Messages in this thread | | | Date | Thu, 12 May 2011 12:23:34 +0200 | From | Tejun Heo <> | Subject | Re: [PATCH 10/11] ptrace: move JOBCTL_TRAPPING wait to wait(2) and ptrace_check_attach() |
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Hello, Oleg.
On Wed, May 11, 2011 at 09:53:33PM +0200, Tejun Heo wrote: > > As for ptrace_check_attach(), it can simply do wait_event(), we > > only need to verify the caller is the tracer. No need to play with > > lock/unlock/retry. > > > > What do you think? > > Hmmm... interesting. Yeah, the state is visible only through wait(2) > and ptrace(2) and for wait(2) TRAPPING is as good as STOPPED/TRACED > and we can wait all we want in ptrace_check_attach(). I'll think more > about it but seems like a nice idea.
Eh, problem. Please consider the following scenario.
* A task is in TASK_STOPPED. current->exit_code contains zero.
* Tracer seizes the task which triggers TRAPPING.
* Tracer wait(2)s which sees TRAPPING but tracee->exit_code is still zero.
I think retrying and ensuring that tracee is in the expected state after going through the regular trapping procedure is better way to handle this. Bypassing wait(2) seems smart and simple but it entails much more subtleties than simple stupid waiting/retrying.
Thanks.
-- tejun
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