only way to get there is via
function tracing, and function tracing should only be enabled by root.

> [260431.892975]  [<ffffffff81114cbb>] ? ftrace_ops_control_func+0x6b/0x110
> [260431.893862]  [<ffffffff8170ce00>] ? ftrace_call+0x5/0x2f
> [260431.894745]  [<ffffffff81097def>] ? local_clock+0x3f/0x50
> [260431.895631]  [<ffffffff8107a325>] ? debug_lockdep_rcu_enabled+0x5/0x40
> [260431.896516]  [<ffffffff8107a325>] ? debug_lockdep_rcu_enabled+0x5/0x40
> [260431.897387]  [<ffffffff811079fb>] ? rcu_eqs_enter_common+0x5b/0x420
> [260431.898264]  [<ffffffff81107e24>] rcu_eqs_enter+0x64/0xa0
> [260431.899133]  [<ffffffff8110c673>] rcu_user_enter+0x13/0x20
> [260431.899999]  [<ffffffff8114e67a>] user_enter+0x6a/0xd0
> [260431.900859]  [<ffffffff81010208>] syscall_trace_leave+0x78/0x150
> [260431.901712]  [<ffffffff8170d4af>] int_check_syscall_exit_work+0x34/0x3d

So my question to you. If you were not running perf or any other
tracing, and this is all just non-root user. How the hell did perf
function tracing get started on your box????

-- Steve
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