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    SubjectRe: [PATCH 1/1] ptrace: make sure do_wait() won't hang after PTRACE_ATTACH
    On Wednesday 09 February 2011 22:25, Oleg Nesterov wrote:
    > > Note that { the task is put into TASK_TRACED state and group stop
    > > resume by SIGCONT is ignored. | the task is put into TASK_STOPPED
    > > state and the following PTRACE request will transition it into
    > > TASK_TRACED. If SIGCONT is received before transition to
    > > TASK_TRACED is made, the task will resume execution. If PTRACE
    > > request faces with SIGCONT, PTRACE request may fail. }
    > To me, the first variant looks better. But, only because it is closer
    > to the current behaviour. I mean, it is better to change the things
    > incrementally.
    > But in the longer term - I do not know. Personally, I like the
    > TASK_STOPPED variant. To the point, I was thinking that (perhaps)
    > we can change ptrace_stop() so that it simply calls do_signal_stop()
    > if it notices ->group_stop_count != 0.
    > > The ptracer may resume execution of the task using PTRACE_CONT
    > > without affecting other tasks in the group.
    > And this is what I do not like. I just can't accept the fact there
    > is a running thread in the SIGNAL_STOP_STOPPED group.
    > But yes: this is what the current code does, I am not sure we can
    > change this, and both PTRACE_CONT-doesnt-resume-until-SIGCONT and
    > PTRACE_CONT-acts-as-SIGCONT are not "perfect" too.

    Can you enumerate reasons why each of them are not perfect?
    I want to understand your thinking better here.

    > > There exists a
    > > fundamental race condition between SIGCONT and the next PTRACE call
    > Yes, and this race is already here, ptracer should take care.

    From the API POV, there is no race, if we assume Oleg's interpretation
    that "stopped/not-stopped" and "traced/not-traced" states are
    completely orthogonal:

    As long as task is in "traced" state and it is in ptrace-stop, SIGCONT
    delivered to it does not make it run. Only next PTRACE_CONT (or SYSCALL)
    will. Neither will SIGCONT delivered to any other thread group member:
    even though this will terminate group-stop state and all untraced
    tasks will start running, all tasks which are in ptrace-stop will not:
    they will wait for the next PTRACE_CONT (or SYSCALL).

    I realize that currently it doesn't work like this, because
    group-stop and ptrace-stop are intermingled concepts right now.
    My point is, it can be made to work that way, and become free
    of this particular race.

    > > In either case, the fundamentals of ptrace operation don't really
    > > change. All ptrace operations are still per-task and ptracer almost
    > > always has control over execution of the tracee. Sure, it allows
    > > ptraced task to escape group stop but it seems defined clear enough
    > > and IMHO actually is a helpful debugging feature.
    > Heh, I think we found the place where we can't convince each other.
    > What if we toss a coin?

    I'm with Oleg on this. If debugger wants to terminate group-stop,
    it should just send SIGCONT, not depend on the obscure feature (it is not
    documented, right?) that PTRACE_CONT somehow affects group-stop state.

    > > > > What do you do about PTRACE requests while a task is group stopped?
    > > > > Reject them? Block them?
    > > >
    > > > Yes, another known oddity. Of course we shouldn't reject or block.

    Why they need to be rejected or blocked? Think again about
    "strace sleep" interrupted by SIGSTOP (or SIGTSTP):

    * sleep runs in nanosleep
    * SIGSTOP arrives, strace sees it
    * strace logs it and allows it via ptrace(PTRACE_SYSCALL, ..., SIGSTOP)
    * sleep process enters group-stop
    * nothing happens until some other signal arrives
    * say, SIGCONT arrives
    * strace logs it and allows it via ptrace(PTRACE_SYSCALL, ..., SIGCONT)

    I believe your question is "what if tracer wants to do a ptrace op
    on tracee while it is in group-stop" (step 4 above)?

    The answer is simple:
    the same as if tracer wants to do a ptrace op on tracee while it is running,
    that is - ptrace() should return error. For the tracer (in my example,
    strace) there is no difference in state after ptrace(PTRACE_SYSCALL, ..., SIGSTOP)
    and ptrace(PTRACE_SYSCALL, ..., <0 or SIGWINCH or any other sig>):
    in both cases tracer must wait for tracee to enter ptrace-stop before
    any ptrace op is allowed.

    Jan, from gdb developer's POV, do you have a problem with this?

    > > Heh, I'm not asking for proof that it is more useful. :-) But I'm still
    > > curious why you think it's important because the benefits aren't
    > > apparent to me. Roland and you seem to share this opinion without
    > > much dicussion so maybe I'm missing something?
    > I can't!
    > I hate this from the time when I noticed that the application doesn't
    > respond to ^Z under strace. And I used strace exactly because I wanted
    > do debug some (I can't recall exactly) problems with jctl. That is all.

    Recently I had exactly this experience too. It's frustrating.

    > > To me, it
    > > isn't too objectionable to allow debuggers to diddle with tracees
    > > behind the real parent's back. In fact, it would be quite useful when
    > > debugging job control related behaviors. I wouldn't have much problem
    > > accepting the other way around - ie. strict job control even while
    > > being debugged, but given that it is already allowed and visible, I
    > > fail to see why we should change the behavior. It doesn't seem to
    > > have enough benefits to warrant such visible change.
    > All I can say is: sure, I see your point, and perhaps you are right
    > and I am wrong.
    > I'd really like to force CC list to participate ;)

    You just succeeded :)


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