RE: [patch 3/14] x86, ptrace, bts: stop bts tracing early in do_exit

[Metzger, Markus T]

>-----Original Message-----
>From: Oleg Nesterov [mailto:oleg@xxxxxxxxxx]
>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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