linux-kernel - RE: [patch 3/14] x86, ptrace, bts: stop bts tracing early in do

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Date:	Mon, 30 Mar 2009 07:55:39 +0100
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	Oleg Nesterov <oleg@...hat.com>,
	Markus Metzger <markus.t.metzger@...glemail.com>
CC:	Ingo Molnar <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"roland@...hat.com" <roland@...hat.com>,
	"eranian@...glemail.com" <eranian@...glemail.com>,
	"Villacis, Juan" <juan.villacis@...el.com>,
	"ak@...ux.jf.intel.com" <ak@...ux.jf.intel.com>
Subject: RE: [patch 3/14] x86, ptrace, bts: stop bts tracing early in	do_exit

>-----Original Message-----
>From: Oleg Nesterov [mailto:oleg@...hat.com]
>Sent: Monday, March 30, 2009 3:22 AM
>To: Markus Metzger


>> ds_release_bts(struct bts_tracer *tracer)
>> {
>> 	struct task_struct *task =
>> 		tracer->ds.context->task;
>>
>> 	do {
>> 		set_task_state(task, TASK_UNINTERRUPTIBLE);
>
>Oh, this is not right,

Agreed.

I wonder if it is ever safe to change another task's state.
There's a lot of code that sets the task state seemingly
unprotected - but always for current.


>> Isn't this a general problem for ptrace?
>>
>> Ptrace uses a similar pattern in ptrace_check_attach().
>> It stops the traced task, but that task might wake up immediately after
>> that check. It might be scheduled in any time during a ptrace operation.
>
>Yes. ptrace() can assume the tracee is TASK_TRACED, or it is dying/dead.
>
>If you need to make sure it is still traced, you can re-check ->state
>under ->siglock. Until you drop this lock, the tracee (if still traced)
>can't escape from ptrace_stop/do_signal_stop, even if scheduled.

I can't hold the siglock for the entire ptrace operation, can I?

Instead, I put all bts stuff into a context struct and I use use-counts
to make sure the tracer is not released while it is used by a ptrace operation.

regards,
markus.

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