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    SubjectRe: [PATCH 1/1] ptrace: make sure do_wait() won't hang after PTRACE_ATTACH
    On 02/14, Tejun Heo wrote:
    >
    > On Mon, Feb 14, 2011 at 06:20:52PM +0100, Denys Vlasenko wrote:
    > > >> 23:02:15.622112 nanosleep({30, 0}, NULL) = ? ERESTART_RESTARTBLOCK (To be restarted)
    > > >> 23:02:23.781165 --- SIGSTOP (Stopped (signal)) @ 0 (0) ---
    > > >> 23:02:23.781251 --- SIGSTOP (Stopped (signal)) @ 0 (0) ---
    > > >>     (I forgot again why we see it twice. Another quirk I guess...)
    > > >> 23:02:23.781310 restart_syscall(<... resuming interrupted call ...>) = 0
    > > >> 23:02:45.622433 close(1)                = 0
    > > >> 23:02:45.622743 close(2)                = 0
    > > >> 23:02:45.622885 exit_group(0)           = ?
    > ...
    > > > This can be fixed by updating strace, right?  strace can look at the
    > > > wait(2) exit code and if the tracee stopped for group stop, wait for
    > > > the tracee to be continued instead of issuing PTRACE_SYSCALL.
    > >
    > > But tracee didn't stop _yet_. Signal is not delivered _yet_, debugger
    > > can decide at this point whether to deliver it:
    > > ptrace(PTRACE_SYSCALL, $PID, 0x1, SIGSTOP)
    > > or ignore:
    > > ptrace(PTRACE_SYSCALL, $PID, 0x1, 0)
    > >
    > > strace has to deliver SIGSTOP if it wants to make program run exactly
    > > as it would run without strace. So it tries to do so.
    > > Currently, ptrace machinery doesn't react as strace, its user, expects it to.
    >
    > Okay, maybe I'm missing something but so once SIGSTOP is determined to
    > be delivered, then the tracee enters group stop and that's the second
    > SIGSTOP notification you get.

    Yes, this is correct.

    But my head spins ;) I have already lost the picture.

    > At that point, strace should wait for
    > the tracee to be continued by SIGCONT. That should work, right?

    Again, given that strace is the real parent, in this particular
    case I think strace can work as you suggest.

    Oleg.

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